Adsorption materials for removal of heavy metals and petroleum hydrocarbons from contaminated leachates
Paper in proceedings, 2006
Adsorption function and capacities, for heavy metals and selected hydrocarbons, of several low-cost and alternative bio-sorption materials have been investigated. The materials studied were residual products from the forest industry (saw dust, pine bark and fiber ash) and natural materials (peat moss, shrimp shells and seaweed). Batch tests and column experiments were carried out with both artificial solutions and highly contaminated leachate from an industrial landfill.
Fiber ashes and peat showed the highest sorption capacity for metals among the materials studied in comparative batch tests. In these tests, artificial single metal solutions in concentration ranges of 1−10 mg/l, and liquid to solid ratios of 20 and 200 were used. The fiber ash removed lead by 99%, copper by 100%, zinc by 99% and chromium by 82%. Peat removed lead by 98%, copper by 94%, zinc by 73% and chromium by 88%. Metal removal from the landfill leachates was also studied in batch tests, where lead was reduced by ash and peat by 99% and 96% respectively, copper by 100% and 92%, zinc by 95% and 33%, cadmium by 88% and 40%. A net release of Al, Cr, Ca, Ba and K from ash was observed, whereas the peat removed chromium by 66% and aluminium by 85%. The lower performance of the adsorbents for complex solutions as real leachates, suggests competitive sorption of ions although the mechanisms of sorption are not yet fully understood.
In initial batch studies for organic pollutants, the adsorption for diesel oil by ash and peat was 98% and 97% respectively, 97% and 92 % for the n-alkane C16, and 91% for n-C12 for both materials. Bark adsorbed diesel oil by 83%, and the lower value could be explained by the larger particle size of the bark.
Several column tests with peat, a peat-ash mixture and bark have been carried out to investigate the adsorbents’ behavior and sorption capacity under flowing conditions. Both simulated contaminated groundwater and real landfill leachates were used as eluents. Sorption capacities for the metals and break-through volumes of the solutions were obtained.
For most of the metals studied (Cd, Pb, Zn, Cu, Ni) ground peat appeared to be the best adsorbent, both in degree of sorption and service time. The column with peat-ash mixture adsorbed Cd, Ni and Pb to a lower degree, but retained the metals for a longer time than the peat column. The adsorption rate was significantly higher when artificial solutions were used and the metals were adsorbed for a longer time: Cu and Ni for approximately 20% and Zn for 50% longer time. These results point out that laboratory tests can overestimate adsorbent’s performance and experiments should be specific and using real leachates. High iron content showed no effect on the sorption capacity of Cd and Cu, but other metals as Ni, Pb, Zn were sorbed for a shorter time. To better understand the basic mechanisms and processes, column tests need to be complemented with batch tests for basic studies of adsorption and competitive mechanisms.