Diego M. Prieto

Arsenate Retention by Epipsammic Biofilms Developed on Streambed Sediments: Influence of Phosphate

Natural geological conditions together with the impact of human activities could produce environmental problems due to high As concentrations. The aim of this study was to assess the role of epipsammic biofilm-sediment systems onto As (V) sorption and to evaluate the effect of the presence of equimolar P concentrations on As retention. A natural biofilm was grown on sediment samples in the laboratory, using river water as nutrient supplier. Sorption experiments with initial As concentrations 0, 5, 25, 50, 100, 250, and 500 μg L−1 were performed. The average percentage of As sorbed was 78.9 ± 3.5 and 96.9 ± 6.6% for the sediment and biofilm-sediment systems, respectively. Phosphate decreased by 25% the As sorption capactity in the sediment devoid of biofilm, whereas no significant effect was observed in the systems with biofilm. Freundlich, Sips, and Toth models were the best to describe experimental data. The maximum As sorption capacity of the sediment and biofilm-sediment systems was, respectively, 6.6 and 6.8 μg g−1 and 4.5 and 7.8 μg g−1 in the presence of P. In conclusion, epipsammic biofilms play an important role in the environmental quality of river systems, increasing As retention by the system, especially in environments where both As and P occur simultaneously.

Arsenic Transfer from As-Rich Sediments to River Water in the Presence of Biofilms

The influence of epipsammic biofilms on As release from river sediments was evaluated in a microcosm experiment where biofilms were grown on sediments containing 106 mg kg−1 As, collected in the Anllons River, and compared with control systems without biofilms. The As transfer to the water column was low (<0.11% of total As in the sediment) and was further reduced by 64% in the presence of biofilms. was the predominant species in the overlying water in both systems. concentration was higher (up to 12% of total dissolved As) in the control systems than in the systems with biofilms, where this species was almost absent. This fact is of toxicological relevance due to the usually higher mobility and toxicity of the reduced species. Control systems exhibited higher As mobility in water, in sulphate solution, and in weak acid medium and higher bioavailability in diffusive gradient in thin films (DGT) devices. Arsenic retained by the biofilm was equally distributed between extracellular and intracellular compartments. Inside the cells, significant concentrations of , monomethylarsonic acid (), and dimethylarsinic acid () were detected, suggesting that active methylation (detoxification) processes are occurring in the intracellular compartment.

Reutilization of granite powder as a component of permeable reactive barriers for the treatment of Cr(VI)-contaminated waters .

Las barreras permeables reactivas (BPRs) son sistemas eficaces y de bajo coste para la descontaminacion de aguas subterraneas. Diversos materiales han sido ensayados como sustratos reactivos en la construccion de BPR’s, siendo de interes la utilizacion de materiales residuales cuando cumplen unos requisitos de permeabilidad y reactividad. En este trabajo se ha evaluado la utilizacion para este fin de serrines procedentes del proceso de corte del granito, mezclados con compost, como componentes de relleno de BPRs. Para ello, se estudio la capacidad de adsorcion de Cr(VI) de serrines graniticos, comparandola con la de compost de corteza de pino, compost de residuos solidos urbanos y mezclas en diferentes proporciones de los serrines y compost. Los serrines graniticos, empleados sin mezcla con otros componentes reactivos, presentan como inconveniente para su uso como relleno de BPRs su moderada permeabilidad y capacidad de adsorcion de Cr(VI). La adicion de compost de corteza de pino compostada mejoro el comportamiento de los serrines, incrementando su conductividad hidraulica, al tiempo que aumento la capacidad de adsorcion de Cr(VI) y disminuyo su desorcion. Por el contrario, el compost derivado de residuos solidos urbanos no tuvo estos efectos beneficiosos, ya que redujo la permeabilidad y solo afecto ligeramente a la capacidad de adsorcion y desorcion de Cr. En resumen, las mezclas de serrines graniticos y compost de corteza de pino al 50 o 75% (v/v) de compost fueron las mas adecuadas para su utilizacion como BPRs desde el punto de vista de su efectividad y coste.

A microcosm study of permeable reactive barriers filled with granite powder and compost for the treatment of water contaminated with Cr (VI)

The permeable reactive barrier (PRB) is a technology developed for the removal of contaminants in groundwater. It consists of a screen perpendicular to the flow of contaminated groundwater filled with a material capable of adsorbing, precipitating or degrading pollutants. Several materials have been tested for their use as reactive substrates for the construction of PRBs. Waste materials are of particular interest for this purpose due to the possibility of their reuse and their generally lower cost. With this aim, the Cr (VI) retention capacity of filler material consisting either of pine bark compost (PB) or a 50% mixture of compost and granite powder (PB50) was evaluated using an experimental device specifically designed for this study, which reproduces a permeable reactive barrier at the laboratory scale. Percolation experiments were carried out with a solution of 100 mg L-1 Cr (VI) in 0.01M KNO3, followed by a leaching step with the saline background. The results show that compost is a highly efficient filler for permeable reactive barriers with almost 100% retention of Cr, whereas the retention efficiency of the mixture of PB50 oscillated between 18 and 46% during the experiment. The Cr retained by the filling material is strongly fixed, since no desorption was detected by leaching with the saline background, and concentrations in the standard Toxic Characteristic Leaching Procedure (TCLP) extracts were lower than 1 mg L-1. This behaviour minimizes the risk of release of the Cr retained by the material of the barrier in the event of it being traversed by water not contaminated with Cr. Modelling with Visual Minteq indicates that in the experiments with PB, the reduction of Cr (VI) to Cr (III) occurs and that Cr (III) is associated with dissolved organic matter, which is a form of lower toxicity than the initial Cr (VI) species. In turn, in the experiments with PB50, Cr (III) and Cr (VI) coexist and the oxidised form is not associated with dissolved organic matter, which suggests greater toxicity. The results indicate that pine bark compost is a potential candidate for use as filler material permeable reactive barriers.

Fe and Al leaching in soils under laboratory-controlled burns

The influence of the severity of soil heating on aluminium and iron leaching was analysed, as well as its relationship with the solubility of organic matter. For this purpose, laboratory experiments were carried out under microcosm conditions where unaltered soil blocks were submitted to thermal shock and subsequently to rainfall simulation (300 mm), collecting the surface (0 cm) and subsurface runoff (12 cm). Soil heating was carried out using infrared lamps reaching temperatures of 222 and 401 oC at a depth of 1 cm, similar to those reached in moderate and high intensity fires respectively. The analysis of the drainage waters showed that the leaching of Al increased with the severity of the thermal shock, whereas for Fe perceptible effects were only seen at the highest temperature. Geochemical modelling with Visual MINTEQ indicated that Al and Fe were fundamentally mobilised and associated with dissolved organic matter. Weak interactions (electrostatic binding) predominated in the case of Al and strong interactions (bidentate complexes with dissolved organic matter) in the case of Fe. Only towards the end of the leaching experiment, when the concentration of dissolved organic carbon decreased, was there a perceptible mobility of Al and Fe in the inorganic form, in the subsurface leachate of the moderate temperature treatment.