Malcolm W Clark

Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model

Abstract Mangrove forest sediments can provide a sink for trace metals because the mangroves create a baffle that promotes the accumulation of fine-grained organic matter-rich sediment, which is usually sulphidic due to the presence of sulphate-reducing bacteria. Direct adsorption, complexing with organic matter, and the formation of insoluble sulphides all contribute to the trapping of metals. The concentration and chemical speciation of the metals are influenced by the distribution of geochemically distinct horizons within the sediment. In horizons with a pH>7 and an Eh +100 mV (oxidation horizons), most metals are present as exchangeable or oxide-bound species. In most cores, two oxidation and two reduction horizons can be recognised, but dark mottles of low Eh ( +100 mV) sediment can be found around mangrove roots and burrows in the reduction horizons. The depth to each horizon, differs between cores and can change in response to seasonal shifts in the position of the water table. A model is presented that accounts for the development of the oxidation and reduction horizons within the Avicennia -dominated mangrove forest sediment and describes the major controls on metal cycling within the sediment.

Chemistry of seawater neutralization of bauxite refinery residues (red mud)

Bayer process red muds are highly alkaline, and their disposal and potential for reuse are complicated by high concentrations of sodium, high pH, and high alkalinity. Several methods of neutralization of red muds have been reported including: infiltration of rainwater and atmospheric CO2, treatment with strong acids, gypsum addition, and seawater neutralization. Using seawater to neutralize red mud at the Queensland Alumina Ltd. (QAL) refinery at Gladstone, QLD, began as a fresh water conservation measure but led to many benefits, including reduced freshwater use at the refinery, increased settling rates at the pond and reduced alkalinity and sodicity in the solid wastes and entrained liquor. The study reported here describes and quantifies the geochemical consequences of seawater neutralization of bauxite refinery residues. Methods and results presented allow description of the chemical, mineralogical, and physical characteristics of seawater-neutralized red mud. Experiments then identify and quantify th...

Phosphate Removal from Aqueous Solutions using Neutralised Bauxite Refinery Residues (Bauxsol

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.

Alkalinity conversion of bauxite refinery residues by neutralization

Red mud remains the largest environmental issue for the alumina industry due to its high pH (>13), fine-grained nature (>90% is <10 microm), elevated sodium concentration (>50 g/kg), and soluble alkalinity (approximately 30 g/kg as equivalent CaCO(3)), which reduce the transport and reuse options of red mud. The neutralization of red mud provides potential reuse options because neutralization lowers pH, increases grain-size (e.g., coagulation), and precipitates or converts alkalinity. This paper investigates the geochemistry of 3 treatments of a red mud to affect neutralization and potentially convert materials from a waste material to a resource. This study investigates two commonly used neutralization techniques, a CO(2)-neutralized red mud (CNRM), a Basecon-neutralized red mud (Basecon), and a more novel approach of a CO(2)-neutralization followed by a Basecon-neutralization (Hybrid) to understand the effects that these treatments have on neutralization process. Data indicate that the neutralization techniques form two distinct geochemical groups when discriminated on total alkalinity alone, that is treatments with, and treatments without alkalinity precipitation. However, each treatment has distinct alkalinity speciation (hydroxide-dominant or carbonate/bicarbonate dominant) and residual Ca, Mg and Al in the treatment solution. Similarly, solids produced differ in their reaction pH and ANC, and contrary pH and ANC, a contrary to other studies, Dawsonite was not seen to precipitate during any neutralization. However, despite this approximately 17 g/kg CO(2) was sequestered during CNRM and hybrid neutralizations and all treatments increased either the transport or reuse options of red mud in some way.

Selective chemical extraction and grainsize normalisation for environmental assessment of anoxic sediments: validation of an integrated procedure

An integrated selective extraction and size normalisation procedure for use in metal partitioning and diagenetic studies of anoxic sediments and soils is presented. Data obtained by this procedure can readily be combined with other primary data (e.g. sulfur concentrations, carbonate concentrations, cation exchange capacity, etc.) and derived parameters (e.g. degrees of pyritisation and sulfidisation) that enhance interpretation of the behaviour of trace metals in anoxic sediments. Achieving size normalisation during extraction, allows direct comparison of sediments from dissimilar sedimentary environments, and simplifies assessment of the processes that determine whether a sediment is a source of or a sink for trace metals. Aspects of a study of trace metals in sediments from the Brisbane River estuary, Australia, are used to illustrate applications of the integrated procedure.

Carbon Capture and the Aluminium Industry: Preliminary Studies

Environmental Context. Carbon dioxide concentrations in the atmosphere are rising every year by 1.5–3.0 ppm and there is now a general acceptance that increased efforts must be made to reduce industrial sources of this greenhouse gas. Carbonation of red mud wastes produced by aluminium refineries has been carried out to study the capacity of these wastes to capture carbon dioxide. Removal is very rapid, with the added carbon dioxide recorded as a large increase in bicarbonate alkalinity. Although these results can only be considered preliminary, the experiments indicate that these wastes can potentially remove up to 15 million tonnes of carbon dioxide produced in Australia per annum. Furthermore, the carbonated waste can be used in other industrial processes to add further value to these waste materials. Abstract. Carbonation of raw red mud produced by aluminium refineries and a chemically and physically neutralized red mud (Bauxsol™) has been carried out to study the capacity of these wastes to capture carbon dioxide. After only 5 min of carbonation of raw red mud, total alkalinity dropped 85%. Hydroxide alkalinity was almost totally consumed, carbonate alkalinity dropped by 88%, and bicarbonate alkalinity increased to 728 mg L–1. After 24 min carbonation, the bicarbonate alkalinity reached its maximum value of 2377 mg L–1, and hydroxide and carbonate alkalinity were virtually absent. After 30 and 60 min carbonation, bicarbonate alkalinity started to decrease slightly as the pH of the slurry increased. After 5 min carbonation of Bauxsol™, total and bicarbonate alkalinity dropped 89% and 9%, respectively. After 20 min carbonation, bicarbonate alkalinity dropped another 11%, but after 30 min carbonation bicarbonate alkalinity increased 26% to levels found in the original Bauxsol material, and pH was stable. Based on these experiments, a calculation of the amount of carbon dioxide that could be removed annually at aluminium refineries in Australia is potentially 15 million tonnes, and suggests that further studies are necessary to maximize this carbon removal process. Furthermore, carbonation produces a product, which can potentially be used in other industrial and agricultural activities to remove toxic metals and nutrients.

