Critical Review: Biogeochemical Networking of Iron in Constructed

Jun 25, 2019 - In this review, the biogeochemistry of Fe in natural wetland systems will be .... includes several abiotic and biotic subprocesses whic...
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Critical Review Cite This: Environ. Sci. Technol. 2019, 53, 7930−7944

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Critical Review: Biogeochemical Networking of Iron in Constructed Wetlands for Wastewater Treatment Shubiao Wu,*,† Jan Vymazal,‡ and Hans Brix§,∥ †

Aarhus Institute of Advanced Studies, Aarhus University, Høegh-Guldbergs Gade 6B, DK-8000 Aarhus C, Denmark Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kymýcká 129, 165 21 Praha 6, Czech Republic § Department of Bioscience, Aarhus University, Aarhus 8000C, Denmark ∥ WATEC Aarhus University Centre for Water Technology, Aarhus University, Aarhus 8000C, Denmark Downloaded via BUFFALO STATE on July 18, 2019 at 05:50:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Iron is present in all types of wastewater; however, besides acid mine drainage, where it is a major constituent of concern, it is usually neglected in other types of wastewaters. In all kinds of constructed wetlands, iron plays important role in removal of organics and phosphorus, and it has an impact on transformation of nitrogen, sulfur, and metals. The biogeochemistry of iron is well understood in natural wetlands, but knowledge about iron impact on microbiological and chemical transformations during wastewater treatment in constructed wetlands is very limited. So far, the sparse research in this area provides limited information on observed interactions with several varying parameters across the studies, making it difficult to draw fundamental and mechanistic conclusions. A critical review of the complex biogeochemical networking of iron in CWs is therefore necessary to fill the gap in knowledge on the role of iron and its biogeochemical multi-interactions in wastewater treatment processes of CWs. This review is the first with specific focus on iron, discussing its mitigation and retention in CWs with different configurations and operational strategies, and presenting both seasonal dynamics and the potential remobilization of Fe. It also comprehensively discusses the interactions of redox-controlled iron turnover with the biogeochemical processes of other elements, for example, carbon (C), nitrogen (N), phosphorus (P), sulfur (S), and heavy metals. The health response of wetland plants to both deficiency and toxicity of Fe in CWs designed with specific treatment targets has also been evaluated. Due to the complexity of various wastewater compositions and microredox gradients in the root rhizosphere in CWs, future research needs have also been identified.

1. INTRODUCTION

seen as a simple and direct indicator for the increasing transparency of the knowledge “black box” in this field (Supporting Information (SI) Figure S1). For example, microbial transformations of carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) have been extensively investigated and reviewed.3,6−10 Moreover, reviews on dynamics of heavy metals11−13 and emerging organic pollutants, for example, antibiotics and pharmaceutical contaminants14,15 in remediating process of CWs, have also been performed. All these reviews undoubtedly have enabled the mechanisms of these transformations to be more evident in the last few decades. Because iron (Fe) is an essential element, the mitigation and mobilization of Fe in CWs not only participates in many physiological processes, for example, plant photosynthesis, but Fe reduction−oxidation turnover also

Constructed wetlands (CWs) are engineered eco-systems that are constructed and operated for the purpose of wastewater treatment by manipulating the simultaneous physical, chemical, and biological processes occurring in natural wetlands.1 Because of their cost-effective and eco-friendly way of treating wastewater, CWs have been developed as an alternative to conventional centralized wastewater treatment systems in the last few decades. From the technical application point of view, along with the growing attention to CW technology, the design and construction of CWs have been extended from traditional basic models to various new configurations, technical amendments, and operations to improve the performance for pollution removal.2,3 Thus, the application of this technology has significantly expanded from polishing tertiary and secondary effluent of centralized wastewater treatment plants to treatment of various industrial effluents.4,5 From the point of view of exploring scientific knowledge, the flourishing publication record of individual experimental research as well as review in the Web of Science might be © 2019 American Chemical Society

Received: Revised: Accepted: Published: 7930

February 15, 2019 June 14, 2019 June 25, 2019 June 25, 2019 DOI: 10.1021/acs.est.9b00958 Environ. Sci. Technol. 2019, 53, 7930−7944

