Mitigation options - Arsenic treatment technologies

Technologies for arsenic removal rely on basic physical and chemical processes that are summarised in the following sections. More details can be found in the scientific literature and more information and references in one of the several reviews of arsenic removal technologies (e.g. Mohan and Pittman, 2007).

The review here focuses on decentralised (community or household) arsenic removal methods. Particular emphasis is on technologies which have been validated through independent verification programmes (Johnston, 2002; USEPA, 2005). The following chapters present and summarise the principal steps and procedures for arsenic removal.

Pre-treatment (oxidation)

Arsenic in groundwater is mainly present in two oxidation states, As(III) and As(V), depending on the environmental conditions in the aquifer. Most arsenic removal technologies are most effective at removing As(V) (arsenate), since As(III) (arsenite) is predominantly non-charged below pH 9.2. Therefore, many treatment systems include an oxidation step to convert arsenite to arsenate. Oxidation alone does not remove arsenic from solution; it must be coupled with a removal process such as coagulation/precipitation, adsorption or ion exchange.

Air oxidation:
Atmospheric oxygen is readily available as an oxidising agent; however, the kinetics of air oxidation of arsenic are very slow (taking weeks), and the reaction needs to be catalysed. Metals such as iron or manganese, which are naturally present in groundwaters, catalyse the oxidation of As(III), but oxidation is normally not complete without additional oxidants or the repeated addition of Fe(II).

Chlorine:
Chlorine is widely available and is a rapid and effective oxidant for arsenite. Dosing can be difficult, since locally available chlorine can be of uncertain quality in developing countries. When enough chlorine is added for effective disinfection of water from microbial contamination, arsenite oxidation is normally complete. Doses generally range from 1.0 to 5.0 mg/L, with the goal of approximately 0.5 mg/L residual chlorine to provide protection against microbial contamination after treatment.

Manganese compounds:
Potassium permanganate (MnVII) effectively oxidises As(III), along with Fe(II) and Mn(II). Filtration of water through a bed of solid Mn(IV) oxides can rapidly oxidise arsenite to arsenate without the need for adding a liquid or gas oxidant. Oxidation is efficient over a wide range of pH and does not release excessive manganese into solution. Other more advanced oxidants (e.g. ozone, ultraviolet lamps) are not considered here, as they are difficult to use in developing countries.

Adsorption and ion exchange

Fig. 7.2 READ-F household ion-exchange filter used in Bangladesh (see also document on “Verified Arsenic Removal Technologies in Bangladesh”)

Ion exchange is a reversible chemical reaction between an insoluble solid and a solution during which ions may be interchanged. The ions can be relatively easily exchanged. Adsorption, on the other hand, involves the formation of a bond between a dissolved ion and the solid-phase surface. These bonds are not so easily broken. Various solid materials have a strong affinity for dissolved arsenic. Arsenic is strongly attracted to sorption sites on the surfaces of these solids, and is effectively removed from solution.

Ion exchange resins:
Ion exchange is a physico-chemical process by which an ion in the solid phase is exchanged for an ion in the feed water. The solid phase is typically a synthetic resin which has been chosen to preferentially adsorb the particular contaminant of concern. To accomplish this exchange of ions, feed water is continuously passed through a bed of ion-exchange resin beads in a down-flow or up-flow mode until the resin is exhausted. A good example is the READ-F ion exchange filter (Fig. 7.2). Most commonly, the resins are composed of a matrix of polystyrene cross-linked with divinylbenzene. Charged functional groups are attached to the matrix by covalent bonding. These functional groups determine the resin’s affinity to certain ions such as arsenate. Conventional sulphate-selective resins are particularly suited for arsenate removal. Nitrate-selective resins also remove arsenic, but arsenic breakthrough occurs earlier (USEPA, 2003c). Only arsenate can be removed using ion-exchange filters, as arsenite is not charged. A pre-oxidation step might therefore be necessary.

Arsenic removal: Various strong-base anion exchange resins are commercially available which can effectively remove arsenate from solution, producing effluent with less than 1 μg/L arsenic (Clifford, 1999). Arsenate removal is relatively independent of pH and influent concentration. On the other hand, competing anions, especially sulphate, can have a strong effect. In low-sulphate waters, ion-exchange resins can easily remove over 95% of arsenate and treat from several hundred to over a thousand bed volumes, before arsenic breakthrough occurs. However, when sulphate is present and saturates the exchange sites, it can lead to desorption of large amounts of exchanged arsenate – so-called “arsenic dumping”. Accordingly, the USEPA recommends that ion-exchange resins only be used for low-sulphate waters (USEPA, 2000b).

