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Critical Review
The relevance of phosphorus and iron chemistry to the recovery of phosphorus from wastewater: a review Philipp Wilfert, Prashanth Suresh Kumar, Leon Korving, Geert-Jan Witkamp, and Mark C.M. Van Loosdrecht Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 07 May 2015 Downloaded from http://pubs.acs.org on May 7, 2015
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The relevance of phosphorus and iron chemistry to
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the recovery of phosphorus from wastewater: a
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review
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Philipp Wilfert‡,a,b, Prashanth Suresh Kumar‡,a,b, Leon Korvinga,*, Geert-Jan Witkampa,b, Mark C.
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M. van Loosdrechtb
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a
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8911 MA, Leeuwarden, The Netherlands
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b
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Netherlands
Wetsus, European Centre Of Excellence for Sustainable Water Technology, Oostergoweg 7,
Dept. Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The
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‡
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*Corresponding author: [emailprotected]; +31-58-2843160; Wetsus, European Centre
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Of Excellence for Sustainable Water Technology, Oostergoweg 7, 8911 MA, Leeuwarden, The
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Netherlands
These authors contributed equally to this work
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KEYWORDS: Phosphorus recovery, wastewater, iron phosphate, chemical phosphorus removal,
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adsorption.
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ABSTRACT
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The addition of iron is a convenient way for removing phosphorus from wastewater, but this is
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often considered to limit phosphorus recovery. Struvite precipitation is currently used to recover
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phosphorus, and this approach has attracted much interest. However, it requires the use of
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enhanced biological phosphorus removal (EBPR). EBPR is not yet widely applied and the
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recovery potential is low. Other phosphorus recovery methods, including sludge application to
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agricultural land or recovering phosphorus from sludge ash, also have limitations. Energy-
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producing wastewater treatment plants increasingly rely on phosphorus removal using iron, but
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the problem (as in current processes) is the subsequent recovery of phosphorus from the iron. In
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contrast, phosphorus is efficiently mobilized from iron by natural processes in sediments and
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soils. Iron–phosphorus chemistry is diverse, and many parameters influence the binding and
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release of phosphorus, including redox conditions, pH, presence of organic substances, and
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particle morphology. We suggest that the current poor understanding of iron and phosphorus
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chemistry in wastewater systems is preventing processes being developed to recover phosphorus
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from iron–phosphorus rich wastes like municipal wastewater sludge. Parameters that affect
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phosphorus recovery are reviewed here, and methods are suggested for manipulating iron–
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phosphorus chemistry in wastewater treatment processes to allow phosphorus to be recovered.
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1
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1.1 Background Phosphorus (P) is an essential nutrient and is very important for global food production. In
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2000, 19.7 Mt of P was mined as phosphate rock. The major part, 15.3 Mt P, was used to produce
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fertilizers.1 The demand for P will further increase in future due to a growing global population,
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dietary changes and a rising share of biofuels.2 Apart from partial recycling of P by applying
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manure to agricultural land, the usage of P around the world is linear, with very few recycling
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routes and huge inefficiencies in its production and use.1–3 Ecological, geopolitical and economic
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concerns demand P recovery.1–5 Hence, a cyclic use of P and thus development of technologies
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that allow the recovery of P from secondary sources is required. Globally, about 1.3 Mt P/year is
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treated in municipal wastewater treatment plants (WWTPs).1 We focus in this review on
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municipal wastewater as a major secondary source of P. The implications of the interactions
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described for P and iron (Fe) are also relevant to other wastewaters and even surface water.
Introduction
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Phosphorus is removed from wastewater to prevent eutrophication in effluent receiving surface
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waters.6,7 The most popular P removal techniques are enhanced biological phosphorus removal
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(EBPR) and the more widely used chemical phosphorus removal (CPR) using Fe or aluminum
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salts (table S1 in supporting information).8–13 Iron salts are usually preferred. They are cheaper
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than aluminum salts.11,14 Also in EBPR plants, Fe is often dosed to support P removal (table S1 in
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supporting information). Apart from P removal, Fe plays an important role in modern wastewater
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treatment in general. It is used to prevent hydrogen sulfide emissions during anaerobic digestion
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and acts as a coagulant to improve sludge dewatering.15–17 Wastewater pumping stations dose Fe
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to control odors and corrosion18 and this practice may even aid the removal of P in WWTPs.19
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Furthermore, significant amounts of Fe (typically: 0.5–1.5 mg Fe/L)20 can already be present in
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the influent of WWTPs. For instance, data from 19 WWTPs in the Waterschap Vechtstromen in
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The Netherlands showed influent Fe concentrations between 1 and 10 mg/L resulting in an
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average Fe/P molar ratio of about 0.26 (unpublished data). These examples illustrate that Fe is
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omnipresent in modern WWTPs (Table S2 in supporting information) and thus, that significant
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amounts of P can be Fe bound, also in WWTPs that do not rely on Fe based CPR.
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The presence of Fe is often perceived as negative when evaluating P recovery options.12,21–26
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However, we will show that P is efficiently mobilized from various iron–phosphorus compounds
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(FePs) in environmental systems. This apparent mismatch can be explained by the current lack of
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understanding of the Fe and P chemistry. We will evaluate the literature that we believe is
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important to help understanding Fe and P interactions in WWTPs. We will also present possible
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directions that research and technology related to P recycling from wastewater could take, as
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inspired by the science of environmental mobilization mechanisms.
