The Effects of Solution Concentration, Drying, Aging Time, and Mixture of Goethite with As(V)-Fe(OH)3 on the Chemical Extraction of Arsenate Associated with Iron (Hydr)oxides

Abstract

Extraction of arsenic (As) from adsorbed (ad) and coprecipitated (cpt) As-Fe(OH)3, As-Fe3O4 using phosphate buffer, sodium hydroxide (NaOH), hydrochloric acid (HCl), and oxalate buffer was studied. In addition, the effects of the solution concentrations, sample drying, aging time, and goethite and fresh As-Fe(OH)3 (cpt) mixture on the extraction efficiencies of As were investigated. For fresh As-Fe(OH)3 (ad) and As-Fe(OH)3 (cpt), 100% of the As was extracted by either 1 mol/L (M) phosphate (pH 5.0) or 0.5 M NaOH. Lower As recoveries from As-Fe(OH)3 associations were obtained by lower concentrations of phosphate (pH 5.0, 0.05 and 0.1 M) and NaOH (0.1 M). Drying and aging of the As-Fe (hydr)oxide associations significantly inhibited the extraction ratio and rate of adsorbed and coprecipitated As relative to fresh samples. The slow intraparticle diffusion of phosphate and arsenate in aggregated or crystalized As-bearing Fe (hydr)oxide particles is possibly the dominant cause. Due to readsorption of As onto goethite, the coexistence of goethite with fresh As-Fe(OH)3 (cpt) resulted in an incomplete release of As (∼70%) with either 1 M HCl or 0.2 M oxalate (pH 3.25). However, when a mixture of 0.1 M phosphate and 1 M HCl was used, 100% of the As was recovered, indicating that the addition of a small amount of phosphate into the HCl or oxalate solutions is helpful to completely extract all of the As coprecipitated with amorphous Fe (hydr)oxides.