Elsevier

Chemosphere

Volume 84, Issue 8, August 2011, Pages 1032-1043
Chemosphere

Review
Removal processes for arsenic in constructed wetlands

https://doi.org/10.1016/j.chemosphere.2011.04.022Get rights and content

Abstract

Arsenic pollution in aquatic environments is a worldwide concern due to its toxicity and chronic effects on human health. This concern has generated increasing interest in the use of different treatment technologies to remove arsenic from contaminated water. Constructed wetlands are a cost-effective natural system successfully used for removing various pollutants, and they have shown capability for removing arsenic. This paper reviews current understanding of the removal processes for arsenic, discusses implications for treatment wetlands, and identifies critical knowledge gaps and areas worthy of future research. The reactivity of arsenic means that different arsenic species may be found in wetlands, influenced by vegetation, supporting medium and microorganisms. Despite the fact that sorption, precipitation and coprecipitation are the principal processes responsible for the removal of arsenic, bacteria can mediate these processes and can play a significant role under favourable environmental conditions. The most important factors affecting the speciation of arsenic are pH, alkalinity, temperature, dissolved oxygen, the presence of other chemical species – iron, sulphur, phosphate –, a source of carbon, and the wetland substrate. Studies of the microbial communities and the speciation of arsenic in the solid phase using advanced techniques could provide further insights on the removal of arsenic. Limited data and understanding of the interaction of the different processes involved in the removal of arsenic explain the rudimentary guidelines available for the design of wetlands systems.

Highlights

Constructed wetlands are capable to remove As. ► The main removal mechanisms are precipitation, coprecipitation and sorption. ► Bacteria can mediate these removal processes under favourable conditions. ► Factors affecting As speciation include pH, DO, Fe, S, P, TOC, wetland media. ► Knowledge of the different processes involved is required to improve wetlands design.

Introduction

Arsenic (As) is mostly found in the earth’s core and in clay- and sulphide-rich portions of the earth’s crust (Henken, 2009b). Being a metalloid in group 15 on the periodic table (along with antimony, bismuth, nitrogen and phosphorus), arsenic is well known for its chronic toxicity, particularly when exposure occurs over prolonged periods. Arsenic exposure via drinking-water is related to lung, kidney, bladder and skin cancer. For example, drinking-water arsenic concentrations in excess of 50 μg L−1 have been associated with increased risks of cancer in the bladder and lung, whilst drinking-water arsenic levels even below 50 μg L−1 have been associated with precursors of skin cancer (IPCS, 2001). Therefore, the presence of arsenic in water supply poses a serious risk to human health.

Surface and ground waters in many parts of the world have been found to naturally contain As concentrations that make these waters unsuitable for human use. Significant concentrations of As have been reported in various countries such as Bangladesh, Chile, USA, China, and India. In Bangladesh, for example, about 100 million people currently drink water with As concentrations up to 100 times the World Health Organisation (WHO) drinking water guideline, which is 10 μg L−1 (Mohan and Pittman Jr., 2007). Two of Northern Chile’s main rivers, the Loa River and the Lluta River, have As concentrations of around 1400 and 240 μg L−1 respectively (Romero et al., 2003, Dirección General de Aguas, 2008).

To remove As from potential drinking water sources, a variety of conventional and non-conventional technologies have been studied, and these technologies have been reviewed by several authors (Mohan and Pittman Jr., 2007). However, it is known that conventional engineered treatment technologies are costly and create problems of sludge generation and disposal (Kosolapov et al., 2004, Cohen, 2006, Nelson et al., 2006). In addition, these systems often become sources of As-rich effluents and are typically located in remote isolated areas (such as mining sites), thus precluding the transportation of the effluents to large centralised treatment facilities. As such, to prevent As pollution of watercourses, it is essential to find onsite, decentralised treatment systems that are robust and have low maintenance requirements and operating costs.

Constructed wetlands are low-energy ‘green’ systems that have been increasingly applied in wastewater treatment since the mid-1980s (Sun and Saeed, 2009). Since the late 1990s, the application of wetland systems has accelerated, primarily due to rising costs of fossil fuel-derived energy sources and worldwide concern about the emission of greenhouse gases and climate change (Lee et al., 2009). Currently, the applications of wetland systems are mostly in the treatment of domestic sewage, especially in rural areas in developed countries in Europe and the USA (Cooper et al., 1996, Scholz and Lee, 2005, Kadlec and Wallace, 2009).

Constructed wetlands have considerable potential to remove metals and metalloids, including arsenic (Ye et al., 2003, Buddhawong et al., 2005). Some studies have been carried out to investigate the removal of metals in wetlands (Kleinmann and Girts, 1987, National Rivers Authority, 1992, Sobolewski, 1999, Sjöblom, 2003), but most have focused on acid mine drainage (AMD) treatment, primarily to remove sulphate, iron (Fe) and manganese (Mn) (Wallace and Knight, 2006). Despite their potential, few experimental studies have been specifically designed to investigate As removal in wetland systems. Kadlec and Wallace (2009) reviewed some key aspects of As behaviour in treatment wetlands, but the review was largely based on unpublished data or data found in the North American Treatment database NABD (US EPA, 1998). Other reviews are available in the literature on the removal of metals using constructed wetlands (Dunbabin and Bowmer, 1992, Sheoran and Sheoran, 2006, Yeh, 2008, Marchand et al., 2010), but they provide a general overview of metals and metalloids. Therefore, arsenic removal is only briefly covered, with little information available on the processes responsible for transformation and retention of arsenic, and the factors which control these processes.

