Chemistry of arsenic removal during coagulation and Fe–Mn oxidation

Abstract

Recent studies have indicated that arsenic in drinking water is as hazardous as radon in homes and secondhand tobacco smoke. Arsenic removal during coagulation or Fe–Mn oxidation is examined to aid utilities that desire to improve arsenic removal. Fundamental mechanisms of arsenic removal are discussed, optimization strategies are forwarded, and some new insights are provided to guide future research. Specifically, As(III) removals by coagulation are primarily controlled by coagulant dose and relatively unaffected by solution pH, whereas the converse is true for As(V). When compared on the basis of moles iron or aluminum hydroxide solid formed during coagulation, iron and aluminum coagulants are of demonstrably equal effectiveness in removing As(V) at pH <7.5. However, iron‐based coagulants are advantageous if soluble metal residuals are problematic, if pH is >7.5, or if the raw water contains As(III). Arsenic removal during Fe–Mn oxidation is controlled by the quantity of iron removed [Fe(OH)3 formed] and is relatively independent of the quantity of manganese removed (MnOOH formed).

Removal of arsenic from water by electrocoagulation

“…Ferric chloride (FeCl 3 ) was used as a coagulant. The chemical coagulation experiments were conducted using jar test apparatus as specified in the literature (Edwards, 1994;Hering et al, 1996). The summary of experimental conditions employed is presented in Tables 1 and 2.…”

“…The commonly used technologies include coagulation and precipitation with iron and aluminum salts (Edwards, 1994;Hering et al, 1996), adsorption onto activated alumina, activated carbon and activated bauxite (Gupta and Chen, 1978), ion exchange and reverse osmosis (Clifford, 1999). Some recent treatment technologies based on oxidation and adsorption are green sand filtration (Christen, 2000), iron oxide coated sand (Joshi and Chaudhuri, 1996), manganese dioxide coated sand (Bajpai and Chaudhuri, 1999), ferruginous manganese ore (Chakravarty et al, 2002), ferrihydrite (Jain et al, 1999), clay minerals (Manning and Goldberg, 1997) and zero-valent iron (Fendorf et al, 1997;Farrell et al, 2001;Su and Puls, 2001).…”

“…Studies on arsenic indicate that hydrous metal oxides, such as ferric hydroxides, ferrihydrite, and goethite strongly adsorb arsenic (Edwards, 1994;Hering et al, 1996;Fendorf et al, 1997;Ravene et al, 1998;Jain et al, 1999). Recently arsenic removal by zero-valent iron has also been reported (Farrell et al, 2001;Krishna et al, 2001;Su and Puls, 2001).…”

In situ chemical fixation of arsenic-contaminated soils: An experimental study

“…Moreover, studies by other researchers have indicated that precipitation of Fe-As and Mn-As phases is very unlikely. The introduction of ferric ions for removal of arsenic from hydrometallurgical processed solutions and in water treatment has been studied by many researchers (Robins et al, 1987;Edwards, 1994;Cheng et al, 1994;Hering et al, 1996). These studies indicated that coagulation and adsorption are the major mechanisms for arsenic removal.…”

“…Furthermore, ferric ion has been used as an effective coagulation reagent in the treatment of arseniccontaminated wastewater (Cheng et al, 1994;Edwards, 1994;Hering et al, 1996). A study by Krause and Ettel (1988) showed that arsenic can form relatively insoluble ferric iron compounds, which have solubility products on the order of 10−20 to 10−24.…”