Removal and recovery of heavy metal from multi-component metal effluent by reduction crystallization

• Main Author: Phetla, Tebogo Pilgrene
• Published: 2012
• Subjects: Wastewater treatment; Sewage purification; Heavy metals absorption and adsorption; Heavy metal recovery; Chemical reduction; Crystallography
• Online Access: http://hdl.handle.net/10210/4940

M.Tech. === The removal and recovery of heavy metals from effluents has been a subject of significant importance due the negative impact these toxic metals have on human health and the environment as a result of water and soil pollution. Precipitation is the mostly widely used wastewater treatment method because it is the most economical and easier to implement and operate on a large scale. However, traditional precipitation methods using lime, sulfides or hydroxides recover metals in the form of a sludge which is not reusable and has to be disposed in landfills creating a potential environmental hazard and resulting in loss of valuable minerals. The current focus in effluent treatment is now on the recovery and re-use of these heavy metals rather than removal and disposal. This study investigated the use of hydrazine as a reducing agent to remove and recover Ni2+, Cu2+, Co2+ and Fe2+ from effluent by reduction crystallization. In this process chemically reduced aqueous metal ions were plated on to a base substrate (nickel powder) with no electrical current required for deposition. A feasibility study was carried out to test the efficiency and find the optimum operating conditions for this method and generate an understanding of the chemical and particulate process occurring. The results obtained indicate that hydrazine is an effective reducing agent for removal and crystallization of Ni2+, Cu2+, Co2+ and Fe2+/ Fe3+ into their elemental states with nickel powder as a seeding material. Over 99 % of metals were removed from the effluent in all the systems (Ni-only, Ni-Cu, Ni-Fe and Ni-Fe). Breakage, aggregation and molecular growth were identified as the predominant mechanisms occurring during the reduction crystallization process in Ni-only, Ni-Cu, Ni- Co systems and there was evidence of nucleation in Ni-Fe solution. These finding were confirmed by analysing the scanning electron micrographs of the powder obtained. A nearly spherical structure powder with wide distribution in particle size and evidence of fragmentation was obtained in all the experimental runs. vii The residual concentrations obtained were far below the required limit for effluent discharge into sewer where 20 mg/L Ni, 20 mg/L Cu and 20 mg/L Fe and the total metal concentration of 50 mg/L for Fe, Cr, Cu, Ni, Zn and Cd is stipulated. Reduction crystallization using hydrazine as a reducing agent can be utilized for controlling environmental pollution and eliminating hazardous metals from the environment.