Use of zero-valent iron for nitrate removal from the soil water in loess areas

Permeable reactive barriers (PRB) based on zero-valent iron have been implemented successfully for nitrate removal from groundwater in developed countries, but they have not been used for field-scale applications in China even though groundwater nitrate pollution is very serious in most areas of China. Batch experiments were conducted using 200~300 mesh reducing iron powder to test the effects of parameters including: initial pH of the water; pretreatment of the iron with dilute acid; the weight ratio of iron to nitrate-N; addition of active carbon; and the concentration of the major anions present in the water. The results indicated that zero-valent iron could reduce the nitrate under the alkaline conditions of loess soils. The effect of the original pH value was not great because the buffer effect of the soil counteracted the effect of the influent water's pH. The nitrate removal rate was increased when the iron was pretreated with dilute acid or if the ratio of iron to nitrate-N was higher. However, the advantage of pretreating the iron was eliminated if the weight ratio of iron to nitrate-N was above 100 and the removal rate of nitrate remained constant even if the ratio was higher, because the proportion of nitrate removed reached 100%. Nitrate reduction was enhanced by the major anions in loess soils such as Cl-, SO2-4 and HCO-3, which contribute to iron corrosion. At the same concentrations, the promotion effect decreased in the order: SO2-4﹥HCO-3﹥Cl-. The nitrate removal rate could also be enhanced by adding active carbon. The primary product of nitrate reduction was ammonia. Nitrite was only present at the beginning of the reaction and decreased rapidly. The concentration of dissoluble Fe(Ⅱ) and Fe(Ⅲ) in the solution was less 0.2 mg·L-1.