Sorptive trace-element removal in alumina packed beds

Adsorption onto packed beds of high capacity, porous alumina particles is a promising technology for the removal of trace elements from contaminated wastewaters. In this research, a parametric investigation of the chemical and physical factors governing the performance of the treatment technology was conducted. Special emphasis was given to the effect of chemical controls such as pH and presence of ligands in enhancing packed bed performance. The study systematically isolated the effect of various chemical (solution and surface chemistry) and physical factors (mass transfer and mass transport) affecting bed performance, in increasingly complex experimental systems ranging from batch equilibrium sorption to batch rate of uptake to column experiments. Equilibrium sorption experiments show that high surface area, porous activated aluminas exhibit good sorptive removal efficiencies for the two ions (cadmium and selenite) used in the study. Ligands such as chloride, thiosulfate and EDTA modified sorption behavior of cadmium significantly; the sorption behavior (with and without ligands) required the use of a surface complexation approach for suitable representation. Good mass transfer characteristics were obtained for both ions in rate of uptake experiments, either with or without chemical modification. Ligand addition (thiosulfate or EDTA) significantly enhanced the rate of cadmium uptake, though with a corresponding decrease in sorption equilibrium. The rate of uptake data suggested that chemical modification of the waste (ligand addition with possible pH modification) could improve the performance (i.e., bed volumes treated before breakthrough) of the proposed technology. Column experiments confirmed that the addition of thiosulfate and EDTA yielded a significant increase in the number of bed volumes treated of a high concentration cadmium solution. Selenite sorption was excellent, yielding high sorbent usage rates. Regeneration studies suggested that selenite could be easily desorbed from the particles by flushing with a high pH, high P_CO2 solution. Cadmium desorption (using pH and ligand concentrations was as control variables) was not very successful. The study demonstrates the potential of the proposed technology for meeting current and future water quality standards as well as highlights the importance of considering chemical controls in designing the treatment technology for maximum performance.