Effects of pH, particle size and porosity of raw rice husk and its silica on removing lead and hexavalent chromium from aqueous solution

Adsorbent properties and aqueous characteristics are very important parameters in the removal efficiency (RE) of environmental pollutants. The main goal of this study was to investigate the effects of pH, particle size and porosity of raw rice husk and its silica on removing lead and hexavalent chromium (Cr+6) from aqueous solution. The raw rice husk was collected from north of Iran and the rice husk silica was prepared at 800ºC  after acid leaching. The effects of the adsorbent particle sizes (0.07-0.1, 0.1-0.5, 0.5-1.0 and 1.0-1.5 mm), porosity and pH from 2 to 8 were investigated by varying any of the process variables while keeping the other variables constant (adsorbent dosage = 1.5 g.l-1, contact time = 60 min, chromium and lead initial concentration= 5 mg.l-1). The results of this study showed that the RE of Cr+6 is intensively pH - dependent. Using 1.5 g.l-1  adsorbents, particle size = 0.5 - 1.0 mm, 5.0 mg.l-1 initial concentration of Cr+6 and 60 min contact time, the maximum RE obtained by raw rice husk and its silica at pH 2 were up to 98.8 and 88.4%, respectively. However, at the same condition with changing pH (pH 7), the maximum RE decreased up to 69.4 and 60.4%, respectively. Moreover, a positive strong significant correlation was detected between decreasing the adsorbents particle sizes and lead removal efficiency in the two adsorbents (p < 0.01). The scanning electron microscope images of the two adsorbents showed that silica derived from raw rice husk has more porosity. In conclusion, the acidic condition of aqueous for Cr+6 and neutral condition for lead, increases the adsorbents porosity,  while decreased adsorbent particle sizes causes an elevation in the RE of the two pollutants