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1.2 Critical evaluation of current phosphorus recovery options
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use of sludge, production of struvite in EBPR plants and recovery of P from sludge ash. After
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hygienization, sludge (often termed biosolids) can be applied to agricultural land. This practice is
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a widespread, low cost option for P recycling. About 50% of all sludge in the USA27 and about
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40% of all sludge in the 27 EU countries28 was applied in agriculture in 2004 and 2005
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respectively. Public concerns about pathogens, heavy metals, and organic micro-pollutants in
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biosolids are widespread.29–32 But several studies showed that associated risks are low.33,34
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Increasing regulations may further reduce concentrations of certain compounds35,36 but at the
Currently, P recovery methods from wastewater, applied on practical scales, include agricultural
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same time emerging contaminants create new concerns.37 The presence of Fe in biosolids lowers
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the water soluble P fraction.38–41 This can be considered positive, because it may prevent P loss by
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surface runoff.34,42 Some authors perceive the presence of Fe in biosolids as negative as it resulted
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in a reduced plant availability of P.21,22,40,43 However, other studies show Fe bound P can still be
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plant available.44–46 The biggest problem of biosolid application is perhaps the fact that there are
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areas with surpluses of P on agricultural land due to manure surpluses.47,48 Transporting sludge
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from such areas to areas with P deficits is problematic because of the transport costs and logistics
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involved. Thus, a pure and high value P recovery product is preferred over a complex product like
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sludge.
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Several options exist for P recovery to produce high value products.12,49–52 Currently, struvite
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precipitation is attracting the most interest despite of limited P recovery potential. This technique
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requires a combination of EBPR and sludge digestion, ideally in combination with a P stripping
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process.53 But in many countries Fe based CPR plants dominate (Table S1 in supporting
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information). Furthermore, the efficiency to recover P as struvite is typically only 10–50 % of the
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total influent P load.51,52,54 This is due to the presence of P fractions that are not extracted during
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anaerobic digestion (P fixed in biomass or bound to metals like Fe).
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In a few countries, a significant proportion of the sludge is incinerated in mono-incinerators.28
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Recovery of P from sludge ash has advantages: (1) economies of scale due to centralized
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incinerators, (2) nearly all P removed can be recovered, (3) destruction of unwanted compounds
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and (4) P is present in a concentrated form. Various promising thermo- and wet-chemical
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technologies have been developed to recover P from sludge ash.26,50–52,55–58
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technologies Fe plays a role too. It is influencing the extractability of P58 or the water solubility of
For these
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P in the final product.56 These techniques depend on expensive infrastructure for incineration.
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Phosphorus recovery alone will not be a sufficient reason to build sludge incinerators.
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2 Iron is key to wastewater treatment plants of the future
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2.1 A future treatment plant
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an even more important role in WWTPs (Figure 1). Adding Fe is a key step in upcoming WWTPs
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as energy and P factories. Energy-producing WWTPs already exist.59 Such plants often apply the
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A-B process, using a very high loaded biological treatment (adsorption or A-stage) followed by a
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bio-oxidation process or B-stage to remove nitrogen.60 During the A-stage, soluble chemical
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oxygen demand (COD) in the wastewater is used for microbial growth and (bio)flocculation
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removes the biomass, and colloidal and particulate COD from the wastewater. Iron addition is the
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cheapest option for the required coagulation and flocculation of the COD and for P elimination in
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the A-stage.60,61 Anaerobic digestion of A-stage sludge produces a large amount of biogas.60
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Meanwhile, the A-B process has been further improved by using anaerobic ammonium oxidation
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(anammox) to remove nitrogen in the side streams of several WWTPs at elevated temperatures
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(25–40 °C).62–65 The anammox process does not need COD for nitrogen removal, while reducing
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the energy demand simultaneously. The use of anammox at lower temperatures of 10-20 °C (cold
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anammox) in the main treatment lines of WWTPs is being researched.66 Using anammox in the
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main line could potentially allow a WWTP to produce energy at a net rate of 86 J/(person d). A
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typical WWTP, using a classical activated sludge process, consumes 158 J/(person d).67
The presence of Fe is important in wastewater treatment already today. In future, Fe could play
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In the future WWTP (Figure 1), P and COD removal can be achieved by adding Fe in the A-
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stage. Nitrogen is removed using cold anammox. The settled sludge would be digested to produce
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biogas and subsequently, P could be recovered from the digested sludge. Phosphorus recovery
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could be done by selectively bringing iron-bound P into solution using a chemical or
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biotechnological P recovery process that is yet to be developed. The sludge would then be
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dewatered and the P precipitated and recovered as struvite or apatite.
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Alternatively, P could be removed using an adsorption stage after the cold anammox. Owing to
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environmental concerns like eutrophication, more stringent regulations on P discharge limits68,69
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may anyway require P polishing of the effluent. To achieve low P concentrations in the effluent,
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iron based adsorbents have already been used70,71 due to the high affinity of iron oxides for ortho-
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phosphate (Portho).72–74 Adsorption also offers the possibility of P recovery and the re-use of the
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adsorbents.75
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Most of the wastewater treatment techniques described above are already being used or tested at
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the pilot scale. Currently, the only missing process (as in current treatment processes) is
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economically feasible P recovery from FePs-containing sludge. We envisage to develop a P
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recovery process which is inspired by environmental mechanisms.
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Figure 1: Proposed processes for an energy-producing wastewater treatment plant in which P is
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recovered.
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2.2 Environmental cycling: inspiration for recovering phosphorus?
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potential for using sludge in agriculture21,22 or P recovery.12,23,24,26,76 Current processes for
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recovering P from FePs-containing sludge and ash require large changes in pH, pressure, or
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temperature (e.g., the Krepro, Seaborne, Mephrec, Ashdec, and Ecophos processes).25,52,56–58,77
A combination of Fe and P is often considered to have a negative impact when evaluating the
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Usually, it is not economically feasible to use these processes. In contrast, P is mobilized very
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efficiently from FePs in aquatic and terrestrial ecosystems.78–81 A biomimetic process could
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therefore be a more attractive alternative.
