Abstract:
Heavy metal pollution of stormwater runoff, the main source of ground and potable water, is currently attracting increasing scientific attention. Heavy metal removal methods, such as precipitation, coagulation, ion exchange, and reverse osmosis, are either too expensive or inefficient. A new carbon-rich adsorbent known as biochar has been demonstrated to possess a high capacity for carbon sequestration but also has potential to remove a variety of organic and inorganic pollutants. Therefore, the goal of this research was to investigate the adsorptive potential of ten biochars: pine sawdust, corncob, poultry litter, sewage sludge, green waste 550ºC, green waste 450ºC, green waste 350ºC, paunch grass, pine bark and broiler litter, from eight feed stocks for three heavy metals (cadmium, copper and zinc). The research also focuses on single metals, multi-metal competition and transport of heavy metal ions commonly found in urban stormwater runoff. Adsorptive removal efficiency of heavy metal ions was studied according to contact time, heavy metal concentrations, adsorbent dosages and pH values. A series of batch experiments were conducted to remove cadmium, copper, and zinc contained in single and multi-metal aqueous preparations. Column experiments were also conducted to determine the adsorption capacities of different biochars to establish the breakthrough point and curve of these metals. The results indicated that all the metals in this study were efficiently adsorbed by the biochars. However, with an increase in adsorbent dosage from 0.01 to 0.5g, adsorption capacity for each single metal decreased and there were no clear trends for multi-metals. After 12hours, no further adsorption occurred for single metals using pine sawdust, corncob and green waste (550˚C) and, during the first 2-5 hours, 70-90% adsorption was achieved. An increase in the initially selected concentration range of 10 to 100 mg∙L-1 for the batch studies indicated an initial, rapid increase in metal adsorption, which then plateaued and finally approached equilibrium. Adsorption capacities for cadmium, zinc and copper followed an order: pine saw dust > poultry litter > corncob > green waste 550˚C > sewage sludge > pine bark ≈ paunch grass > broiler litter > green waste 450˚C > green waste 550˚C ( cadmium ions), pine saw dust > corncob > poultry litter > sewage sludge > green waste 550˚C > paunch grass ≈ pine bark ≈ broiler litter > green waste 450˚C > green waste 350˚C (zinc ions) and poultry litter > corncob > pine sawdust > sewage sludge > broiler litter ≈ green waste 550˚C ≈ pine bark > green waste 350˚C ≈ green waste 450˚C (copper ions). In the single metal adsorption experiments, copper ions were the most adsorbed metal for all biochars and during the multi-metal competition adsorption stage copper ions again showed high sorption affinity for the biochar. A similar adsorption efficiency rate was achieved as that of the single metal adsorption experiments but the removal of cadmium and zinc was less effective. The results of the column experiments showed that for zinc and cadmiun ions breakthrough occurred approximately at 30 min and the complete breakthrough curve was achieved at approximately 270 min. However, the breakthrough point and curve for copper ions were 100 min and 400 min respectively. Among the range of pH (3 to 8) tested for single metals, the most suitable pH range for cadmium and zinc ion adsorption was 5.5 to 6.5, but those of copper ions ranged from 4.5 to 5.5. During the multi-metal adsorption stage, final pH values decreased slightly with any increase in the initial concentrations and, the change in pH values between the initial and final pH’s, increased. Results from the column experiments indicated a reduction in the effluent pH values with increases in time. An analysis of the filtrates and residues from the adsorption equilibrium experiments indicated the importance of two distinct mechanisms active in the adsorption process. These included cations exchange mechanisms with involvement of potassium, calcium and magnesium ions, and surface complexations of free carboxyl and hydroxyl functional groups. Keywords: Batch adsorption, biochar, cadmium, column adsorption, copper, heavy metals, urban stormwater runoff, zinc