Regeneration: Exhaustion occurs when all sites on the resin beads have been filled by contaminant ions. At this point, the bed is regenerated by rinsing the column with a regenerant, a concentrated solution of the ions initially exchanged from the resin. The number of bed volumes that can be treated before exhaustion varies with resin type and influent water quality (USEPA, 2000b). Ion-exchange resins are easily regenerated by flushing with concentrated salt solutions (1.0 M NaCl is commonly used). Brine can be reused 20–30 times, in spite of increasingly concentrated arsenic levels in the regenerant. Spent regenerant is loaded with arsenic and needs to be treated or disposed of safely (USEPA, 2000d). A hybrid anion exchanger (HAIX) containing hydrous ferric oxide has been used to remove arsenic from drinking water in West Bengal for 10 years now. The initial investment in this material appears to be offset by the long filter life. Please see German et al. (2014) for further details.

Advantages

• High adsorption capacity
• Commercially available
• Regeneration possible

Disadvantages

• Moderately expensive
• Risk of “arsenic dumping” of waters with high sulphate concentrations
• Interference from sulphate and total dissolved solids
• Water rich in Fe and Mn might require pre-treatment to prevent filter clogging
• Regeneration produces arsenic-rich brine

Activated alumina:
Activated alumina (AA) is a commercially available granular form of aluminium oxide which can be used as a filter medium to remove a range of contaminants from water, including arsenic. The contaminant ions are exchanged with the surface hydroxides on the alumina. When adsorption sites on the AA surface become filled, the bed must be regenerated. Activated alumina has a much higher affinity for As(V) than for As(III). Therefore, depending on the prevalence of As(III), filtration might need to be preceded by an oxidation. Arsenic removal: The arsenic adsorption capacity of AA (mg As/g AA) varies significantly with water pH and influent arsenic concentrations and speciation. Arsenate removal capacity is highest within a narrow range of solution pH from 5.5 to 6.0, in which the alumina surfaces are protonated, and in which other anions are not concentrated enough to compete with arsenic (USEPA, 2000b). In large systems, pH adjustment is often applied to optimise treatment. Regeneration: Regeneration of AA beds is usually accomplished using a strong basic solution of concentrated NaOH. Arsenic is more difficult to remove during regeneration than other ions such as fluoride. Therefore, higher base concentrations are used, typically, 4% sodium hydroxide. After regeneration with strong base, the AA medium must be neutralised using strong acid (e.g. 2% sulphuric acid). Arsenic-rich wastes must be processed before disposal (USEPA, 2001).

Advantages

• High arsenic removal efficiency
• Commercially available
• Regeneration possible
• Tested in community and household application

Disadvantages

• Moderately expensive
• Strong acid and base needed for regeneration
• Arsenic-rich waste produced
• Optimal arsenic removal within a limited pH range

Fig. 7.3 SIDKO community arsenic removal filter installed in Bangladesh

Iron-based solids:
Iron, especially in the ferric state (Fe(III)), has a strong affinity for arsenic. It also has an affinity for other ions. Phosphate, arsenate and silicate bind equally strongly, followed by negatively charged ions (Balistrieri and Chao, 1990; Hsu et al., 2008; Hug, 2014): phosphate = arsenate ≈ silicate > (bi)carbonate > humic acid >fluoride > sulphate > chloride

This sequence indicates that arsenic will compete for binding sites with phosphate and silicate, but not with ions such as fluoride, sulphate or chloride. Granular iron-based media have been developed relatively recently for arsenic removal (e.g. Driehaus et al., 1998). Several commercial iron-based materials are available, including granular ferric hydroxide (e.g. AdsorpAs®, see SIDKO filter, Fig. 7.3). Iron-based solids can effectively remove arsenate, arsenite and phosphate from water. Before the water is passed over the active medium, it is aerated and pre-filtered to oxidise and remove iron flocs (USEPA, 2003b). Sands coated with iron oxides have been synthesised by various researchers and tested for their arsenic removal capacity. UNESCO-IHE has developed a household filter which uses coated sand from Dutch iron removal plants (Petrusevski et al., 2008).

Advantages

• High arsenic removal efficiency
• Works well over a broad range of pH
• Removes both As(V) and As(III): pre-oxidation may not be needed
• Commercially available
• Tested in community and household application

Disadvantages

• Moderately expensive
• Regeneration is possible but usually not done
• Arsenic-rich waste produced

Zero-valent (metallic) iron:

Fig. 7.4 SONO filter using metallic iron for arsenic adsorption

When metallic, or zero-valent, iron corrodes, it produces dissolved ferrous iron (Fe(II)). The ferrous iron reacts with oxygen to form ferric iron (Fe(III) that precipitates as iron hydroxide (Fe(OH)3), which acts as a sorbent for arsenic. Reactive oxygen species produced during iron corrosion also oxidise As(III) to the more strongly sorbing As(V) (Leupin and Hug, 2005). A household filter (the_SONO_filter, Fig. 7.4) has been developed which makes use of metallic iron to remove arsenic from drinking water in Bangladesh (Hussam and Munir 2007). This filter consists of two buckets placed on top of each other, with the top bucket containing sand, iron filings and brick chips and the bottom bucket containing sand, charcoal and brick chips. It has been verified through the BETV-SAM programme.

