ReviewRemoval processes for arsenic in constructed wetlands
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 and arsenate , 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.
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