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Chemical precipitation is the most economic approach and is widely used for the treatment of heavy metal-laden wastewaters. In the past, most of the related references emphasized on the removel control and complexation effect for a certain kind of heavy metal ion in water and wastewater. However. However, study on the competitive reaction of various species due to the coexistence of several heavy metal sorts is quite scarce. Thus, the treated wastewater may not meet all the effluent standards of various heavy metals under a certain pH operational condiction. The prupose of this research is to investigate the effect of environmental controlled parameters(such as initial concentration of heavy metal, mole ratio between the coexitent species, pH value, and the presence of chelating agent) on the removal efficiency of heavy metals when the chemical precipitation followed by coagulative sedimentation process is used by batch type of experiments. It is expected that the relation between the coexistent heavy metals and the optimum controlled conditions can be observed through the theoretical calculations of aquatic chemcial equilibrium and the results of background experiments in order to offer the important information on treatment control.
It was found that the co-precipitation/adsorption by Fe(OH)3(s) is the major mechanism for the removal of heay metal ions with the coexistence of copper and nickel, ro, zinc and nickel, by the addition of FeCl3 as coagulant at pH9. The settling characteristic of the suspended particulates can be enhanced and the supernatant after quiescent settling meets the effluent standards. The requirement of coagulant dosage is based on nickel, however, the cadmium is instead in the cadmium/nickel coexistent wastewater.
In the presence of strong chelating agent (such as EDTA), the substitution reaction by Fe3+ and the co-precipitation/adsorption mechanism are significantly restricted by the addition of FeCl3 as coagulant. Thus, the limitation of treatment efficiency by this approach should be noticed when the heavy metal-laden wastewater with high strength chelating agent is treated. In the continuous-flow reaction system, the successive efficiency of chemical precipitation/coagulative sedimentation process can be enhanced by the proper hydraulic controlled conditions and solid-liquid separation treatment.
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