In 2012, the United States environmental protection agency (USEPA) prepared a list of 129 organic and inorganic pollutants found in wastewater that constitute serious health hazards. This list includes the following heavy metals: cadmium, chromium, lead, mercury, nickel and zinc. The effects of bed depths from 0.5 to 2.0 m, liquid flow rates from 0.1 to 0.5 L.min-1, biosorbent particle sizes of 0.5 to 2 inches (1.27 - 5.08 cm) and metal concentrations from 10 to 50 mg-Zn-Ni/L on biosorption service were investigated. The best performance in biosorption of ZN2+ and Ni2+ binary solutions was at the smallest biosorbent particle size (0.5 inch), highest bed depth (2 m), lowest flow rate (0.1 L.min-1) and smallest initial concentration (10 mg/L).
For biomass regeneration, the effects of the type of desorbing agents (hydrochloric acid, nitric acid and sulphuric acid), concentrations (0.1 to 0.5 M) and flow rates (0.05 to 0.1 L/min) on recovery of Zn2+ and Ni2+ binary mixture were investigated. The best performance in desorption of Zn2+ and Ni2+ binary solutions were 0.1 M H2S04 and a 0.05 L/min inlet flow rate. Moreover, the reusability of wheat straw was examined by carrying out five successive biosorption/desorption, and the results indicated that wheat straw retained its zinc and nickel adsorption capacity in all five sorption/desorption cycles.
Electro-deposition was also used to remove metal ions from concentrated ZN+2 and Ni+2 binary solutions (about 340 ppm) from the desorption step. It was found that the electro-deposition could reduce the metal concentrations down to wastewater discharge limit of 2 mg-Zn-Ni/L.
Lastly, economical evaluation showed that the recovery of the heavy metals using Integrated Biosorption and Electro-deposition has great advantages from both technical and economical perspectives over the chemical precipitation technology.