Critical Review

Environmental Science & Technology

−2 to +6, although +2 (Fe2+ in soluble form) and +3 (Fe3+ in insoluble form) are the most common oxidation states encountered. Iron biogeochemistry in nature is complex and covers various oxidation and reduction processes. 2.1. Oxidation. In reductive condition ferrous iron is the dominant form which is fully dissolved in the water. However, in oxidative conditions under the exposure to oxygen in the atmosphere, the water may turn reddish brown in color.23 This formation process of brown substance in the water is due to the spontaneous oxidation of ferrous iron to ferric iron and subsequent hydrolysis to ferric hydroxide, Fe(OH)3 or oxyhydroxide, FeOOH in the presence of oxygen (eqs 1, 2, 3). But this reaction is strongly dependent upon the environmental redox potential and pH19,24 (SI Figure S2).

affects the geochemical cycle of other elements in CWs.16 Even though the removal performance of iron in some CWs, particularly those treating acid mine drainage, has been observed,17−19 the comprehensive knowledge on the dynamic mitigation of iron and retention in various wetland configurations and intensive operations are not well understood. How iron cycling interacts with the microbial transformations of the above-mentioned four basic elements and trace metals in CWs also needs to be more clearly explained. So far the sparse research in this area provides limited information on observed interactions with several varying parameters across the studies, making it difficult to draw fundamental and mechanistic conclusions. The essential function of Fe to wetland plant metabolism and chlorophyll synthesis has been well recorded in the scientific literature.20 In natural wetlands with anoxic soil conditions, ferrous Fe is often the dominant form of Fe which is readily available for plant uptake. However, in CWs with forced aeration or vertical unsaturated flow, the oxidized conditions can easily form low soluble iron precipitates and cause iron deficiency, resulting in leaf chlorosis and reduced growth.21 Fe toxicity to wetland plants may also need to be considered when iron-rich substrates are used for specific treatment purposes. Therefore, the health of wetland plants can be challenged by either deficiency or toxicity of Fe in different configured CWs, thus affecting the role of plants in wetlands. The formation of iron plaque on the root surface of various aquatic and wetland plants has been reported.22 Even though scientists have put extensive efforts toward improving the understanding of the role of iron plaque in sequestration and translocation of nutrients and metals, a conclusion still is yet to be achieved due to several contrasting results. For example, the uptake of nutrients and metals by plants can be either positively or negatively influenced by not only the extent of iron plaque formation, but also by other factors such as plant species, pH of the rhizosphere, or water composition. The phase and reactivity of Fe in these plaques also plays an important role in controlling the desorbility of the adhered phosphorus and metals from plaque to plants. Therefore, this scenario profoundly illustrates the necessity of a review on the role of iron-related plaque on root surfaces in the sequestration and translocation of various trace metals in CWs, particularly with different plant species and configurations. In this review, the biogeochemistry of Fe in natural wetland systems will be first described, in order to give an overall knowledge background before getting into the comprehensive discussion later. The presence of Fe in various wastewater treated in CWs and its basic redox-controlled processes in CWs with different configurations and operational strategies will then be summarized. Subsequently, the interactions of iron cycling with geochemical processes of other elements (e.g., C, N, P, S) will be discussed. The health response of wetland plants to both deficiency and toxicity of Fe in CWs designed with specific treatment targets will be evaluated. The role of iron plaque in sequestration and translocation of various trace metals will also be discussed. Finally, future research perspectives associated with Fe in CWs will be pointed out.

Fe2 + + 0.25O2 + H+ → Fe3 + + 0.5H 2O

(1)

3H 2O + Fe3 + → Fe(OH)3 + 3H+

(2)

2H 2O + Fe3 + → FeOOH + 3H+

(3)

The bacteria mediated biological iron oxidation and subsequent precipitation are intermediated between those required for physical−chemical iron removal.25 For example, the “true” iron bacteria occur in iron-rich waters of neutral or alkaline pH. They are autotrophic, aerobic bacteria in which the oxidation of iron is an important source for their metabolic energy.26 This group is most often associated with filamentous and stalked forms that are encrusted with iron (Leptothrix, Clonothrix, and Gallionella). Some species of Leptothrix are facultative iron bacteria that can oxidize both ferrous and manganous salts, whereas Gallionella (Spirophyllum) is restricted obligately to iron. Characteristic reaction (eq 4) of the few chemoautotrophic bacteria that deposit hydroxides and oxides is26 4Fe(HCO3)2 + O2 + 6H 2O → 4Fe(OH)3 + 4H 2CO3 + 4CO2 (4)

