Mitigation options - Arsenic treatment technologies - Revision history

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← Older revision		Revision as of 08:41, 10 August 2026
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	==References==		==References==
	For references, please visit the page [[References - Geogenic Contamination Handbook]].		For references, please visit the page [[References - Geogenic Contamination Handbook]].
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		+	==Related articles on GAP Wiki==
		+	*[[Mitigation options]]
		+	*[[Mitigation options - Exploiting alternative water resources]]
		+	*[[Mitigation options - Fluoride treatment technologies]]

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 ===Co-precipitation with naturally occurring iron===
-[[File:Figure_7.5.jpg|400px|thumb|right|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.
+[[File:Figure_7.5.jpg|400px|thumb|right|Fig. 7.5 Sand filter for arsenic removal in Vietnam ''(Terms of use: Cite original source from Handbook)'']]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).

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 ===Iron-based solids===
-[[File:Figure_7.3.jpg|400px|thumb|right|Fig. 7.3 SIDKO community arsenic removal filter installed in Bangladesh]]
+[[File:Figure_7.3.jpg|400px|thumb|right|Fig. 7.3 SIDKO community arsenic removal filter installed in Bangladesh ''(Terms of use: Cite original source from Handbook)'']]
 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
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 ===Zero-valent (metallic) iron===
-[[File:Figure_7.4.jpg|400px|thumb|right|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.
+[[File:Figure_7.4.jpg|400px|thumb|right|Fig. 7.4 SONO filter using metallic iron for arsenic adsorption ''(Terms of use: Cite original source from Handbook)'']]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<br>

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 ==Adsorption and ion exchange==
-[[File:Figure_7.2.jpg|400px|thumb|right| 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.
+[[File:Figure_7.2.jpg|400px|thumb|right| Fig. 7.2 READ-F household ion-exchange filter used in Bangladesh ''(Terms of use: Cite original source from Handbook)'']]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===

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		+	''NOTE: Article from the Geogenic Contamination Handbook''
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	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.		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.

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	'''Advantages:''' Additional removal of other chemical contaminants and pathogens; Arsenic removal over a wide pH range<br>		'''Advantages:''' Additional removal of other chemical contaminants and pathogens; Arsenic removal over a wide pH range<br>
	'''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		'''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
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		+	==References==
		+	For references, please visit the page [[References - Geogenic Contamination Handbook]].

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 ===Zero-valent (metallic) iron===
-[File:Figure_7.4.jpg|400px|thumb|right|Fig. 7.4 SONO filter using metallic iron for arsenic adsorption]]<br>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.
+[[File:Figure_7.4.jpg|400px|thumb|right|Fig. 7.4 SONO filter using metallic iron for arsenic adsorption]]<br>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<br>
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 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 (Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) 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.
+Alum (Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) 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. FeCl<sub>3</sub> and Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) 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. FeSO<sub>4</sub>) 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.
-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).
+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).