Study On Sorption Of The Heavy Metal In Plating Wastewater By Bentonite

With the development and economic and rapid urbanization, many industries, including the electroplating, metal finishing, tannery industries, produce aqueous effluents containing heavy metal contaminants, causing serious soil and water pollution. Nowadays, various techniques have been used to remove the heavy metals from waste water, including chemical precipitation, ion exchange, electrolytic methods, chemical coagulation, membrane processes and so on. This paper emphasizes the application of bentonite in treating electroplating wastewater. The mechanisms of bentonite adsorbing heavy metals are also studied.Bentonite is a kind of clay mineral that primary constituent is montmorillonite. It has unique layer structure and powerful adsorption ability. Na-exchanged bentonites have improved cation exchange capability and spacing and capabilities to adsorb hydrophobic contaminants. They are widely used as sorbents for environment protection and pollution control application, especially for wastewater treatment.The content of montmorillonite of Handan is 40%-70%. The methods of purification and Na-exchanging of Ca-bentonite were investigated. The procedure and conditions are optimized. The experiment tested the physical and chemical properties of bentonite, such as surface area and cation exchange capacity. Ca-bentonite and Na-exchanged bentonite were synthesized, characterized and studied by various analysis techniques, such as XRD, IR, SEM, DTA. The result showed that the content of montmorillonite in the above-treated bentonite increased to more than 90%. The spacing distance (d₀₀₁) value of Na-montmorinollite reduces from 15.224(?) to 12.73(?).Adsorption of Ni²⁺, Cu²⁺, Cr⁶⁺ and Zn²⁺ on Ca-bentonite and Na-exchanged bentonite were investigated. The factors of affecting adsorption were studied. The experimental results showed that Na-exchanged bentonite sorbed higher amounts of cations than the Ca-bentonite and the concentration of heavy metal ions was substandard.The experiment had obtained the best adsorption condition of the Ca-bentonite and Na-bentonite separately adsorbing Ni²⁺, Cu²⁺, Cr⁶⁺ and Zn²⁺. Under this conditions the removing rate were gained. The removal rate of Ni²⁺, Cu²⁺, Cr⁶⁺ and Zn²⁺ by Ca-bentonite and Na-exchanged bentonite is 84.7%, 87.2%, 66.85%, 75.1% and 96.34%, 98.51%, 97.93%, 96.4%, respectively. The adsorption of Cu²⁺ and Cr⁶⁺ on Ca-bentonite and Na-exchanged bentonite were represented by Freundlich adsorption isotherm.Adsorption capacities of each mineral are different. The adsorption capacities of different adsorbents for Ni²⁺ are in order of Na-exchanged bentonite>Ca-bentonite> illite>Maoming kaolinite>coal-bearing kaolinite>Suzhao kaolinite.