Various strains of nitrate-reducing bacteria have been found in the aquatic environment, with ecophysiology and the energetic benefit to utilize ferrous iron as electron donor to reduce nitrate into nitrogen gas.27,28 The process of denitrification based on nitrate-dependent Fe(II) oxidation was discovered two decades ago, but mainly in lake and freshwater sediments. The stoichiometry of the nitrate-dependent Fe(II) oxidation can be described as eq 5.29 10Fe2 + + 2NO3− + 24H 2O → 10Fe(OH)3 + N2 + 18H+ (5)

2.2. Reduction. Reduction of Fe(III) in anaerobic aquatic environments may involve both microbial dissimilatory reduction and chemical reduction.30,31 Under natural conditions, the reduction of ferric oxide may proceed chemically (eqs 6,7) by the involvement of sulfide or organic materials such as phenols and carboxylic acid.32 So, sulfate-reducing, sulfite producing bacteria may be indirectly involved in the reduction of ferric oxide.33,34 When sulfide oxidation occurs, in addition to Fe(III) reduction, the same overall reaction will produce Fe(II) without any apparent sulfate reduction (eqs 6 and 7). Two types of ferric oxide-reducing bacteria can be distinguished: a group producing fermentation products and a group using fermentation products for the generation of energy. Facultative and strictly anaerobic bacteria belong to the first group, many of which are also able to reduce nitrate.32

2. IRON BIOGEOCHEMISTRY Iron, making up at least 5% of the earth’s crust, is one of the earth’s most plentiful resources. Iron is a transition element and exists in a wide range of oxidation states varying between 7931

DOI: 10.1021/acs.est.9b00958 Environ. Sci. Technol. 2019, 53, 7930−7944

Critical Review

Environmental Science & Technology

Figure 1. Mitigation and retention of Fe in horizontal subsurface flow CWs.

3. PRESENCE OF FE IN VARIOUS WASTEWATERS TREATED IN CWS The presence of Fe in CWs for wastewater treatment comes from two main sources: influent wastewater and wetland matrix. Because of the unavoidable contact of water with the underlying geologic formations, iron becomes a common constituent in many wastewaters, for example, domestic and municipal sewage, landfill, or acid mine drainage. The content of Fe in various wastewaters ranges widely (SI Table S1). Fe concentrations in municipal/domestic sewage, agricultural/ highway runoff, effluents from textile, tannery, and oil refineries are usually low. Iron concentration in these low-level wastewater effluents does not represent any environmental risks. High content of Fe is often reported from various industrial effluents, landfill leachate, and mine drainage, but depends on time taken for build up to occur. Taking the acid mine drainage as an example, ferrous iron generation and subsequent oxidation to ferric iron from iron pyrite oxidation during the mining process can easily occur under oxidative conditions under the exposure to oxygen. In mining drainage, the Fe concentration can reach up to thousands mg/L. It is important to note that for both low Fe and high Fe content effluents, for example, municipal sewage and acid mine drainage (AMD), the removal of iron may not be the primary focus for the construction of CWs.19 However, the mitigation and retention of Fe is highly important due to its interactions with other wastewater constituent such as P, S, and trace metals.16

The reduction of ferric oxide may release phosphate and trace elements that are adsorbed to the ferric oxide. So the reduction of ferric oxide may enhance the availability of these compounds in the soil.33 Moreover, Fe(III)-reducing organisms can inhibit sulfate reduction and methane production by outcompeting sulfate reducers and methanogens for electron donors.35 2FeOOH + 4H+ + H 2S ↔ 2Fe2 + + So + 4H 2O

(6)

6FeOOH + 10H+ + So ↔ 6Fe2 + + SO4 2 − + 8H 2O (7)

