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).