An investigation of the mechanism of chromium interactions with activated carbon.

摘要

The focus of this research was to develop an understanding of the reaction mechanism of chromium-carbon interactions that takes place during accumulation, and regeneration of chromium with activated carbon. The first stage of the study developed a hypothetical reaction mechanism model based on Cr{dollar}sp{lcub}+6{rcub}{dollar} and Cr{dollar}sp{lcub}+3{rcub}{dollar} chemistry, and activated carbon in acidic and alkaline aqueous media. Stage two validated the reaction model via both batch and continuous experiments.; Batch experiments investigated the reduction kinetics of Cr{dollar}sp{lcub}+6{rcub}{dollar} to Cr{dollar}sp{lcub}+3{rcub}{dollar}. The rate of reduction was found to be dependent on pH and carbon concentration in accord with the Equation:{dollar}{dollar}rm {lcub}-{rcub}{lcub}d lbrack Crsp{lcub}+6{rcub}rbrack/dt{rcub}sb{lcub}reduction{rcub}= 8 times 10sp{lcub}-2{rcub} lbrack Crsp{lcub}+6{rcub}rbrack lbrack carbonrbrack sp{lcub}3/4{rcub} lbrack Hsp+rbrack{dollar}{dollar}; Continuous flow experiments quantified both accumulation and regeneration of Cr{dollar}sp{lcub}+6{rcub}{dollar} and Cr{dollar}sp{lcub}+3{rcub}{dollar}. Under acidic conditions, Cr{dollar}sp{lcub}+6{rcub}{dollar} was reduced to Cr{dollar}sp{lcub}+3{rcub}{dollar}, with simultaneous oxidation of the carbon sites that created additional sites for accumulation. Under acidic regeneration conditions (0.88 {dollar}<{dollar} pH {dollar}<{dollar} 1.0), carbon accumulation capacities increased noticeably during the first 3 to 4 cycles. The removal of chromium was found to take place via two competitive reactions. The first mechanism decreased effluent pH, and predominated at the beginning of the accumulation process. This removal mechanism is governed by:{dollar}{dollar}rm lbrack Csb{lcub}x{rcub}(OH)sb2sp{lcub}+2{rcub}rbracksp* + HCrOsb4sp- Longleftrightarrow Csb{lcub}x{rcub}(OH)sb2CrOsb4 + Hsp+{dollar}{dollar}The second mechanism increased the effluent pH and predominated toward the end of the accumulation. This removal mechanism is governed by:{dollar}{dollar}rm Csb{lcub}x{rcub}(OH)sb2sp+rbrack sp* + HCrOsb4sp- Longleftrightarrow Csb{lcub}x{rcub}OHsb2CrOsb3sp+ + OHsp-{dollar}{dollar}Regeneration showed that acid and alkaline regeneration recover chromium in the tri- and hexavalent state respectively. Both regenerants exhibited an incomplete recovery of the chromium species when used separately. Alkaline followed by acidic regeneration resulted in greater than 90% chromium recovery from activated carbon. Preliminary economic evaluation analysis showed that a 10% savings can result from the use of activated carbon versus the similar use of ion exchange resins.