Under acidic conditions, several autotrophic bacteria (e.g., Thiobacillus ferrooxidans, T. thiooxidans, Sulfolobus acidocladarius) can oxidize iron disulfides, and sulfur (So) to sulfate, with Fe(III) serving as the electron acceptor (eqs 8 and 9). Ferric sulfate, the product of the oxidation, reacts with water to form ferric hydroxide and sulfuric acid (eq 10). This reaction is spontaneous and leads to a net increase in acid formation in the environment.36 FeS2 + Fe2(SO4 )3 → 3FeSO4 + 2S

(8)

2S + 6Fe2(SO4 )3 + 8H 2O → 12FeSO4 + 8H 2SO4

(9)

2Fe2(SO4 )3 + 12H 2O → 4Fe(OH)3 + 6H 2SO4

(10)

The process of ammonium oxidation can also be coupled to dissimilatory reduction of iron oxides under anaerobic conditions in wetland soils, where microorganisms can use Fe(III) as an electron acceptor to oxidize NH4+ to NO2−.37 This process is also known as Feammox. Although the Feammox process also occurs under anaerobic conditions similar to anammox, these two processes are different. Many species have been identified as anammox bacteria in the group of the phylum Planctomycetes, while there is only one microorganism so far has been identified to perform the Feammox process, namely Acidimicrobiaceae sp. A6 (ATCC, PTA-122488).38,39 The stoichiometry of the Feammox reaction with ferrihydrite as the ferric iron source is expressed as eq 1138,40

4. RETENTION AND REMOBILIZATION OF FE IN CWS The process of Fe retention in CWs mainly involves sedimentation of Fe3+ precipitates and plant uptake. As the diagram in example of horizontal subsurface flow CWs of Figure 1 shows, the sedimentation process includes several abiotic and biotic subprocesses which depend on the flow path of water along the length of wetland as well as the depth. The Fe uptake performance of wetland plants and immobility in roots will be discussed in next section. Seasonal and diurnal changes of Fe reduction and oxidation in the wetland bed, as well as remobilization are also included in this section to enrich our understanding the dynamic mitigation and retention of Fe in CWs. 4.1. Sedimentation. Fe removal are well reported in fullscale free water surface flow CWs and horizontal subsurface flow CWs for the treatment of AMD and municipal sewage

3Fe2O3 ·0.5H 2O + NH4 + + 10H+ → NO2− + 6Fe2+ + 8.5H 2O

(11) 7932

DOI: 10.1021/acs.est.9b00958 Environ. Sci. Technol. 2019, 53, 7930−7944

Critical Review

Environmental Science & Technology Table 1. Performance of Fe Removal in Different CWsa scale HF full full full full full full full full full pilot pilot lab FWS full full full full full full full full full lab lab VF lab lab lab lab

wastewater

inflow

municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage domestic sewage artificial water artificial water AMD

2.417 0.930 0.980 0.280 0.520 1.400 7.600 0.43−0.76 1.75 2 2 12

municipal sewage municipal sewage industrial effluent mine drainage mine drainage mine drainage coal ash leachate sewage+industrial sewage+industrial AMD sewage+industrial

2.200 15.300 0.27 170 44 205 4.69 7.73 13.7 12 9.1

AMD AMD AMD AMD

107 107 107 107

outflow

efficiency

plants

reference

1.072 0.460 4.077 0.050 0.140 0.120 0.800 1.1−2.35 0.35 0.18 0.16

55.6 50.5 −358(1) 82 97 91 90 (−50)-(−445) 80.2 92 95 20−100

P. Phalaris P. Phalaris P. Phalaris S. viminalis S. viminalis P. Canadensis P. Canadensis P. australis P. australis P. australis T. latifolia J. eff usus

42 42 42 43 43 43 43 44 45 46 46 34

0.510 2.200 0.07 30 0.9 6.3 0.002−0.49 0.24 0.38−0.67 0.21

79 86 74 82 98 97 90−94 74 95 20−100 82

P. communis P. communis Mixed T. latifolia T. latifolia T. latifolia Mixed E. crassipes E. crassipes J. eff usus E. crassipes

43 43 47 48 49 49 50 51 52 34 53

116 57 15 1.85

−8(2) 46(3) 86(4) 98(5)

P. P. P. P.

41 41 41 41

australis australis australis australis

a The unit for inflow and outflow is mg/L and for efficiency is %. HF means horizontal flow CWs. FWS means free water surface flow CWs. VF means vertical flow CWs. AMD means acid mine drainage. (1) Filtration beds are sealed with clay with high Fe content (22 g/kg). (2) With substrate of gravel. (3) With substrate of cocopeat. (4) With substrate of zeolite. (5) With substrate of limestone.

