Metal contamination in the environment is one of the persisting global issues since it not only disturbs the environmental quality, but also the environment and human health. The major contribution to this problem arises mainly from anthropogenic activities such as industries. Metal scarcity has become more severe lately where some elements have been predicted to be eradicated from the earth crust in several decades. Recently, researchers have focused their attention to recover these metals from the waste stream and reuse it in industrial production processes.
The use of agricultural wastes as a potential low cost adsorbent for heavy metal removal from wastewater is one of the most versatile technologies. In this study among the different adsorbents tested, groundnut shell gave high removal efficiencies with fewer requirements for further post treatment for Cu, Pb and Zn removal. Furthermore, the batch experiments on the main effects of process parameters (pH, adsorbent dosage, contact time and initial metal concentration) showed a major effect on the metal uptake and removal efficiency. For material regeneration, 0.2 M HCl was the most effective desorbing solution that did not alter the efficiency, up to three adsorption and desorption cycles.
The use of sulfate reducing bacteria (SRB) in bioreactors is another technology that can be applied for the treatment of metal contaminated wastewater. The SRB reduce sulfate into sulfide which further reacts with metals to form metal sulfide precipitates. The inverse fluidized bed (IFB) bioreactor is a configuration which shows prominence in utilizing SRB technology for metal contaminated wastewater treatment. Two IFB bioreactors were operated at different pH (7.0 and 5.0). The sulfate reducing activity (SRA) at pH 7.0 was higher than at pH 5.0, illustrating that pH is the main factor that affects SRA. However, thiosulfate showed a higher efficiency than sulfate as an alternate electron acceptor. The sulfide produced using thiosulfate as the electron acceptor was 157.0 mg/L, while only 150.2 mg/L was produced using sulfate and it required an adaptation period at pH 5.0 prior to successful operation.
Moreover, the IFB showed a high efficiency for Cu, Ni and Zn removal from synthetic wastewater. The removal of Cu and Zn was more than 90% at pH 7.0 and 5.0 at an initial metal concentration of 25 mg/L. On the other hand, Ni was not removed at an initial concentration of 25 mg/L, as it exerted toxic effects towards SRB.
There are various types of metal contaminated waste streams which pose as a good candidate for metal recovery include electronics waste (e-waste). This e-waste has a high potential as secondary source of metal to recover especially base metals such as Cu, Ni and Zn. Printed circuit boards (PCBs) of personal computers were evaluated as the potential secondary source of Cu, Ni and Zn using hydrometallurgical and sulfide precipitation methods. The optimal conditions for metal leaching were 0.1 M HNO3 with a liquid to solid ratio of 20 using PCBs of 0.5 - 1.0 mm particle size at 60 °C, which resulted in 400 mg Cu/g PCBs. With sulfide precipitation at a stochiometric ratio of 1:1 (Cu:S2-), the recovery of Cu was very effective up to 90% from the leachate which accounted to approximately 0.41 g Cu/g PCBs, while Ni and Zn recovery were, respectively, 40% (0.005 g Ni/g PCBs) and 50% (0.006 g Zn/g PCBs) from the leachate in an upflow leaching column.