Simultaneous removal of arsenate and fluoride from water by Al-Fe (hydr)oxides

Al-Fe (hydr)oxides with different Al/Fe molar ratios (4:1, 1:1, 1:4, 0:1) were prepared using a coprecipitat method and were then employed for simultaneous removal of arsenate and fluoride. The 4Al: Fe was superior to other adsorbents for removal of arsenate and fluoride in the pH range of 5.0-9.0. The adsorption capacity of the Al-Fe (hydr)oxides for arsenate and fluoride at pH 6.5±0.3 increased with increasing Al content in the adsorbents. The linear relationship between the amount of OH^- released from the adsorbent and the amount of arsenate or fluoride adsorbent by 4Al: Fe indicated that the adsorption of arsenate and fluoride by Al-Fe (hydr)oxides was realized primarily through quantitative ligand exchange. Moreover, there was a very good correlation between the surface hydroxyl group densities of Al-Fe (hydr)oxides and their adsorption capacities for arsenate or fluoride. The highest adsorption capacity for arsenate and fluoride by 4Al: Fe is mainly ascribed to its highest surface hydroxyl group density besides its largest pH_pzc. The dosage of adsorbent necessary to remove arsenate and fluoride to meet the drinking water standard was mainly determined by the presence of fluoride since fluoride was generally present in groundwater at much higher concentration than arsenate.