This review aims to summarise what is currently known about the physicochemical processes for As removal in constructed wetlands, including major environmental factors that affect these processes. Microorganism-mediated mechanisms, which can also remove As by direct and indirect means, are discussed. Major knowledge gaps that currently impede wetland modelling and design for As removal are identified, together with research directions and tools that could potentially address these gaps.

Section snippets

Arsenic chemistry

Arsenic is a highly reactive metalloid that can be found in oxidation states −3, 0, +3 and +5. In natural waters, arsenic occurs as arsenite AsO3-3 and arsenate AsO4-3, referred to as As(III) and As(V). As(III) mostly exists in reducing groundwaters and hydrothermal waters, whilst As(V) is more often present in surface waters and oxidising groundwaters (Henken and Hutchison, 2009). The main factors that control arsenic speciation are the oxidation state and pH.

As(III) commonly hydrates to

Arsenic removal mechanisms in constructed wetlands

Being considered complex bioreactors due to interactions between microbial communities, plants, soil and sediments, subsurface flow wetlands may remove pollutants via various physical, chemical and biological processes (Kadlec and Wallace, 2009). The occurrence and rate of these processes depends on the nature of the pollutants and environmental conditions.

Metal removal processes in wetlands have been reviewed by different authors (Kleinmann and Girts, 1987, National Rivers Authority, 1992,

The effect of environmental factors on arsenic removal

A variety of environmental factors can affect the removal of As in constructed wetlands, and changes in one factor often affects another (such as pH and alkalinity; temperature and dissolved oxygen). However, many of these factors can be controlled during the design/operation of the wetlands or during any pretreatment process, such as through varying the type of wetland substrate, providing an additional carbon source, or adjusting the pH of either the influent water or of the wetland substrate.

Synthesis of As removal pathways

The main removal pathways of As in constructed wetlands are precipitation, coprecipitation and sorption. Even though these are chemical processes, they can be microbially-mediated. Depending on environmental conditions, arsenic can precipitate mainly as arsenosulphides (reduced species) and as arsenates (oxidised species), coprecipitate with sulphides or Fe oxides, or it can be sorbed onto the wetland substrate, metal oxides and/or organic matter.

The most important factors that affect the

Design and modelling of constructed wetlands for As removal

Currently, there is no official guideline on how a wetland should be designed specifically for the removal of arsenic. Information about the design of wetlands for metals removal is also rare, but some tentative design guidelines have been proposed using simple pollutant removal models such as the zero-order model or the first-order kinetic decay models (Kadlec and Wallace, 2009). Before sufficient experiment data are collected for As removal, the design for lab- or pilot-scale experimental

Key research needs

It is apparent from the literature that constructed wetlands have the potential to remove metals and metalloids. However, little is known about their efficiency, nor about means of optimising arsenic retention. Most studies describing the application of constructed wetlands in the removal of metals and metalloids come from studies on the treatment of acid mine drainage using surface flow systems. The efficiency of subsurface flow wetlands has not been sufficiently studied, since wetlands with

Conclusions

To date, the main application of constructed wetlands in the removal of metals and metalloids has been the treatment of acid mine drainage, where arsenic was not the priority pollutant. Arsenic, as a metalloid, presents differences in reactivity and therefore in the removal processes with metals such as Cu and Zn. The literature on As removal in treatment wetlands is very limited, and studies have showed that constructed wetlands have considerable potential to remove arsenic from contaminated

Acknowledgements

The authors would like to thank Chilean Government (Becas Chile) for sponsoring Katherine Lizama A.’s Ph.D. studies.

References (127)

  • J.L. Faulwetter et al.

    Microbial processes influencing performance of treatment wetlands: a review

    Ecol. Eng.

    (2009)
  • W.B. Gagliano et al.

    Chemistry and mineralogy of ochreous sediments in a constructed mine drainage wetland

    Geochim. Cosmochim. Acta

    (2004)
  • X. Gao et al.

    Chemical and mineralogical characterization of arsenic, lead, chromium, and cadmium in a metal-contaminated Histosol

    Geoderma

    (2010)
  • K.H. Goh et al.

    Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the subsurface environment

    Appl. Geochem.

    (2005)
  • R.R. Goulet et al.

    Test of the first-order removal model for metal retention in a young constructed wetland

    Ecol. Eng.

    (2001)
  • M. Gräfe et al.

    Copper and arsenate co-sorption at the mineral-water interfaces of goethite and jarosite

    J. Colloid Interf. Sci.

    (2008)
  • S. Groudev et al.

    Bioremediation of acid mine drainage in a uranium deposit

    Hydrometallurgy

    (2008)
  • K.B. Hallberg et al.

