Mobility and phyto-availability of arsenic in soil-plant system and decontamination techniques of arsenic polluted areas
Caporale, Antonio Giandonato (2011) Mobility and phyto-availability of arsenic in soil-plant system and decontamination techniques of arsenic polluted areas. [Tesi di dottorato] (Inedito)
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Contamination of terrestrial and aquatic ecosystems by arsenic is a very sensitive environmental issue due to its adverse impact on human health. Urgent action must be taken to reduce this impact by providing access to safe water as a basic human right. In the present work, researches have been carried out on novel arsenic sorbents at low cost, that can be easily synthesized, or even representing by-products from production processes and thus available for free or at very low price. In particular, this work describes the sorption of arsenate on Al-Mg and Fe-Mg layered double hydroxides as affected by pH and varying concentrations of inorganic and organic ligands, the effect of residence time on the arsenate desorption by ligands and the kinetics of arsenate desorption by phosphate. It was also studied the arsenate sorption by Fe- and Al-based drinking-Water Treatment Residual samples (by-products coming from drinking-water treatment plants) as a function of WTRs particles size at different initial As concentrations and solid:solution ratios (SSRs). The Fe-Mg-LDH sorbed nearly twice the amount of arsenate compared to the Al-Mg-LDH, due to its greater surface area and lower degree of crystallinity. Moreover, the Fe-Mg-LDH sorbed more arsenate than phosphate, in contrast to the Al-Mg-LDH, which adsorbed more phosphate than arsenate, probably because of the greater affinity of arsenate than phosphate for Fe sites and, vice versa, the greater affinity of phosphate than arsenate for Al sites. The capacity of ligands to inhibit the fixation of arsenate followed the sequence: nitrate < nitrite < sulphate < selenite < tartrate < oxalate << phosphate on Al-Mg-LDH and nitrate < sulphate ≈ nitrite < tartrate < oxalate < selenite << phosphate on Fe-Mg-LDH. The inhibition of arsenate sorption increased by increasing the initial ligand concentration. The longer the arsenate residence time on the LDH surfaces the less effective the competing ligands were in desorbing arsenate from sorbents. A greater percentage of arsenate was removed by phosphate from Al-Mg-LDH than from Fe-Mg-LDH during kinetics of arsenate desorption. Both WTR samples show a high affinity for arsenate. Anyway, the Al-WTR samples, characterized by a higher surface area, were able to sorb much greater amounts of arsenate than the Fe-WTR. The greater the SSR, the higher the amounts of arsenate sorbed on both WTR samples. The influence of Fe-WTR particles size on the arsenate sorption capacity was greatly pronounced when compared to that of the Al-WTRs. The smallest Fe-WTR particles were able to sorb much more arsenate than the bigger ones, whereas, surprisingly, the biggest Al-WTR particles showed the best arsenate sorption capacity with respect to that of smaller Al-WTR particles. The presence of As in soils and/or groundwaters used for agricultural purposes, causes a strong abiotic stress to the cultivated plants, which results in the reduction of biomasses and yields, and the abundance of non-tradable products. It is therefore desirable to identify and develop production techniques capable of limiting the mobility and phyto-availability of As in soil, through the stabilization of the metalloid on the more recalcitrant soil fractions. In the present work it was carried out an experiment on the bean (Phaseolus vulgaris L.), irrigated with different solutions containing arsenite and grown in a As-uncontaminated soil amended by increasing amounts of stabilized compost. The aims of this experiment were to: i) study the influence of the compost application on the mobility and phyto-availability of As in soil; ii) study the influence of the compost on the growth of the bean plants and their uptake of As from contaminated systems. Bean plants growth was significantly affected by As and compost treatments. The higher the As concentration in the irrigation water, the lower was the plants biomass, as a consequence of the phytotoxic effect of As, whereas a higher application of the compost corresponded to a higher plant biomass, indicating the ability of the compost to alleviate the As phytotoxicity. In all treatments, arsenic concentration in roots was higher than in shoots and bean,. Moreover, the compost application reduced the As concentration in all tissues of the amended plants compared to those non-amended. A low As allocation in bean is definitely desirable, because a high content of As in the edible part of the plant could cause contamination of the human food-chain. The concentration of the free-fraction of As in soil decreased significantly by increasing the level of compost application, whereas the higher the compost application the higher was the concentration of specifically sorbed As by soil colloidal particles.
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