Removal of Heavy Metals from Aqueous Solutions by Natural Adsorbents (A Review)

Background & Aims of the Study: The pollution of water resources due to the disposal of heavy metals has been an increasing worldwide concern for the last few decades. The removal of pollutants from water and wastewater is a problem that has grown with rapid industrialization. The discharge of toxic metals into watercourses is a serious pollution problem, which may affect the quality of water supply. By confirming the toxic effects of these metals, various methods have been implemented to remove pollutants from aqueous solutions. The aim of this study was determining the removal efficiency of natural adsorbents for removing heavy metals from aquatic solutions. Materials & Methods: In this review study, 39 experiments from 23 published papers on the mentioned topic between 2000 and 2011 were selected and the effective parameters on removal efficiency were studied and concluded. Results: The average removal efficiency was 88.14%. In more than 65% of studies the removal efficiency was above 90%. In more than 96% of experiments, by increasing the adsorbent dose, the removal of heavy metals increased. Average optimum pH for maximum adsorption of heavy metal ions was 5.47, and the removal efficiency at acidic pH was allocated 76.92%. Twenty four experiments have examined the effect of contact time on the removal efficiency, which with increasing time to reach equilibrium time, the removal efficiency increased. In 16 studies, primary concentration was investigated, which by increasing concentrations of heavy metals, removal efficiency decreased. 58.97% of data were compatible with Langmuir isotherm. Conclusions: According to the results, sawdust can be used as a low cost, natural and abundant availability adsorbent for removal of heavy metal ions from aqueous solution.