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Fungi, bacteria, and plants are able to mobilize Fe bound P and allow P cycling. The
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mobilization of P can be so efficient that it results in environmental damage by causing
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eutrophication in freshwater systems.82 Phosphorus can be released from FePs by iron-
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reducing78,79 or sulfate reducing bacteria.78,79,82 Plants and fungi have developed a wide variety of
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strategies to access Fe and P in FePs.80,83 For example, excretion of carboxylate anions (such as
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oxalate or citrate) that chelate Fe and release P,84,85 exudations of anions (e.g., bicarbonate or
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hydroxide) to desorb P from iron oxides,86,87 or reduction of FePs88 and inducing pH changes to
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release P from FePs.80 Mechanisms presumed to be predominantly related to the mobilization of
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Fe (e.g., excretion of siderophores or iron reduction)89 may also play a role in mobilizing P.88,90
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Dissolved organic matter can assist in the mobilization of P from FePs by chelating Fe91 or by
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facilitating the microbial reduction of Fe.91–94
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Iron plays an important role in controlling the mobilization of P in soil and sediment systems.
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Therefore, a great deal of research has been performed on the role of Fe in the P cycle. The
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results, show that Fe and P cycling is possible, and this implies that recovering P from FePs is
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achievable as well. Insufficient understanding of the Fe and P chemistry in WWTPs has prevented
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the environmental mechanisms responsible for mobilizing P from being transferred to industrial
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processes.
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In section 3, we highlight the need for distinguishing between the different kinds of FePs to
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better understand the binding and release of P. In section 4, we will show that various FePs are
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formed and transformed during wastewater treatment processes but that little information is
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available on the occurrence and behavior of these FePs. In Section 5, we will describe the findings
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on the mobilization of P from FePs that could offer inspiration for the development of new P
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recovery technologies.
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3.1 Diversity of iron–phosphorus compounds
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states varying between -2 to +6 although +2 (ferrous) and +3 (ferric) are the most common
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oxidation states encountered. The solubility of ferrous and ferric ions vary with pH and oxidation
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reduction potential (ORP) (Figure 2). Depending on the pH, the ferrous and ferric ions can get
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hydrolyzed and form various insoluble oxides, oxyhydroxides and hydroxides, collectively termed
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iron oxides.95
Iron and phosphorus interactions
3.1.1 Introduction to iron–phosphorus compounds Iron is a transition metal and its chemistry is very diverse.95 It can exist in several oxidation
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Fig 2: Simplified Pourbaix diagram showing the stable iron species under different conditions
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The FePs found in WWTPs can be either iron phosphate minerals or adsorption complexes
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which involve adsorption of Portho to iron oxides (different methods to characterize FeP
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interactions are listed in table S3).97–100 These FePs have often not been well described. This has
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led to publications on the removal of P using Fe or on the recovery of P from FePs often
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containing unspecific expressions such as “insoluble iron phosphates”, “metal phosphates”, and
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“iron III phosphates”. We will give examples which illustrate that P can be bound to Fe in various
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ways and that the amount and strength of P bound to the Fe differ. This suggests that there is a
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range of mechanisms through which FePs can be altered resulting in P release, underlining the
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importance to differentiate between various FeP.
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3.1.2 Iron oxides and their interaction with Portho At least 16 iron oxides exist.95 Prominent examples of ferric iron oxides are goethite,
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ferrihydrite, lepidocrocite, akaganeite, and hematite. Green rust iron oxides and magnetite are
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examples of iron oxides that contain both ferrous and ferric iron. The different iron oxides have
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different crystalline structures or are amorphous, and these structures largely determine properties
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such as porosity, specific surface area, the number of exposed surface sites, solubility, and
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reducibility. These properties in turn affect the Portho binding properties of the iron oxides and the
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bioavailability of adsorbed Portho.101–105 The surface area of the iron oxide usually correlates with
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its capacity to adsorb Portho (Figure S1 in supporting information). Amorphous or less crystalline
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iron oxides have higher Portho adsorption capacities than more crystalline iron oxides, and this is
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attributed to amorphous iron oxides having higher surface areas.101,103,106 Portho adsorption to iron
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oxides can also differ due to the type and density of surface hydroxyl groups present on the crystal
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faces, which are the functional groups where Portho adsorption occurs.95 Hematites showed Portho
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adsorption capacities varying from 0.19 to 3.33 µmol/m2 due to the differences in their crystal
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faces.104 In contrast, goethites showed a narrower range of Portho adsorption capacities between
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2.16 to 2.83 µmol/m2 owing to their relatively constant crystal face distribution.107 Figure 3 shows
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the Portho adsorption capacities in different iron oxides. The Portho adsorption capacity varies within
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the same type of iron oxides based on the conditions under which they are synthesized and
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used.104,105,108
212 213 214 215
Figure 3. Portho adsorption capacities of different iron oxides. Details of conditions used for adsorption are presented in Table S4 in supporting information.
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Figure 4: Anion binding onto iron oxides as: Portho adsorbed as innersphere complexes109–111 a)
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mononuclear monodentate b) mononuclear bidentate c) binuclear bidentate; d) sulfate adsorption
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is shown as an example for outersphere complex in which water molecules are present between
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the iron oxide surface and the sulfate112 e) example of surface precipitation in which dissolved Fe
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from the iron oxide surface contributes to the formation of multiple layers of FeP precipitates113
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on the surface of the iron oxide.
