Abstract
Shredded card (SC) was assessed for use as a sorbent of potentially toxic elements (PTE) carried from contaminated soil in various leachates (oxalic acid, formic acid, CaCl2, water). We further assessed SC for retention of PTE, using acidified water (pH 3.4). Vertical columns and a peristaltic pump were used to leach PTE from soils (O and A/B horizons) before passing through SC. Sorption onto SC was studied by comparing leachates, and by monitoring total PTE contents on SC before and after leaching. SC buffers against acidic soil conditions that promote metals solubility; considerable increases in solution pH (+4.49) were observed. Greatest differences in solution PTE content after leaching with/without SC occurred for Pb. In oxalic acid, As, Cd, Pb showed a high level of sorption (25, 15, and 58x more of the respective PTE in leachates without SC). In formic acid, Pb sorption was highly efficient (219x more Pb in leachate without SC). In water, only Pb showed high sorption (191x more Pb in leachate without SC). In desorption experiments, release of PTE from SC varied according to the source of PTE (organic/mineral soil), and type of solvent used. Arsenic was the PTE most readily leached in desorption experiments. Low As sorption from water was followed by fast release (70% As released from SC). A high rate of Cd sorption from organic acid solutions was followed by strong retention (~12% Cd desorption). SC also retained Pb after sorption from water, with subsequent losses of ≤8.5% of total bound Pb. The proposed use of this material is for the filtration of PTE from extract solution following soil washing. Low-molecular-mass organic acids offer a less destructive, biodegradable alternative to strong inorganic acids for soil washing.
Highlights
Contamination of the environment with potentially toxic elements (PTE) is a persistent threat in the industrialized world
For use as the mobilizing solvent, this study focuses on the following low-molecular-mass organic acids (LMMOA): 1 mM formic acid and 1 mM oxalic acid
A generally greater degree of sorption was achieved for PTE that had leached from a mineral soil layer
Summary
Contamination of the environment with potentially toxic elements (PTE) is a persistent threat in the industrialized world. Metals dissolved in the extract solution represent an environmental hazard, and metals left in the soil following leaching are likely to be present in chemically stable mineral forms and bound to non-labile fractions [3]. Bases and complexing agents are rarely applied because of the difficulties of treating the wastewater [4]. Strong mineral acids such as HCl, and HNO3 can be used to leach PTE from soil, these strong acids can compromise soil structure. Many studies have focused on the use of amendments to immobilize and reduce availability of PTE through their incorporation into the soil. Amendments can be mixed into the contaminated soil or into a layer that forms a barrier between the pollution source and the environmental receptor. Studies have focused on using a range of materials for this purpose, including simple amendments such as sawdust [6],[7],[8] and brewers’ draff [9], and more comparatively hi-tech materials such as recycled activated alumina and recycled collagen fiber [10]
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