    Microbiology of a wetland ecosystem constructed to remediate mine drainage from a heavy metal mine

    Sci. Total Environ.

    (2005)
  • O.J. Hao

    Sulphate-reducing bacteria

  • Y. Kalmykova et al.

    Peat filter performance under changing environmental conditions

    J. Hazard. Mater.

    (2009)
  • J.K. King et al.

    Mercury removal, methylmercury formation, and sulfate-reducing bacteria profiles in wetland mesocosms

    Chemosphere

    (2002)
  • L. Kröpfelová et al.

    Removal of trace elements in three horizontal sub-surface flow constructed wetlands in the Czech Republic

    Environ. Pollut.

    (2009)
  • J.T. Landrum et al.

    Partitioning geochemistry of arsenic and antimony, El Tatio Geyser Field, Chile

    Appl. Geochem.

    (2009)
  • B.H. Lee et al.

    Application of the self-organizing map (SOM) to assess the heavy metal removal performance in experimental constructed wetlands

    Water Res.

    (2006)
  • J.R. Lloyd et al.

    Stimulation of microbial sulphate reduction in a constructed wetland: microbiological and geochemical analysis

    Water Res.

    (2004)
  • L. Marchand et al.

    Metal and metalloid removal in constructed wetlands, with emphasis on the importance of plants and standardized measurements: a review

    Environ. Pollut.

    (2010)
  • W.M. Mayes et al.

    Wetland treatment at extremes of pH: a review

    Sci. Total Environ.

    (2009)
  • M. Mkandawire et al.

    Accumulation of arsenic in Lemna gibba L. (duckweed) in tailing waters of two abandoned uranium mining sites in Saxony, Germany

    Sci. Total Environ.

    (2005)
  • D. Mohan et al.

    Arsenic removal from water/wastewater using adsorbents – a critical review

    J. Hazard. Mater.

    (2007)
  • H.R. Pfeifer et al.

    Dispersion of natural arsenic in the Malcantone watershed, Southern Switzerland: field evidence for repeated sorption–desorption and oxidation–reduction processes

    Geoderma

    (2004)
  • M.A. Rahman et al.

    Arsenic accumulation in duckweed (Spirodela polyrhiza L.): a good option for phytoremediation

    Chemosphere

    (2007)
  • L. Romero et al.

    Arsenic enrichment in waters and sediments of the Rio Loa (Second Region, Chile)

    Appl. Geochem.

    (2003)
  • T. Saeed et al.

    Enhanced denitrification and organics removal in hybrid wetland columns: comparative experiments

    Bioresour. Technol.

    (2011)
  • K. Sakadevan et al.

    Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems

    Water Res.

    (1998)
  • V.K. Sharma et al.

    Aquatic arsenic: toxicity, speciation, transformations, and remediation

    Environ. Int.

    (2009)
  • A.S. Sheoran et al.

    Heavy metal removal mechanism of acid mine drainage in wetlands: a critical review

    Miner. Eng.

    (2006)
  • J. Ackermann et al.

    Speciation of arsenic under dynamic conditions

    Eng. Life Sci.

    (2008)
  • A.R. Adhikari et al.

    Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada

    Environ. Monit. Assess.

    (2010)
  • APHA et al.

    Standard Methods for the Examination of Water and Wastewater

    (2005)
  • C.A.J. Appelo et al.

    Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic

    Environ. Sci. Technol.

    (2002)
  • National Rivers Authority

    Constructed Wetlands to Ameliorate Metal-rich Mine Water: Review of Existing Literature. R&D Note 102

    (1992)
  • M. Bissen et al.

    Arsenic – a review. Part II: oxidation of arsenic and its removal in water treatment

    Acta Hydroch. Hydrob.

    (2003)
  • N.K. Blute et al.

    Arsenic sequestration by ferric iron plaque on cattail roots

    Environ. Sci. Technol.

    (2004)
  • D.A. Bright et al.

    Methylation of arsenic by anaerobic microbial consortia isolated from lake sediment

    Appl. Organomet. Chem.

    (1994)
  • S. Buddhawong et al.

    Removal of arsenic and zinc using different laboratory model wetland systems

    Eng. Life Sci.

    (2005)
  • J.S. Chang et al.

    Arsenic detoxification potential of aox genes in arsenite-oxidizing bacteria isolated from natural and constructed wetlands in the Republic of Korea

    Environ. Geochem. Health

    (2010)
  • J.M. Cloy et al.

    Retention of As and Sb in ombrotrophic peat bogs: records of As, Sb, and Pb deposition at four Scottish sites

    Environ. Sci. Technol.

    (2009)
  • P.F. Cooper et al.

    Reed Beds and Constructed Wetlands for Wastewater Treatment

    (1996)
  • O.P. Dhankher

    Arsenic metabolism in plants: an inside story

    New Phytol.

    (2005)
  • L. Diels et al.

    Heavy metal immobilization in groundwater by in situ bioprecipitation: comments and questions about efficiency and sustainability of the process

  • Cited by (0)

    View full text