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Portho adsorption onto iron oxides occurs since the Fe beneath the surface hydroxyl acts as a
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Lewis acid and exchanges the surface OH groups for other ligands.95 When Portho is bound directly
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to an iron oxide surface through a ligand exchange mechanism, without any water molecules
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between the Portho and the surface, (Figure 4 a,b,c) the resulting complex is called an innersphere
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complex.114 An innersphere complex can comprise of a single Portho molecule attached through
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one or two oxygen bonds (mono or bidentate respectively) with either one or two Fe atoms (mono
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or binuclear, respectively).115 The type of complex formed determines the relative strength at
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which the Portho is bound. Bidentate complexes have more stable structures than monodentate
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complexes, which implies that it could be easier to release Portho from monodentate than from
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bidentate complexes.110 The types of innersphere complexes differ based on the type of iron
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oxides and the conditions (such as the pH and the initial Portho concentration).109,110,114 Thus, Portho
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adsorption and desorption properties vary for different iron oxides and for the conditions where
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the iron oxides are produced and used. This make adsorption a very versatile process and offers
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the possibility of engineering specific adsorbents based on iron oxides.
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Adsorption is not the only interaction that occurs between Portho and iron oxides. It is possible to
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have surface precipitation (Fig 4 e), which is the formation of three-dimensional entities as
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opposed to the two-dimensional monolayer coverage during adsorption.115,116 Surface
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precipitation can lead to the formation of a solid phase from which P is less readily desorbed
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because the P buried in the surface precipitate is no longer in equilibrium with the solution.113 The
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dissolution of Fe from the iron oxide contributes to the formation of the surface precipitate.113,117
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For instance, nano zero-valent iron (nZVI) particles were shown to have very high Portho
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adsorption capacities (245 mg P/g) even though their surface area (27.6 m2/g) were not very high
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.118 This high capacity to remove Portho was explained as being partly caused by the occurrence of
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precipitation, which was facilitated by the dissolution of Fe from the nZVI particles. The initial
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Portho concentration in the solution influences the type of binding with iron oxide by determining
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the surface coverage of Portho. Surface complexation tends to dominate at low surface coverages,
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and surface precipitation becomes dominant as the surface loading increases.113,115 At a high
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surface coverage with Portho, goethite and strengite (an iron phosphate mineral) have similar points
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of zero charge (PZC), suggesting that surface precipitation occurred on goethite.113
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3.1.3 Iron phosphate minerals Iron phosphate minerals are polyatomic complexes of iron and phosphate.119–121 Unlike
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adsorption complexes where Portho is removed from solution by binding on the surface of a solid
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(e.g. iron oxide),115 iron phosphate minerals are usually formed in the presence of Portho and
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dissolved iron.122–124 However, the exact mechanisms involved in formation of iron phosphate
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precipitates
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(Fe(III)[PO4]·2H2O) are the common examples of iron phosphate minerals, although there exist
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several
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(Fe(II)(Fe(III))5[(PO4)4|(OH)5]·6H2O) and rockbridgeite (Fe(II)(Fe(III))4(PO4)3(OH)5.119 The
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stability of different iron phosphate minerals vary in terms of their formation and solubility with
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respect to pH and redox conditions126 which in turn might have implications on the P release from
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these compounds. Vivianite has been found in WWTPs and its formation and role in recovering P
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from wastewater will be discussed in detail in sections 3.2.3 and 4.1.4.
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3.2 Iron–phosphorus compounds in wastewater treatment processes
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efficiency at which P is removed in a WWTP by adding Fe is influenced by the oxygen
can
others
be
complex.98,125
like
Vivianite
lipscombite
(Fe3(II)[PO4]2·8H2O)
and
(Fe(II)(Fe(III))2(PO4)2(OH)2),
strengite
beraunite
3.2.1 Introduction to chemical phosphorus removal using iron salts Amongst other reasons, iron salts are added to wastewater to also remove P.127,128 The
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concentration (for ferrous salts), the concentrations of competing ions, the presence of organic
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matter, the pH, the alkalinity, mixing, the age of the Fe or iron oxide flocs, the type of P present,
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and whether ferric or ferrous iron salts are used.127 FePs are exposed to dramatic changes in ORP
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and temperature over a period of about one month in a WWTP with an anaerobic digestion
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process. The following examples will show that adsorption, mineral formation, and
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recrystallization may occur at different stages in a WWTP (figure 5).
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Figure 5: WWTP schematic highlighting possible Fe and P interactions at different stages. Iron
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can be dosed at various stages for reasons like sulfide removal, P removal, flocculation and to
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facilitate dewatering of sludge.
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3.2.2 Dosing ferric versus ferrous iron salts The exact mechanisms through which ferric or ferrous iron salts initially remove P are not yet
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understood. The hydrolysis of ferric iron in an aqueous solution is usually very rapid.129 It has
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been suggested that the adsorption of Portho onto iron oxides is an important98,130 or even the major
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mechanism97,131 involved in the removal of Portho from wastewater when ferric iron salts are dosed.
