Study On Adsorption And Its Mechanism Of Chromate On Modified Zeolite And Bentonite

This research includes three parts. First of all, the adsorption of chromate on zeolitemodified by cation surfactant CTAB and its mechanism were investigated; then differentquantity of cation surfactant CTAB was used to modify zeolite to different extent, theamount of CTAB adsorbed by zeolite was determined, and then the adsorption capacity forchromate was compared among these modified zeolites. The relationship betweenadsorption of chromate on modified zeolite and zeta potential in its suspension system wasalso investigated and the adsorption mechanism for chromate was discussed. Secondly,adsorption of CrO₄^2- by Fe/Al modified bentonites was investigated. At the last section,adsorption of F^- by Fe modified bentionites was studied.The main results are summarized as following:The modified zeolite had a greater adsorption capacity for chromate as compared withnon-modified zeolite. The amount of chromate adsorbed by modified zeolite decreased withthe increase in ionic strength, which suggested that the electrostatic interaction between themodified zeolite and chromate was the major mechanism. The Zeta potential of zeolitechanged from negative value to positive value after modified by cation surfactant, and thisindicated that the adsorption of cation surfactant on zeolite induced a change in its surfacecharge properties. The increase in positive value of surface potential was the main reasonfor electrostatic adsorption of chromate on the modified zeolite. Both adsorption ofchromate by modified zeolite and desorption of pre-adsorbed chromate increased with theincrease in pH, and reached a maximum value at pH 5.0 and 6.0, respectively, and thendecreased. While after that, the difference between adsorption and desorption of chromatedecreased with the increased of pH, suggesting that the adsorption through noneelectrostatic mechanisms at low pH value.Results showed that both adsorption of CTAB on zeolite and adsorption capacity ofthe modified zeolite for chromate increased obviously with the increase in the quantity ofCTAB added. The zeta potential of zeolite changed from negative value to positive valueafter modified by cation surfactant, and the value of the zeta potential also increased withthe increase in the quantity of CTAB added. This indicated that the adsorption of cation surfactant on zeolite induced a change in its surface charge properties. The amount ofchromate absorbed by the modified zeolite decreased with the increase in ionic strength,and most of chromate adsorbed on modified zeolite can be desorbed by neutral salt, whichsuggested that the electrostatic interaction between the modified zeolite and chromate wasthe major mechanism for the enhanced adsorption of chromate. When the quantity of CTABadded was in the range from 0.15 to 0.30 mol.kg^-1, the used modified zeolite can be reusedas absorbent for chromate after renewing.Results indicated that modification of bentonite by Fe or A1 resulted in the increasein CrO₄2 adsorption to some extent and Fe modified bentonites adsorbed much moreCrO4^2- than the one modified by A1. The amount of CrO₄^2- adsorbed by Fe/A1 modifiedbentonites increased with the increase in the ion strength and decreased with the increase ofpH. The adsorption of CrO₄(2-) on the Fe/A1 modified bentonites involved in specific andelectrostatic interaction of CrO₄(2-) with the modified bentonites, and the specific adsorptionwas the main mechanism for CrO₄(2-) adsorbed by Fe/A1 modified bentonites under theexperimental conditions.Preliminary results showed that modification of bentonite by Fe resulted in theincrease in F adsorption to some extent and Fe modified bentonites adsorbed more F thanthe latosols. The adsorption increased with the increase in system pH, reached a maximumvalue at about pH5.0, and then decreased with the further increase in pH.