Advantages

• High arsenic removal efficiency
• Continuous generation of ferric adsorption sites prolongs filter lifetime
• Removes both As(V) and As(III)
• Relatively inexpensive

Disadvantages

• Iron corrosion may lead to clogging and low filtration rates
• Limited field experience, mainly in household filters
• Limited commercial availability
• Arsenic-rich waste produced

Precipitation, co-precipitation and coagulation

Precipitation methods reduce dissolved arsenic concentrations by the precipitation of low-solubility solid minerals such as calcium arsenate. But these cannot normally lower arsenic to drinking-water limits. Co-precipitation refers to the precipitation of solid particles in the arsenic-containing water – normally aluminium or iron (hydr)oxides – that can sorb and incorporate arsenic.

Coagulation is the clumping of fine particles in solution to larger ones that can settle. Metal salts, such as alum, ferric chloride or ferric sulphate, are widely used coagulants to remove arsenic from drinking water (USEPA 2000a). These salts initially dissolve upon addition to water and then rapidly form fine precipitated flocs of metal hydroxides. These flocs coagulate and settle out of solution, scavenging many dissolved and particulate materials in the process. Vigorous stirring is required immediately after coagulant addition to ensure uniform mixing. Once the coagulant is dispersed, slow mixing allows the flocs to collide and grow (flocculate) without breaking up. Much of the floc matter will settle by gravity, but filtration is essential to remove small particles which can remain in suspension, as these can contain significant amounts of arsenic. If water is soft and of low alkalinity, it may be necessary to increase alkalinity (e.g. by adding lime addition) to ensure good floc formation.

Alum (Al₂(SO₄)₃) is effective for removing As(V) but ineffective for As(III), so pre-oxidation is often necessary. Alum has a narrow effective range, from pH 5–7; if the pH is above 7, removal may be improved by adding acid to lower the pH. Typical doses are 10 to 50 mg alum per litre.

Ferric (Fe(III)) salts (e.g. FeCl₃ and Fe₂(SO₄)₃) coagulate best between pH 5 and pH 8. Typical doses are 5 to 50 mg/L ferric salts. Ferric salts can remove both As(III) and As(V), but As(V) is retained more strongly, so pre-oxidation is often carried out.

Ferrous (Fe(II)) salts (e.g. FeSO₄) can also be used to remove arsenic, but oxygen (in air) and time are required to let the Fe(II) oxidise to Fe(III), which forms the arsenic-sorbing Fe(III) (hydr)oxide particles. At pH 7, it takes 1–4 hours for Fe(II) to oxidise completely to Fe(III) and to precipitate. Less time is required at a higher pH. During the oxidation of Fe(II) to Fe(III) by oxygen from air, a part of the As(III) is also oxidised to As(V), so the overall removal of As(III) with Fe(II) is better than with Fe(III), if no additional oxidant is used (Roberts et al., 2004). Groundwater often contains naturally dissolved Fe(II). If the natural concentration of Fe(II) is high (>15 mg/L), then this Fe(II) alone might be sufficient to remove the arsenic.

Coagulation also improves turbidity and colour and can also reduce levels of organic matter, bacteria, iron, manganese and fluoride, depending on operating conditions. If concentrations of phosphate or silicate in the source water are high, coagulation may be less effective.

Coagulation is operationally complex and is more commonly practised in centralised water-treatment plants. Chile has been removing arsenic from drinking water by coagulation for a long time – in 1970, the world’s first arsenic removal plant was constructed along the Toconce River. Since then, numerous plants have been built in Chile, most of which use ferric chloride coagulation with chlorine pre-oxidation (Sancha, 2006).

Some household coagulation systems have been developed, typically using an upper bucket for coagulation and flocculation and a lower bucket with filter material (e.g. charcoal and sand) for the removal of suspended solids, including metal (oxy)hydroxide particles containing arsenic (e.g. Cheng et al., 2004). The performance of the Shawdesh_Aqua_Filter, a two-bucket system using ferric sulphate, was verified in the Bangladeshi BETV-SAM project (see “Verification Programmes” below).