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The situation is even more complex when ferrous iron is added because this can be partly or
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fully oxidized to ferric iron. The ferrous salts are usually added to aerated stages of the WWTP to
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allow oxidation to ferric iron. The kinetics of ferrous iron oxidation strongly depend on the
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oxygen concentration and particularly on the pH.132 Half of the ferrous iron in water containing
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dissolved oxygen at 5 mg/L has been found to be oxidized to ferric iron within 45 minutes at pH 7
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and within 0.5 minutes at pH 8.133,134 The presence of other ions (e.g., sulfate or Portho) or
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dissolved organic matter can considerably influence the oxidation kinetics.132,135,136 The kinetics
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of ferrous iron oxidation and hydrolysis in wastewater are not well established. In a WWTP, about
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40 % of the ferrous iron that was added was found to be rapidly oxidized to ferric iron (at
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relatively high pH 8.2 and dissolved oxygen concentration of 4.6 mg/L).137 Similarly, half of the
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ferrous iron in activated sludge matrix could be oxidized within hours but about 10 % of the
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ferrous iron fraction was not oxidized even after 6 days of aeration.138 Measurements on sludge
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taken from the aeration tank of a WWTP in which ferrous iron was used to remove P suggest that
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most of the Fe in the sludge was ferric iron.139 In contrast, 43 % of the total Fe in activated sludge
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before anaerobic digestion was found in the form of the ferrous iron phosphate mineral
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vivianite.100 This data indicates either extensive reduction of ferric iron during wastewater
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treatment or incomplete oxidation of the ferrous iron that has been added to the aerated tanks.
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However, also in the absence of oxygen, Portho could be removed with a ferrous Fe:P molar ratio
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of 1.5 in batch tests using secondary effluents, a maximum Portho removal efficiency (98 %) was
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found at pH 8.134 It has been suggested that the removal of P can be made more efficient if ferrous
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iron is slowly oxidized in situ.134,140,141
306 307
3.2.3 Vivianite formation in wastewater treatment plants During wastewater treatment, initially formed FePs may change because of exposure to
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different ORPs and, therefore, to different microbial and chemical processes.18,100,138,139,142
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Vivianite can be formed when ferrous iron is added to remove P.100,134,143 Mössbauer
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spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analyses,
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showed that 43 % of the Fe in activated sludge from a WWTP in which ferrous sulfate was used
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to remove P, and 60–67 % of the Fe in the digested sludge was bound in vivianite.100 Vivianite is
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sparingly soluble in water (Ksp = 10−36), and it is stable in the absence of oxygen, at pH 6–9, under
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non-sulfidic conditions, and in the presence of high ferrous Fe and Portho concentrations.144 In
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WWTPs in which ferric salts are used to remove P or in WWTPs which apply different treatment
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strategies (e.g., the A-B process), it is not known whether vivianite forms or not and if so to what
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degree. The microbial reduction of ferric iron in anaerobic treatment stages could initially lead to
318
P release from FePs.138,139 However, the reduced Fe could ultimately act as a P sink by forming
319
vivianite, which has a higher P content (Fe:P molar ratio of 1.5) than ferric FeP precipitates found
320
in experiments with wastewater (Fe:P molar ratio of 2.5).98 The formation of ferric phosphate
321
minerals like strengite (Fe:P molar ratio of 1) does not seem to play a significant role in WWTPs.
322
In WWTPs strengite and lipscombite in Fe stabilized digested sludge were found after high Fe
323
dosing (Fe:P of 6.15) only.99 Hence, the formation of vivianite could be the final mechanism for
324
the retention of P in WWTPs.
325
4 Transforming iron–phosphorus compounds
326 327 328
4.1 Oxidizing and reducing conditions
329
FePs (Figure 6).95,145–147 The mobilization and retention of P from FePs in these systems, in
330
response to changes of ORPs, is well documented.78,82,148 Similar processes could also occur in
331
WWTPs.
4.1.1 Introduction Iron plays an important role in retaining P in soil and sediments because of the formation of
332
WWTPs require a large range of ORPs to allow different microbial processes to take place. The
333
ORPs in a WWTP will range from less than −300 mV, during anaerobic digestion or the anaerobic
334
period of an EBPR process, to more than +200 mV during the nitrification process. Here,
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microbial and chemical processes can take place that alter FePs by oxidizing or reducing the Fe or
336
by replacing the P with sulfide or other ions. These modifications can affect the P removal
337
performance and other parameters, such as the dewaterability of the sludge.138 Nevertheless, Fe
338
speciation in response to varying ORPs in WWTPs has not received much attention. In a potential
339
P recovery process, exposing FePs to ORPs that anyway occur in WWTP, could assist in P
340
mobilization. For instance, at low ORPs iron reducing or sulfate reducing bacteria could mobilize
341
Fe bound P. On the other hand, oxidation can mobilize P bound in vivianite. The chemical or
342
biological processes that could mobilize P from FePs could be facilitated by the presence of
343
dissolved organic matter. In this section, we give a short overview on how ORPs can influence P
344
binding to Fe. We will show that changes in the ORPs in both, positive and negative ranges and
345
subsequent changes in microbial processes can assist in either retaining or mobilizing P from
346
FePs.
