Environmental Context.Eutrophication of freshwater and marine ecosystems is a global problem, which is frequently linked to high phosphorus concentrations. The present study investigated the use of Bauxsol™, a modified bauxite refinery residue, to remove dissolved phosphate from water, and has shown that it can be used as a cost-effective adsorbent for treating phosphate-contaminated waters. The results provide water and environmental managers with a new technique for decreasing the phosphate loads in water and wastewater. Environmental benefits include improved water quality, minimisation of excessive plant growth, including potentially toxic blue green algae, and the utilisation of an industrial residue for environmental remediation.
Abstract.Phosphate (PO43–) removal by Bauxsol™, a neutralised bauxite refinery residue, was investigated as a function of time, pH, ionic strength, adsorbent dosage, competing ions, and initial phosphate concentration. The results of adsorption and desorption studies indicate that adsorption of PO43– by Bauxsol™ is based on a ligand-exchange mechanism, although the low reversibility pH-independent desorption observed in acid-treated Bauxsol™ indicates a dominance of chemisorption. It was shown that PO43– adsorption onto both Bauxsol™ and acid-treated Bauxsol™ followed a Langmuir isotherm model, with adsorption capacities of 0.21 and 0.48 mmol g−1 at pH 9.0 and 5.2 respectively. Adsorption of PO43– by Bauxsol™ increased with decreasing pH, with maximum adsorption efficiencies obtained at pH 5.2 ± 0.1 (the lowest pH investigated), higher Bauxsol™ to initial phosphate concentration ratios, and increased time. Studies of the effects of competing ions on the adsorption of PO43– by Bauxsol™ indicated that adsorption decreased in the presence of HCO3− ions, whereas SO42–and Cl− ions had little effect, and Ca2+ and Mg2+ ions increased adsorption. These findings suggest that Bauxsol™ could be used as an efficient low-cost adsorbent for treating phosphate-contaminated waters.
Experimental sediment cores from Lake Ainsworth, Australia, were exposed to an induced 46-day, anoxic/oxic cycle in the laboratory, mimicking the seasonal thermal stratification cycle commonly observed in the lake’s waters every summer. Under oxic conditions the supply of phosphorus (P) and iron (Fe) to the overlying water was slow, however, induced anoxia led to an enhanced release of P and Fe from the sediments to the water column. An inverse relationship between total P, Fe and redox potential suggests that Lake Ainsworth sediments are redox sensitive. Phosphorus speciation analysis of Lake Ainsworth sediments revealed the presence of a large pool of organic P, reactive Fe-bound P, and CaCO3-bound P, the latter fraction decreasing during anoxic conditions. Two sediment-capping agents, a lanthanum modified bentonite clay and Bauxsol (a waste product from the aluminium smelting industry) were assessed for their ability to reduce the levels of P released from Lake Ainsworth sediments during the 46-day, anoxic/oxic cycle. The bentonite clay was highly effective at reducing plant available P in anoxic/oxic conditions, but levels of dissolved Fe were enhanced with its use. Although the use of Bauxsol to remove plant available P is not recommended in anoxic waters, its use in suspension in oxic waters warrants further study.
Radium (Ra) removal by an unconventional sorbent, a modified bauxite refinery residue (MBRR), is investigated for a groundwater extracted in Missouri, USA. The MBRR treatment causes substantial reductions of both gross α and combined Ra activities from 0.955 ± 0.005 and 0.66 ± 0.005 Bq L to below detection limits (0.037 Bq L or 1 pCi L). Column breakthrough occurs at 0.555 Bq L for gross α and 0.185 Bq L for combined Ra (15 and 5 pCi L; USEPA's maximum contaminant levels) after 54 and 40 d run time, respectively. At 84 d the MBRR media continues to remove 24.3% of raw water gross α and 39.7% of the combined Ra. The treatment effluent has an initial pH of 10.9, outside the USEPA guides (6.5-8.5); this may be readily mitigated by posttreatment acid injection, or by raw water blending. The MBRR simultaneously removes other potentially hazardous trace elements (e.g., Cu, Zn, and Fe) to extremely low concentrations. In addition, toxicity characteristic leaching procedure testing of spent MBRR suggests that metals are bound tightly, such that it is nonhazardous, permitting cost-effective disposal to landfill without special confinement or storage. Consequently, MBRR may be utilized as an alternative adsorbent for treating Ra-contaminated groundwater.
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