Alternative Adsorption Materials for Treatment of Contaminated Waters

Licentiate thesis, 2006

The thesis presents a study of adsorbent materials for treatment of contaminated waters and remediation of contaminated soils. Natural and waste product materials showed high adsorption capacity for metals and organic pollutants and proved to be a good alternative to commercial adsorbents. Adsorption of Pb, Cu, Zn and Cr onto peat moss, fibre sludge ash, pine bark, sawdust, shrimp shells and seaweed was studied in batch adsorption systems. Artificial solutions and real landfill leachates were used. All the materials showed high uptake of metals from solutions with a concentration range typical for severely polluted groundwater. The adsorption capacities found for the conditions applied were 1.3 – 1.8 mg•g–1. Materials having the highest uptake for metals, including peat moss, fibre sludge ash and bark were further studied in column experiments for possible implementation in treatment of leachate at a landfill. The adsorption capacities for adsorption from the artificial solution were one order of magnitude higher than those for the real leachate. This was attributed to competition between different metals for adsorption sites in the case of the leachate. These results may indicate that laboratory tests with artificial solutions can overestimate the performance of an adsorbent, and that specific experiments using real leachates are needed when a water treatment application is developed. A peat-ash mixture showed lower adsorption capacity than the peat itself, and these materials are suggested to be used separately. The presence of Fe in the solution reduced the adsorption of all studied metals to half the normal. Thus, Fe precipitation is recommended as a pre–treatment step. Adsorption of Pb, Cd, Ni, Cu and Zn in low concentrations onto peat and ash was studied in detail since adsorption mechanisms were suggested to depend on concentration. Kinetic and equilibrium batch adsorption studies were conducted and data complied with a second-order kinetic model and the Freundlich isotherm model. An empirical model for predicting adsorption of metal ions at a given time was derived from kinetic model constants. Possible mechanisms of adsorption onto biosorbents and mineral surfaces were summarised and suggestions made for the mechanisms of adsorption onto peat and ash. An explanation for the orders of affinity of metal ions to adsorption sites found was proposed on the basis of the chemical properties of the metals. The use of peat and ash in filter beds for contaminated water treatment is discussed. Recommendations are made for the choice of peat material and filter maintenance to provide maximum removal of pollutants and service time of the filter.