347 348
4.1.2 Iron reduction and iron oxidation The chemical or biological reductive dissolution of ferric iron can cause iron-bound P to be
349
released. In general, dissimilatory iron-reducing bacteria are widespread in soil and sediment
350
systems.149–151 These organisms reduce ferric iron in iron oxides or iron phosphate minerals,
351
thereby mobilizing P.94,152,153 However, in the absence of sulfate, ferrous iron compounds were
352
formed that bound most of the released P.78,154 The reducibility of an iron oxide depends on its
353
crystal structure, solubility, and surface area.155,156 Crystalline iron oxides with low surface area
354
(e.g., goethite and hematite) and low solubility are usually less accessible to iron-reducing
355
organisms than amorphous iron oxides (e.g., lepidocrocite and ferrihydrite).156–158
356
Once formed, ferrous iron can precipitate as secondary iron oxides (e.g., magnetite or green
357
rust) or as ferrous iron phosphate minerals (e.g., vivianite).151 In the presence of electron acceptors
358
(e.g., oxygen or nitrate), dissolved or solid ferrous iron compounds may be oxidized. Biogenic
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iron oxides that can be formed in the presence of iron-oxidizing bacteria include goethite,
360
magnetite, ferrihydrite, and green rust.151 Biogenic iron oxides are often amorphous and
361
nanocrystalline,159 and thus showed high Portho binding capacities.160 Biologically formed iron
362
oxides can contain organic matter, which disrupts the crystallization process95,161 and makes the
363
Fe more accessible and therefore more easily reduced. This reduction process might be assisted by
364
humic substances.162,163
365
It has been shown that iron-reducing and iron-oxidizing bacteria are very active in
366
WWTPs.138,139,142 Reduction (presumably enzymatic) of Fe has been measured in activated sludge
367
immediately after storage under anaerobic conditions. The ferrous iron produced stayed mainly
368
within the organic matrix of the sludge despite of humic substances showing lower affinity to
369
ferrous than ferric iron.139,164 The authors hypothesized that the reduction of Fe can cause
370
significant P release from sludge under anaerobic conditions in WWTPs. However, the formation
371
of secondary ferrous iron oxides or vivianite that can bind P was not taken into account. It has also
372
been shown that the microbial oxidation of ferrous iron in activated sludge using nitrate as an
373
electron acceptor plays a significant role in the denitrification stage in WWTPs.138 The authors
374
hypothesized that this anoxic oxidation of ferrous iron could improve sludge dewatering and P
375
retention. The kinetics of iron oxidation and reduction and the transformation of Fe, that is cycled
376
through treatment stages with high and low ORPs, have not been determined yet. Thus, it is not
377
known whether ferrous or ferric, crystalline or amorphous, biogenic or chemogenic Fe compounds
378
dominate at different stages of a WWTP. Humic substances also play a role in the redox chemistry
379
of Fe. This will be discussed in section 4.2.
380 381
4.1.3 Sulfide and iron–phosphorus compounds Sulfide can reduce ferric iron compounds165 and can further react to form various iron sulfide
382
compounds (FeSs).166 It has been hypothesized that this could be the main mechanism through
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which Fe bound P is released from sediments.78,82,148 The reactivity of an iron oxide toward
384
sulfide (as for iron-reducing bacteria) depends on the crystallinity of the iron oxide. Reaction
385
times have been found to range from minutes for poorly crystalline iron oxides (e.g., hydrous
386
ferric oxide, ferrihydrite, and lepidocrocite) to days or years for more crystalline iron oxides (e.g.,
387
hematite and goethite).165,167 The presence of Portho can decrease the reductive dissolution of
388
different iron oxides by sulfide via formation of binculear innersphere complexes.168–170
389
Sulfide has already been used to solubilize P selectively from FePs sludge for P recovery.
390
Sulfide released 75 % of the solid P into solution at pH 4 from sludge collected at a water
391
production plant.171 Similarly, 43 % of the total solid P was found to be released from sludge pre-
392
coagulated with Fe by adding sulfide.172 In another study, iron sulfate was added to precipitate P
393
in sludge liquor and the microbial reduction of the added sulfate produced sulfide.173
394
Subsequently, P was released (1.5 moles of sulfide released about one mol Portho) over a timescale
395
of days, without gaseous hydrogen sulfide formation.
396
To our knowledge, it is not known if sulfide induced P release is influenced by the type of FeP.
397
However, analogous to the difference in reactivity of sulfide to iron oxides, it is likely that the
398
amount of sulfide required to release P from FePs with different crystal structure varies.
399 400
4.1.4 Transforming vivianite Vivianite could be an important ferrous iron phosphate compound in WWTPs (see section
401
3.2.3). Transformation of vivianite by oxidation or by exposing it to sulfide can induce P release.
402
Chemically, about 5–10 % of the ferrous iron in freshly synthesized vivianite has been found to
403
oxidize within minutes when exposed to air and about two thirds of the ferrous iron was oxidized
404
after air bubbling for 53 days.123 In this study, oxidation occurred in the presence of a P sink (an
405
anion exchange membrane). The initial Fe:P ratio (determined by energy dispersive X-ray
406
spectrometry (EDX)) was 1.4 and the final Fe:P ratio was around 6.2. The complete oxidation of
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vivianite and the formation of an amorphous iron phosphorus compound was much faster (16
408
days) when the oxidation was microbially induced.174 No P sink was present, but the Fe:P ratio
409
(determined by EDX) decreased from 1.3 (vivianite) to 2.8. Due to these properties vivianite has
410
been used as a slow release Fe and P fertilizer.123,175,176 Accordingly, vivianite may recrystallize
411
when sludge is exposed to air resulting in P release.
412
During anaerobic digestion, substantial sulfide formation by sulfate reducing bacteria would
413
most likely result in the release of significant amounts of iron-bound P, as reported for anoxic
414
sediments.82 The formation of vivianite during anaerobic digestion is not hampered by FeSs
415
formation since the supply of sulfate is limited in digesters.177,178 When considering the recovery
416
of P from sludge by sulfide, the crystallinity of vivianite should be taken into account. Vivianite
417
could be rather insensitive to sulfide, similar to more crystalline iron oxides.165,167
418
419 420 421
Figure 6: Redox processes and the cycling of P. The arrow keys represent the effect on soluble P: implies P release,
implies P sink,
implies not clear.
