Removal of phosphate ions from aqueous solutions using bauxite obtained from Mulanje, Malawi

Studies on stream water and effluent from selected wastewater treatment plants in Blantyre, Malawi, have reported phosphate concentrations above recommended levels. High phosphate levels in the effluent and streams pose a threat to aquatic life through the stimulation of excessive growth of plants and toxic cyanobacteria. Phosphate removal by bauxite was investigated as a function of pH, contact time, dosage, competing ions and initial phosphate concentration by means of jar tests. Phosphate removal increased with decreasing pH with maximum removal (99.75%) recorded at pH 2.40 ± 0.10. Phosphate removal was attributed to ligand exchange reactions with gibbsite and goethite minerals that are chemically and electrostatically favoured at low pH. Bauxite indicated a high phosphate removal capacity with 98.42% removal recorded for a dosage of 15 g/l. This was attributed to the presence of goethite and gibbsite minerals for which phosphate has a strong affinity. Kinetics studies revealed a fast adsorption reaction with 61 and 65% phosphate removal achieved after 30 min of contact at 20 and 40°C respectively. Phosphate removal was enhanced in the individual presence of calcium and magnesium ions whereas carbonate and sulphate ions reduced it by competing for active sites.Key words: Adsorption, bauxite, phosphate, gibbsite, goethite, eutrophication

pH dependence of sorption of Cd2+, Zn2+, Cu2+ and Cr3+ on crude water and sodium chloride extracts of Moringa stenopetala and Moringa oleifera,/i>

The ability of crude water and sodium chloride extracts of partially defatted powder of Moringa stenopetala (MS) and Moringa oleifera (MO) to remove heavy metals (Cd2+, Zn2+, Cu2+ and Cr3+) fromsingle ion solution was investigated. At initial metal concentration of about 4 ppm, the extracts showed complete sorption for Cd2+, Zn2+ and Cr3+ ions at pH above 7.8, 4.0 and 4.0, respectively, at a dose of 1.0 ml of sorbent in 9.50 ml of metal solution. Cu2+ sorption increases slightly with pH to about 60% for MS at pH 6 and then becomes constant up to pH 8 when sorption rises to completion. Preliminarycharacterization of the actual powder by proton nuclear magnetic resonance showed clear presence of amide (-CO-N-H), benzenoid (Ar-H), saturated alkyl and unsaturated fragments in both MS and MO. The mass spectrum showed the presence of amino (R-NH2) fragments. The remarkable heavy metal sorption ability of M. stenopetala and M. oleifera could thus be attributable to, among other mechanisms,coordination or complex formation between the metal cations and pH dependent oxygen and nitrogen anionic sites of the Moringa proteins

Mechanisms for biosorption of chromium(III), copper(II) and mercury(II) using water extracts of Moringa oleifera seed powder

In continuation of our work on heavy metal remediation using Moringa seed powder, this study examines the mechanisms of metal sorption on water extracts of Moringa oleifera (MO) seed powder using extended X-ray absorption fine structure (EXAFS). Chromium(III) is hydrolysed to form a mixture of [Cr3(OH)4(H2O)10]5+ and [Cr(OHx)2]nn(2x-3)+ complexes, x = 1 or 2. The chromium(III) complexesare octahedral with mean Cr-O bond distance of 1.97(2) Å. Copper (II) forms complexes with ligands with oxygen and/or nitrogen donor atoms, most likely amino and carboxylate groups. Copper(II) retainthe typical Jahn-Teller distortion with Cu-O/N bond distances of 1.97(2) and 2.21(4) Å. Furthermore, a CuC distance at 2.96 Å, and a corresponding Cu-O-C 3-leg scattering path at 3.10 Å are observed as well, strongly supporting that a large fraction of carboxylate groups are bound to the copper(II) ion on the equatorial plane. Mercury (II) forms mainly linear complexes with nitrogen donor ligands, d(Hg-N) =2.15(2) Å, most probably from amino groups in amino acids or proteins. This shows that the softer metal ions, copper(II) and mercury(II), form complexes with oxygen and/or nitrogen donor ligands in the MO extracts, while the harder and more highly charged chromium(III) ion becomes hydrolysed. The study therefore suggests that the successful biosorption of heavy metals by Moringa, a potential heavy metal removing agent, is attributable to its oxygen and nitrogen donating carboxylate and amino groups

Removal of Cd2+, Cr3+, Cu2+, Hg2+, Pb2+ and Zn2+ cations and AsO43- anions from aqueous solutions by mixed clay from Tundulu in Malawi and characterisation of the clay

Water and wastewater studies in Malawi have revealed very high levels of heavy metals in most streams and other water bodies particularly within urban areas. The metals are produced and released during industrial and agricultural activities, and also in vehicular emissions. These metals may pose serious threats to both human health and the environment. This study investigated the potential of mixed clay, obtained from the Tundulu area, in removing, Cd2+, Cr3+, Cu2+, Hg2+, Pb2+ and Zn2+ cations and AsO43- anions from aqueous solutions using batch equilibrium technique. Qualitative mineralogical characterisation of the clay revealed that the clay contains illite, distorted kaolinite, mixed layer minerals and non-clay mineral carbonate fluoroapatite. pHpzc for the raw clay, as determined by potentiometric titrations, was 9.66 while pHpzc of pretreated clay was 9.63. Pretreatment of the clay involved removal of carbonates, iron oxides and organic matter. Initial total metal concentrations ranged from 3 to 5 mg/ℓ. pH metal sorption dependence of the clay revealed Cr3+ removal from pH of 3 to complete removal at pH 5 with over 90% of the removal attributable to adsorption on the clay while the remaining 10% attributable to both adsorption and Cr(OH)3 precipitation. Zn2+ complete removal occurred at pH above 7 with 92% attributable to adsorption while the rest could be from both adsorption and hydroxide precipitation. Cu2+ was removed from pH 4 and completely above pH 6.8 with 50% due to adsorption. Cd2+ removal was between pH of 6 and 9 with 85% due to adsorption to the clay. Lead was completely removed at pH greater than 7.67. Removal of Hg2+ at total Hg2+ concentration of 0.023 mM was pH independent fluctuating between 30 and 60%. No effective removal of AsO43- anion was observed. Water SA Vol.32 (4) 2006: pp.519-52