| This study combined adsorption and dielectrophoresis (DEP) methods for the treatment of heavy metals, it is a new green method which has high removal rate and would not produce secondary pollution.This study firstly selected the best adsorbent from some natural geological materials and investigated the adsorption mechanism of the best adsorbent for removing heavy metal. Then we designed DEP processing devices which were used to investigate the elimination of heavy metals by adsorption/DEP method. The effects of some physical factors were investigated. The different states of the adsorbent particles were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) after adsorption/DEP experiments and the adsorption/DEP mechanism was studied. The main conclusions of this research are as follows:1. Hollow microspheres (the average particle size was6μm and10μm, respectively) and bentonite act as adsorbents for the removal of Cd2+. Then bentonite was chosen for the best adsorbent. From this part several conclusions can be draw.(1). The adsorption thermodynamic experiments shows that the adsorption behavior of Cd2+on10μm hollow microspheres and bentonite is between monolayer and multi-layer adsorption. The adsorption was spontaneous thermodynamically and the metal adsorption process was endothermic in nature. (2). The adsorption kinetics experiments shows that the adsorption process of Cd2+on6μm and10μm hollow microspheres is different, the former is mainly affected by diffusion and the latter is based on chemical control.(3). By calculating the removal rate and the maximum adsorption capacity of Cd2+on three adsorbents, the sequence of the maximum adsorption capacity was as follows:bentonite>10μm hollow microspheres>6μm hollow microspheres, so we chose bentonite to study the process of adsorption and dielectrophoresis (DEP).2. Using bentonite as the adsorbent in the DEP processing devices, the effects of different physical factors on the removal rate of Cd2+、Pb2+were investigated. Then the different states of the bentonite particles were characterized by SEM and EDS after adsorption/DEP experiments. From this part several conclusions can be draw.(1). The influence of the voltage.The removal rate of Cd2+increased first and then decreased as the voltage increased. Optimum voltage was specified as20V; the removal rate of Pb2+reduced first, then increased and continued to reduce,18V was determined to the optimum voltage.(2). The influence of the flow. The removal rate of Cd2+、Pb2+increased as the flow rate decreased. Optimum flow rate was specified as0.0926ml/s,and the residual concentration of Pb2+was0.039mg/L which reached the national drainage standards at the optimum voltage.(3). The influence of bentonite additive amount. The percent removal of two kinds of ion increased with increasing weight of bentonite. Optimum adsorbent dosage of Cd2+was specified as1.4g/L,and the Pb2+was0.5g/L at the optimum voltage and flow rate.(4). The influence of the capture pool numbers. As the capture pool numbers increased, the removal efficiency was higher. In the case of Cd2+,under the condition of the optimum voltage and flow rate, the removal rate of Cd2+was99.97%after processing in three capture pools. And the residual concentration of Cd2+was0.065mg/L which reached the national drainage standards.(5). The exploration of removal mechanism.The surface morphology of bentonite was changed after DEP processing, the curling sheet structure turned to spherical surrounded structure and the particle size became smaller, and more notably,and the cadmium content was higher after DEP processing than adsorbention. The removal rate of Cd2+was about70%higher than adsorption; The lead content was higher after DEP processing than adsorbention,and the curling sheet structure turned to small sheet-like structure and the particle size became smaller. The removal rate of Pb2+was about50%higher than adsorption.3. In the DEP processing devices, the effects of different physical factors on the removal rate of the mixture of Cd2+and Pb2+onto the bentonite were investigated. After characterizing the different states of the bentonite particles by SEM and EDS, the adsorption/DEP mechanism of single ion and mixed ions was studied. From this part several conclusions can be draw.(1). The adsorption rate of the mixed ions was reached its limit as the dosage of bentonite increased. But the removal rate of Pb2+onto the bentonite was higher than that of Cd2+under the same condions.(2). In two processing pools, the optimum voltage in the primary unit and secondary unit was20V;(3). The removal efficiency of mixed ions was increased as capture pool numbers increased. At the optimum voltage and flow rate, optimum flow rate was specified as0.0926ml/s,the removal rate of Cd2+and Pb2+was97.34%and99.97%in three processing pools, repectively.(4). The surface morphology of bentonite changed the particle size became smaller after adsorption/DEP processing, which integrated the results of Cd2+and Pb2+.(5). Either single ion or mixed ions, the removal rate all higher than that of adsorbtion after DEP processing. The difference between the two conditions was greater when two ions co-exist, which means that DEP has a greater impact on mixed ions,which can weaken the competitive inhibition of single adsorption. |
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