(Table 1). Even in recent years innovation of the traditional CW types by integrating some new configurations, technology combinations and intensive operational strategies such as forced aeration has extended the application of CWs from traditional domestic sewage treatment to use for treatment of various strong industrial effluents. However, the monitoring strategy of these expanded applications are still conventionally targeting the removal of organic matter, nitrogen, phosphorus, suspended solids and so on. Exploring the mitigation and retention of Fe in these CWs is still not the focus of regular monitoring. So as shown in Table 1, recorded information from current literature on Fe removal in full-scale vertical flow CWs and new innovated wetlands is quite rare, some types were tried but only in small-scale mesocosms.41 The removal performance of Fe in different wetlands is widely variable. Except for some negative removal performance in Fe-rich materials in embedded wetlands, most of the reported wetlands show a positive effect on Fe mitigation. As shown in Table 2, the removed iron from wastewater was mainly retained in the sediments of CWs. It is still difficult to compare their performance, even in the same type of wetlands, due to the variable inflow concentrations and hydraulic retention time as well as plant species. In general, the treatment of AMD in free water surface flow CWs is mainly performed by promoting metal precipitation through subsequent oxidation/hydrolysis and the sedimentation of the

produced precipitates. Thus, Fe accumulated in the sediments is largely in the residual fraction with relatively low mobility and availability.54,55 Without addition of external organic materials, most surface flow CWs cannot buffer the acidity by limited alkalinity production. Thus, failing operations of treatment of AMD containing Fe concentration of >75 mg/L Fe by these systems alone have been reported (Wieder 1989; Brodie and others 1993). Subsequently a few free water surface flow CWs for treatment of AMD implemented extra organic substrata such as peat, horse manure, cocopeat, compost, or sphagnum to enhance metal complexation.56−58 Although horizontal subsurface flow CWs (HF CWs) also rely on similar processes to remove Fe from wastewater, these primarily depend on chemical and microbial reduction processes within the substrate with more anoxic conditions. The removal of Fe in CWs with subsurface flow is usually low because of the anoxic or anaerobic status of the bed resulting in bacterial reduction of Fe3+ and consequent release of the reduced form of Fe2+. The retention by precipitation is then restricted only to narrow zones adjacent to roots where oxygen may diffuse to the rhizosphere. The high removal performance for HF CWs (Table 1) is due to the low influent concentration of Fe in municipal and/or domestic sewage, which are commonly treated by use of HF CWs. However, alkalinity produced from dissimilatory microbial sulfate reduction in this condition is more favorable for neutralizing pH and the 7933