Electrocoagulation, in which aluminium or iron flocs are produced by passing a current through metal plates in contact with the water to be treated, is an emerging technology. Electrocoagulation offers certain advantages over conventional treatment with salts: removal of As(III) may be superior due to at least partial oxidation, the need for chemical supply and addition is greatly reduced and sludge volumes are smaller (e.g. Kumar et al. 2004; Emamjomeh and Sivakumar 2009a). As electrocoagulation is a relatively new approach for the removal of arsenic (and fluoride), current research is focusing on optimising the many design factors which can influence treatment efficiency and cost (Addy et al., 2011). Common to all (co)precipitation techniques are:

Disposal: The use of coagulants produces arsenic-rich sludge which needs to be safely disposed of, away from drinking-water sources (USEPA, 2000d). Wastes may be thrown into latrines that are well separated from drinking-water wells. However, centralised landfilling is probably the best disposal route. Costs: Coagulation using metal salts requires simple chemicals that are readily available and cost-effective. Filter material generally consists of sand and charcoal, materials which are also cheap and easy to obtain.

Advantages

• Relatively inexpensive
• Simple chemical reagents, widely available
• Usually applied in batch treatment; effectiveness should remain constant over time (i.e. no “breakthrough” or saturation issues)

Disadvantages

• Requires rigorous and time-consuming operation and maintenance
• Usually requires pre-oxidation
• Generates arsenic-rich sludge
• Phosphate and silicate may reduce arsenic removal rates
• Treatment adds ions (sulphate, chloride) to the water, which may affect its taste
• Limited optimal pH range
• Limited field experience with electrocoagulation, processes not yet optimised

Co-precipitation with naturally occurring iron:

Fig. 7.5 Sand filter for arsenic removal in Vietnam

High dissolved iron concentrations in groundwater pumped from anoxic aquifers can be utilised to remove arsenic. When the iron to arsenic mass ratio is greater than 40–50 (Meng et al., 2001), oxidation and filtration of iron will generally reduce arsenic to acceptable levels (USEPA, 2000c; USEPA, 2006). If groundwater also contains high phosphate concentrations, the iron:arsenic ratio should be even higher (Hug et al., 2008). If this criterion is met, then the system can function from its first use.

In Vietnam, household sand filters are commonly used for iron removal. An upper chamber is filled with locally available sand, while a lower chamber serves to store the filtered water. Groundwater pumped from a tube well trickles through the sand filter into the underlying storage tank (Fig. 7.5). Arsenic removal is governed by the precipitation of iron (hydr)oxides, which form a coating on the surface of the sand grains. Arsenic is then absorbed by the iron (hydr)oxides and remains immobilised under oxic conditions. The efficiency of the method is dependent on the concentration of the naturally occurring iron, as well as on the concentration of competing ions (especially with phosphate >2 mg/L) (Luzi et al., 2004; Roberts, 2004). Fe/As ratios of ≥50 or ≥250 are required to ensure arsenic removal to concentrations below 50 or 10 µg/L, respectively. In Vietnam, where 93% of tube wells contain >1 mg/L iron and <2 mg/L phosphate, the sand filters’ median arsenic removal efficiency was 91%. Estimates for Bangladesh indicate that a median residual level of 25 µg/L arsenic could be reached in 84% of the contaminated groundwaters (Berg et al., 2006).

Advantages

• Relatively inexpensive
• Achievable using locally available materials
• No consumables or regeneration needed
• Efficiency improves with time, as ferric iron accumulates in sand filter
• Taste and appearance of water is markedly improved through iron removal

Disadvantages

• Arsenic removal is limited, requires high Fe/As ratio
• Poor performance where phosphate concentrations are high
• Lack of standard design parameters can lead to inefficient “homemade” systems
• Stored water may be vulnerable to faecal contamination

Membrane methods

Selectively permeable synthetic membranes can remove a variety of contaminants, including arsenic. Reverse osmosis and nanofiltration are two membrane technologies suitable for arsenic removal, operating with membrane pore sizes of less than 0.01 micron, which is sufficient to remove metal ions. These membranes need to be operated with pressure gradients ranging from about 3 to 10 bar (Johnston et al., 2002).

Membrane techniques require that inflowing water be of relatively high quality to prevent membrane fouling, meaning that a preceding filtration step is often necessary. Arsenic removal is possible over a wide pH range.

The percentage of treated water that can be produced from the feed water is known as the recovery. In municipal systems, recovery can be up to 85% for nanofiltration and 30–85% for reverse osmosis. In household systems, this value is typically significantly lower (e.g. 10–25%), which can be seen as a disadvantage, as a large amount of raw water is needed to produce the desired amount of treated water (USEPA, 2003a).

Advantages

• Additional removal of other chemical contaminants and pathogens
• Arsenic removal over a wide pH range

Disadvantages

• Complex and maintenance-intensive process
• Membrane fouling needing pre-treatment and chemical cleaning
• Operation at high pressures
• Low recovery rate
• High capital and operating costs