The pH-dependence and reversibility of uranium and thorium binding on a modified bauxite refinery residue using isotopic exchange techniques

The pH-dependence and reversibility of uranium and thorium binding onto a modified bauxite refinery residue (MBRR) were studied in laboratory uptake/leaching experiments. Natural (238)U and (232)Th isotopes were contacted with MBRR in an 8day loading period (equilibrium pH≈8.5) then leached in pH-dependent experiments, where the pH was decreased from 8 to 3 over several hours following addition of exchange isotopes (232)U and (229)Th. Relative concentrations of the thorium isotope pair ((232)Th and (229)Th) indicate that Th is very strongly bound to MBRR, and although at pH 3, some de-sorption is observed (232)Th (≈3%) and (229)Th (≈2.5%), released thorium is partially re-adsorbed during an overnight equilibration. During the initial equilibration, approximately 50% of the (238)U was adsorbed, and a U adsorption maximum occurs between pH 5 and pH 6, where <0.5% of the U remains in solution. However, at a pH between 5 and 3, some 60% of the bound U releases, hence the pH range of maximum U retention on the MBRR is relatively narrow. When equilibrated overnight, the MBRR releases additional U, suggesting a kinetically controlled de-sorption linked to mineral dissolution. Plots of U isotope exchange between (232)U and (238)U are linear, and suggest that U adsorption is mostly reversible. Data for adsorption in mixed systems of U and Th suggest that Th and U compete for similar binding sites.

Phosphate removal by hydrothermally modified fumed silica and pulverized oyster shell

A calcined and hydrothermally annealed material for phosphate removal was prepared in a hollow cylindrical shape from fumed silica generated by a ferroalloy factory, and pulverized oyster shell. Phosphorus removal from wastewater by this material calcined at a range of temperatures (700-900 degrees C) and hydrothermally annealed at temperatures from 130 to 180 degrees C, for 8-16 h, was investigated and the most suitable physicochemical conditions were determined. XRD, SEM, EDS, and XRF techniques were used to characterize the microstructures and compositions of the materials produced, and UV-Vis spectrophotometry using the ammonium phosphomolybdate blue method was used to determine the phosphate concentration in the wastewater. The results indicate that calcium carbonate in oyster shell reacted with SiO(2) in fumed silica and formed hydrated calcium silicate after hydrothermal treatment, and the hydrated calcium silicate reacted with phosphate ions in wastewaters to form hydroxyapatite precipitate. The optimal conditions for material production were calcination at 800 degrees C, and hydrothermal annealing at 150 degrees C for 12 h. Materials made under these conditions showed 74% or 92% phosphate removal after 2 or 4 h, respectively.

Effects of Bauxsol on the immobilisation of soluble acid and environmentally significant metals in acid sulfate soils

The effects of Bauxsol, an abundant industrial by-product, on the immobilisation of soluble acid and a range of potentially environmentally toxic metals in artificial and natural acid sulfate soils were investigated. The acid neutralising capacity of Bauxsol increased with decreasing pH, which is probably provided not only by basic metal hydroxides, carbonates, and hydroxycarbonates but also by protonation of variably charged particles (e.g. gibbsite and hematite) present in Bauxsol. Simulation experiment results show that the removal of 9 tested environmentally significant heavy metals can be enhanced by addition of BauxsolTM; an exception was Co. The removal of the added soluble heavy metals by the BauxsolTM-soil mixtures shows a preferential order of Pb > Fe > Cr > Cu > Zn > Ni > Cd > Co > Mn. For the natural acid sulfate soil without added synthesised metal solution, the retention of the investigated environmentally significant metals is in the following decreasing order : Al > Zn > Fe > Co > Mn.

Management implications of metal transfer pathways from a refuse tip to mangrove sediments

Mangroves have often been thought of as wasteland, and because of this attitude, many mangrove forests have been used as sites for refuse tips, sewage outfalls and as illegal dumping grounds. The transfer of metals from a refuse tip to mangrove sediments is investigated and four pathways of heavy metal migration are identified. These are direct seepage across the tip-cell floor to groundwater, tidal over-topping of the bund wall and capillary suction of leachates to groundwaters, direct seepage through the cell wall of leachates to surficial sediments, and surface runoff during rainfall events. Metal input by surficial runoff indicates that significant quantities of metals have been transferred to the mangroves via this mechanism. However, metal transfer via surface runoff is probably small in comparison to the other three mechanisms discussed. The variable nature of the transfer mechanisms operating, the variability in sediment texture both vertically and laterally, and the number of transfer pathways makes potential management of the site difficult. Any management plan for this site must consider both the feedback mechanisms that operate, and that the whole sediment column is involved in metal transfer.