Effects of hardness and alkalinity on the removal of arsenic(V) from humic acid-deficient and humic acid-rich groundwater by zero-valent iron

The effects of hardness (Ca 2+) and alkalinity (HCO 3 −) on arsenic(V) removal from humic acid (HA)-deficient and HA-rich groundwater by zero-valent iron (Fe 0) were investigated using batch experiments. Arsenic, in general, is removed from groundwater possibly by adsorption and co-precipitation with the iron corrosion products. However, in the co-presence of HCO 3 − and Ca 2+, the removal rate of arsenic increased with increasing concentrations of either Ca 2+ or HCO 3 −. It was observed that the removal of arsenic was significantly enhanced by the formation of CaCO 3 as a nucleation seed for the growth of large iron (hydr)oxide particles. In the co-existence of Ca 2+, HCO 3 − and HA, the presence of HA diminished the positive role of Ca 2+ due to the formation of Fe-humate complexes in solution and delaying of the formation of CaCO 3. As a result, the formation of the large iron (hydr)oxide particles was inhibited in the earlier stage which, in turn, affected the removal of arsenic. However, after the formation of CaCO 3 and the subsequent growth of such particles, the presence of large iron (hydr)oxide particles resulted in the rapid removing of arsenic and Fe-humate by adsorption and/or co-precipitation.