Bioremediation of Acid Mine Water Using Rice Husk and Tree Bark

Acid mine drainage (AMD) is a perennial problem particularly in abandoned mines, occurring due to the oxidation of sulphidic mine wastes. AMD is usually characterized by high concentration of dissolved metal ions, sulphate and low pH. The efficacies of substrates such as rice husk and the bark of Eucalyptus tereticornis (Sm.) are assessed for the removal of metal ions such as iron, zinc and copper from acid mine water obtained from an abandoned copper mine. Over 99 % of Fe, 98 % of Zn and 95 % of Cu removal could be achieved from acid mine water, with a concomitant increase in the pH value by three units using rice husk as an adsorbent. With respect to tree bark as an adsor-bent, about 96 % of Fe, 75 % of Zn and 92 % of Cu could be removed from the acid mine water with an attendant increase in the pH value by two units. The free energy of adsorption of the metal ions onto rice husk and tree bark is found to be negative in both the cases, while the adsorption process is endothermic in the case of rice husk and exothermic for tree bark. The adsorption process is found to adhere to the Lagergren pseudo-first order rate equation. The reduction of sulphate con-centration in the acid mine water is achieved using a typical sulphate reducing bacteria namely De-sulfotomaculum nigrificans. Desorption experiments indicate that about 88 % of Zn, 87% of Cu and over 77 % of Fe could be recovered from rice husk and over 70 % of Fe, 82 % of Zn and 77 % of Cu could be desorbed from the metal loaded tree bark using 5 M HCl. The possible mechanisms of metal ion and sulphate removal from acid mine water using rice husk and tree bark are discussed.