Several studies suggest that solution concentrations of Al in organic surface soils are controlled by complexation with organic matter. We applied the mechanistic Windermere humic aqueous model (WHAM) to describe the solubility of Al and organic matter as observed in a batch equilibrium study with a forest floor Oe horizon. WHAM is unique in that it considers interactions of soil organic matter with protons and metals. We also compared WHAM with a previously proposed linear regression model that describes Al solubility. A range of soil Al contents was established by adding different amounts of Al in batch prior to titration with acid or base. The soil Al content was described by the bound Al ratio (BAR), defined as the equivalent ratio of organically bound Al and carboxyl groups. The bound Al ratio and pH ranged from 0.1 to 3, and from 1.7 to 6.3, respectively. Solutions were undersaturated with respect to Al(OH)3, except at BAR ≥ 2 and pH ≥ 4.5. Aluminum solubility increased with increasing BAR. Organic matter solubility was greatest at low BAR and high pH. WHAM reproduced the observed pH and Al concentrations, using parameters derived from experimental data. At BAR < 0.7, pH–pAl relationships were approximately linear. At BAR ≥ 0.7, there was a nonlinear increase in ΔpAl/ΔpH with pH. WHAM simulated the changing slope of the pH–pAl curves satisfactorily and reproduced observed trends in dissolved organic C (DOC) concentrations. This supports the hypotheses and assumptions concerning mechanisms for binding Al to soil organic matter in a forest floor, as embodied in WHAM.
Recovery of chromium from a plating sludge is discussed. The process consists of two parts: roasting the sludge with sodium carbonate (Na2CO3) and calcium oxide (CaO), followed by leaching. The optimal parameters for roasting are a temperature range of 950 to 1000°C and Na2CO3 and CaO contents of 2.4 and 2.8 mol per mole of chromium oxide (Cr2O3) respectively.
Demineralized water is recommended for the leaching of sodium chromate (Na2CrO4) from the agglomerate formed during roasting. The leached water is recycled through a strongly acidic cation exchanger followed by a weakly basic anion exchanger. A solution of chromate or chromic acid having a concentration up to 60 kg Cr/m3 is obtained.
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