DOI: 10.1021/acs.est.9b00958 Environ. Sci. Technol. 2019, 53, 7930−7944

Critical Review

Environmental Science & Technology

iron form occurred. Thus, higher concentrations of iron oxides were found in samples of root occupied depths of Phragmites australis and samples taken at larger distance from the inflow.62 Interestingly, iron concentration in the sediment was found to decrease with longer operation time by Vymazal and Svehla (2013), after surveying seven full-scale horizontal subsurface flow CWs of 2−16 years old in the Czech Republic. If considering the accumulation of the sediments in the bed due to filtration effect depends on operation time, the total amount of Fe was still accumulated in the bed after longer operation times. Therefore, the results in this study demonstrated that measurements of sediment accumulation as well as element concentration should be taken into consideration.19 4.2. Seasonal and Diurnal Dynamics. The dynamic mitigation and exchange of Fe at the sediment−water interface in free water surface flow CWs can be seasonally dependent.63 Goulet and Pick (2001) found a strong seasonal difference in Fe retention in Monahan surface flow CWs in Kanata, Ontario. During spring and fall, a partitioning process of dissolved Fe into particulate form was observed, which was related partially to the seasonal changes of pH, alkalinity, and temperature. It is well-known that pH in surface flow CWs is greatly affected by photosynthesis and respiration. Photosynthesis by algae and submerged macrophytes during spring, summer and fall can increase pH and therefore would indirectly stimulate the process of transforming Fe into particulate form. However, during the winter period with low temperatures the presence of an ice cover on the top of the surface flow CWs limited the transfer of oxygen from the atmosphere and created an anoxic condition in the sediments. Under these conditions, it has been observed that the reduction of particulate Fe oxyhydroxides resulted in the release of dissolved Fe in Monahan surface flow CWs.64 An insignificant seasonal influence on Fe cycling in another surface flow CW for stormwater runoff quality control located in Kanata, Ontario, Canada was found by Fortin et al. (2000). In this wetland, dissimilatory microbial sulfate reduction and the subsequent precipitation of FeS have been identified as an important Fe removal pathway, particularly in low-temperature seasons.65 Due to the fact that large sulfate-reducing bacteria populations were still actively encountered in cold waters,65 the removal of Fe in this surface flow CW was active throughout the whole year, even when the water temperature decreased drastically in the cold winter of Canada. Besides the seasonal effect, diel changes in the form of Fe3+ and Fe2+ may also occur in free water surface flow CWs. Photoreduction of Fe has been demonstrated to be one of the reasons for diel shift of Fe3+ to Fe2+ in many aquatic systems.66 However, in surface flow CWs the diel changes in the proportion of Fe3+ and Fe2+ might be more linked to microbial Fe3+ reduction in sediments than photoreduction in the top surface waters.67 For example, Wieder (1994) observed an obvious diel shift of Fe3+ and Fe2+ in the effluent of a series of free water surface flow CWs for treatment of AMD with relatively stable influent chemical composition. The results from hourly sampled influent and effluent showed that prior to sunrise, 75−100% of the soluble Fe in effluent was determined to be Fe2+, whereas prior to sunset, 62−88% of the soluble Fe was determined to be Fe3+. These findings indicated that a random sampling scheme over the period of a day in a wetland may not sufficiently represent the phenomenon of Fe transformations.67

Table 2. Content of Fe (mg/kg) in Sediments of CWs content FWS 21−32 176−617 704−1041 0.9−1.7 31 300−35 200 HF 41−62 18 028 17 585 19 694 21 567 11 773 16 412 8659 467−504 23 000−35 000 19 000−21 000 VF 14 000 0.2−5.9% w/w of dry mattera

wastewater municipal sewage acid mine drainage acid mine drainage industrial effluent coal ash leachate municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage municipal sewage acid mine drainage domestic sewage domestic sewage domestic sewage domestic sewage

plants

place

reference

P. communis

Poland

43

T. latifolia

U.S.

49

J. ef fusus

Germany

34

Mixed

Pakistan

47

Mixed

U.S.

50

P. Canadensis

Poland

43

P. arundinacea

19

J. ef fusus

Czech Republic Czech Republic Czech Republic Czech Republic Czech Republic Czech Republic Czech Republic Germany

P. australis

Belgium

44

P. australis

Belgium

59

P. australis

Belgium

59

France

60

P. arundinacea P. arundinacea +P. australis P. australis P. arundinacea +P. australis P. arundinacea +P. australis P. australis

19 19 19 19 19 19 34

Surface sludge deposits of 14 French vertical flow constructed wetlands.

a

removal of metals by sulfide precipitates (McIntire and Edenborn 1990). Therefore, the combination of free water surface flow followed by horizontal subsurface flow CWs are often used for treatment of AMD on much larger scales.48 The processes of iron removal have been demonstrated to be different along the water flow pass of the wetland, namely “position” effect. For treatment of AMD, Lesley et al. (2008) found that the dominant removal process of iron in the oxidation lagoon prior to wetlands and the first part of free water surface flow wetlands is more abiotic and depends significantly on the oxidative condition and the formation of iron hydroxides. In these parts, the iron oxide particles can be easily settled to the bottom of the wetlands or retained in fine gravels. Biotic removal processes may become more important once iron concentrations are significantly low in the later part of the wetland bed, for example, concentrations of