Phytoremediation of Heavy Metals in Sewage Sludge by Brassicae Family Plants; Turnip, Sunflower and Mustard

Abstract

Poor enforcement of standards for release of heavy metals into public sewers has led to the presence of the metals in sewage sludge at wastewater treatment plants. The sludge if used as manure would risk transferring heavy metals to human beings and animals through the food chain. The objective of this study was to establish the concentration of heavy metals in the sludge at Kariobangi Wastewater Treatment Plant, compare heavy metal phytoremediation potential of local plants: mustard plant (brassica juncea), sunflower plant (helianthus annuus), turnip (brassica rapa) and evaluate efficacy of citric acid as a chelating agent in phytoremediation of heavy metals. Turnip, sunflower and mustard plants of the Brassicae family were grown in three conditions; namely, red loamy soil, sludge and sludge with citric acid as chelating agent for four weeks and their roots and shoots tested for metals. Metal concentrations in sludge were 10,878, 568.5, 157, 96 mg/kg for Fe, Mn, Zn and Cu. The concentration of Mn and Cu were above the maximum regulatory limit of the U.S. EPA of 80 and 50 mg/kg respectively, while that of Zn was below the maximum allowed regulatory limit of 200 mg/kg. The plants accumulated metals from loamy soil at a higher concentration than in the soil, suggesting that the accumulation were primarily to meet physiological needs. The roots of the plants studied accumulated between 1,513 and 1,813 mg/kg of Fe, 347 and 444 mg/kg of Mn, 143 and 166 mg/kg of Zn and 16 to 32 mg/kg of Cu, when grown in sludge. The plants indicated significant translocation of the heavy metal to the shoots of 51 to 91% of the metal accumulation in the roots. Citric acid increased uptake of metals by turnip plant and sunflower plants by average of 37 and 24% respectively for roots, and 53% for turnip shoots but stunt growth of plants and retarded metal uptake in mustard plant suggesting intolerance of the plants to acidity.