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4.2 Humic substances
425
humic substances, plays an important role in Fe and P cycling in soil and aquatic systems. Humic
426
substances have received attention because of their omnipresence and relevance to Fe and P
427
chemistry. Humic substances include humic acids, fulvic acids, and humins.164 These are
428
relatively large, refractory and complex molecules that are products of organic matter degradation.
429
Humic substances lack well-defined compositions but usually contain large numbers of oxygen-
430
containing functional groups, such as carboxyl and hydroxyl groups.164 This characteristic
431
explains some of their interactions with Fe and P. Humic substances contributed about 20 % of the
432
total dissolved organic carbon in secondary effluent180 and 10–20 % of the total organic carbon in
433
sludge dry matter.181 It has been estimated that 22 % of the Fe in activated sludge could be bound
434
to organic matter.139 Accordingly, pyrophosphate extractions showed that approximately 30 % of
435
Fe in digested sludge could be bound to organic matter.182,183 Since humic substances are present
436
in abundance in WWTPs, they can considerably effect Fe and P speciation. Hence, their effects
437
need to be considered during research on P recovery processes from wastewater. Especially, since
438
the effect of humic substances on FePs has shown controversial results. In the next section, we
439
will briefly discuss how humic substances interact with Fe and the various ways in which they can
440
affect Fe and P interaction.
441 442
4.2.2 Humic substances interaction with iron and phosphorus The bond between Fe and humic molecules is relatively strong and can prevent the hydrolysis
443
and polymerization of Fe.184 Mössbauer spectroscopy185 and synchrotron-based spectroscopy184
444
have indicated that ferric iron can occur as oxides and non-oxides together with OM. It has also
445
been shown that different bonds between Fe and humic substances have different strengths186 and
446
that mononuclear and polymeric iron humic complexes occur.184,187,188 The type of complex
4.2.1 Introduction Organic matter contributes 40–80 wt. % of the total solids in sludge.179 Organic matter, like
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formed influences Fe speciation, and the processes that lead to the different species being formed
448
include Fe hydrolysis, polymerization, and the binding of arsenate, which has similar structure
449
and reactivity as Portho.184,189–191
450
The presence of humic substances decreased the Portho adsorption capacity of goethite.192–194 It
451
has also been suggested that humic substances have either limited or positive effects on the
452
binding of Portho to Fe.195–197 It has been hypothesized that the Portho adsorption capacity of Fe
453
could increase because of the formation of iron–humic–phosphorus complexes.196,198 Such
454
complexes have been found to have about eight times higher Portho adsorption capacities than pure
455
iron oxide phases.195 This could be due to the Fe being finely distributed on the organic
456
surfaces.195 In studies using Mössbauer spectroscopy, it has been confirmed that iron oxides can
457
be evenly distributed over the surfaces of humic compounds.199 Yet, to the best of our knowledge,
458
there is no direct proof for the existence of such iron–humic–phosphorus complexes. However,
459
the binding of arsenic by humic–iron compounds has been proven using extended X-ray
460
absorption fine structure analyses. 189
461
The presence of humic substances could increase the Portho adsorption capacity of iron oxides by
462
preventing crystallization of amorphous iron oxides.185,200–202 However, it has also been shown
463
that organic matter does not have a significant influence on the crystallization of iron oxides and
464
does not affect the adsorption of P.203 Ferrous iron can be bound by humic substances, influencing
465
oxidation properties of ferrous iron, the crystallization of iron oxides, and the bioavailability of
466
ferrous iron.204,205 It has been found that humic substances can dissolve P by chelating Fe from
467
ferric FePs.91 Ferric iron can be kept in solution when it has been complexed with humic acids and
468
may, in that state, bind Portho.184,196,198
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Iron-reducing bacteria can use humic substances as electron acceptors during the oxidation of
470
organic compounds.92 The rate at which Fe is reduced may be increased by the presence of humic
471
substances and usually inaccessible iron oxides may be made available.93 The ability of humic
472
substances to transfer or shuttle electrons to ferric iron has led to the hypothesis that even
473
fermenting bacteria, sulfate-reducing bacteria, or methanogens could reduce ferric iron.162,163,206
474
When humic substances act as electron acceptors, they can be restored after exposure to
475
oxygen.207 Fig 7 summarizes the possible effect of humics on Fe and P interactions.
476
477 478
Fig 7: Effect of humic substances on Fe and P interaction
479 480 481
4.3 The effect of pH
482
the speciation of Portho, the surface charge of iron oxides and the solubility of iron oxides and iron
483
phosphate minerals. We will discuss the effect of pH on Fe and P interactions in two contexts.
484
Firstly, the effect of pH on adsorption of Portho on and the desorption of Portho from iron oxides
485
respectively. This will be followed by a short discussion on existing techniques to recover P from
486
FePs in sludge to show controversial experiences that have been made in these studies.
