Arsenate and arsenite adsorption and desorption behavior on coprecipitated aluminum:iron hydroxides

Although arsenic adsorption/desorption behavior on aluminum and iron (oxyhydr)oxides has been extensively studied, little is known about arsenic adsorption/desorption behavior by bimetal Al:Fe hydroxides. In this study, influence of the Al:Fe molar ratio, pH, and counterion (Ca2+ versus Na+) on arsenic adsorption/desorption by preformed coprecipitated Al:Fe hydroxides was investigated. Adsorbents were formed by initial hydrolysis of mixed Al3+/ Fe3+ salts to form coprecipitated AIR hydroxide products. At AIR molar ratios <= 1:4, Al3+ was largely incorporated into the iron hydroxide structure to form a poorly crystalline bimetal hydroxide; however, at higher Al:Fe molar ratios, crystalline aluminum hydroxides (bayerite and gibbsite) were formed. Although approximately equal As-V adsorption maxima were observed for 0:1 and 1:4 Al:Fe hydroxides, the As-III adsorption maximum was greater with the 0:1 Al: Fe hydroxide. As-V and As-III adsorption decreased with further increases in Al:Fe molar ratio. As-V exhibited strong affinity to 0:1 and 1:4 Al:Fe hydroxides at pH 3-6. Adsorption decreased at pH > 6.5; however, the presence of Ca2+ compared to Na+ as the counterion enhanced As-V retention by both hydroxides. There was more As-V and especially As-III desorption by phosphate with an increase in Al:Fe molar ratio.