4.3.1 Introduction The pH can have a considerable effect on Fe and P interactions since it affects several factors like
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4.3.2 Desorption of Portho from iron oxides The surface potential of the adsorbent as well as the Portho becomes more negative as the pH
489
increases.208 Beyond the PZC of the iron oxide, electrostatic repulsion leads to a decrease in Portho
490
adsorption.209 Furthermore, an increase in pH increases the hydroxide ion concentration, which
491
results in Portho desorption. The hydroxide ion is the hardest Lewis base among the common
492
inorganic ions, so it is an effective reagent for desorption.210 Desorption of Portho from iron oxides
493
has been studied somewhat less than adsorption. Not all of the adsorbed Portho is easily released by
494
competing ions.107,108 The proportion of the adsorbate ion that is not easily desorbed could be
495
explained by the formation of surface precipitates, the slow restructuring of the solid, or diffusion
496
limitations related to the porosities of the iron oxides.108,113,116,211 XRD measurements have shown
497
that the crystallinity of goethite increased after one adsorption-desorption cycle (with NaOH), and
498
this affected Portho adsorption negatively.211 However, no change in crystallinity and reusability
499
(after 10 cycles) was observed after desorption using akaganeite.211
500 501
4.3.3 Inducing pH changes to recover phosphorus Wastewater and sludge is usually at pH 6–8 in WWTPs179 but much higher or lower pH are
502
applied in some processes to recover P. It has been suggested that at pH 13, P may be released
503
from FePs sludge using a microbial electrolysis cell.212,213 Phosphorus extraction from FePs
504
containing sludge, taken from a WWTP using Fe electrolysis for P removal, was more selective
505
and greater in alkaline compared to acidic conditions (92 compared to 70 % of total P
506
extracted).214 In other studies, relatively little P was released under alkaline extraction conditions
507
from FePs sludge215 (13 % extracted at pH 13) and iron-rich sludge ash216 (3–28 % extracted
508
using 1 M NaOH). These contradictory results further underline the importance of characterizing
509
FePs. The re-precipitation of released P (as calcium or magnesium phosphorus compounds) could
510
influence its net release. Strong acidification will dissolve and release P from iron oxides and iron
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phosphate minerals thereby mobilizing most of the P in sludge and ash samples.215,217–220
512
Acidification is part of current P recovery techniques (such as Ecophos, ICL, PHONAX,
513
Seaborne, and Recophos) but can also bring heavy metals and other metals into solution.
514
5 Approaches to recover phosphorus from iron
515
Future energy producing WWTPs will rely on the removal of P and COD by Fe addition. An
516
economically feasible process for recovering P from FePs does not yet exist. Many different FePs
517
may be formed in WWTPs because of the wide range of microbial and chemical processes that
518
occur. The development of processes for recovering P from FePs demands more research,
519
especially on Fe and P interactions in WWTPs. The generated knowledge will help to identify the
520
best stages for introducing P recovery processes and will prepare a base for additional focused
521
research. Furthermore, this research will help to better understand and to improve wastewater
522
treatment processes, in general. For instance, it may be possible to induce formation of a specific
523
FeP from which P is easily extractable. A wide range of processes for releasing P from FePs in
524
nature exist, these processes depend also on the types of FePs present. The most relevant
525
mechanisms are summarized below:
526
-
The reduction of Fe may trigger initial P release from ferric FePs, but the vivianite
527
subsequently formed can act as a net P sink. In contrast, the oxidation of Fe may cause net
528
release of P bound in vivianite. Biological and chemical oxidation and reduction of FePs
529
occur in WWTPs. The use of these processes to develop a P recovery process remains to be
530
addressed.
531
-
Microbial reduction and oxidation of Fe plays an important role in the binding and release of
532
P. Different Fe compounds have different availabilities to the microbes that are responsible
533
for the oxidation or reduction of the Fe. These processes may be facilitated (e.g., by the
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presence of humic substances) or hampered (e.g., by the crystal structure of the ferric FePs)
535
by other parameters.
536
-
Sulfide selectively releases P bound to ferric and ferrous FePs. Sulfide is formed to a limited
537
extent during anaerobic digestion of sludge. However, further stimulation of sulfate
538
reducing activity (e.g. after anaerobic digestion) would require COD input and would reduce
539
the net energy yield of the WWTP. Additionally, sulfide is corrosive and toxic. Therefore,
540
although sulfide addition could be useful to recover P, the dosing of sulfide needs to be
541
optimized and economic feasibility needs to be considered as well. The reaction
542
mechanisms between sulfide and FePs and the type of FeP in WWTPs have to be
543
investigated in detail to evaluate the potential of sulfide for P recovery from FePs.
544
-
Under very alkaline or acidic conditions P is released from most FePs. However,
545
contradictory results have been found under alkaline conditions, suggesting that the release
546
depends on the types of FePs that are present in sludge.
547
-
The presence of high concentrations of OM in WWTPs complicates the Fe and P chemistry
548
involved. The role of OM in the Fe and P biogeochemistry is not clear. It can, however, be
549
assumed that OM significantly influence Fe and P speciation in WWTPs. Thus, OM should
550
be included in future research on the development of a biomimetic process to recover P from
551
FePs.
552
-
Another approach to recovering P is to simplify the complex FePs interactions by
553
engineering iron-based adsorbents. Iron-based adsorbents are already used to remove P from
554
WWTP effluent. The regeneration of these adsorbents could be an effective approach to
555
recovering P. Currently, this aspect receives insufficient attention. The diversity of FePs
556
chemistry can be used to influence the binding and release characteristics of P, for example,
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by varying the crystallinity, pore size distribution or surface area of the iron oxide based
558
adsorbent.
559
We believe that a process for recovering P using Fe should be developed in two steps. First,
560
suitable FePs should be identified and characterized. Second, specific tools for mobilizing P
561
from these compounds should be identified. Developing a biomimetic process to recover P from
562
FePs would be an important step towards WWTPs acting as energy and nutrient factories.
563
6 Acknowledgements
564
This work was performed in the TTIW-cooperation framework of Wetsus, European Centre Of
565
Excellence For Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch
566
Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of
567
Fryslân, the City of Leeuwarden and the EZ/Kompas program of the ‘Samenwerkingsverband
568
Noord-Nederland’. The authors would like to thank the participants of the research theme
569
“Phosphate Recovery” for their financial support and helpful discussions.
570
7 Supporting Information Available
571
Tables S1 to S4 and figure S1 are included in the supporting information. The supporting
572
information is available free of charge.
573
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