Effect of citric acid on metals mobility in pruning wastes and biosolids compost and metals uptake in Atriplex halimus and Rosmarinus officinalis

Availability of copper in mine tailings with humic substance addition and uptake by Atriplex halimus

The chemical characteristics of mine tailings, organic amendments (doses), and plants are the critical factors that must be evaluated and monitored to ensure the sustainability of phytostabilization. The aim of this study was to evaluate the mobility of copper (Cu) in mine tailings (MT) of the Zone Central of Chile to which commercial humic substances were added, examining their effect on the uptake of Atriplex halimus. Two commercial humic substances (HS1 and HS2) extracted from leonardite (highly oxidized lignite), of different pH and total organic carbon, were evaluated by adsorption curve for Cu. In columns, soluble Cu, pH, and electrical conductivity in leachates were evaluated for MT, MT + HS1, and MT + HS2, and HS1 and HS2 in doses of 120 mg kg^-1. In pot assay, seeds were germinated directly in MT and cultivated for 140 days with the addition of HS2 in 120 and 240 mg kg^-1. Mine tailing presents high concentration of Cu (2016 ± 223 mg kg^-1, pH 6.3 ± 0.1). The results of sequential extraction indicate that Cu is associated with the sulfide fraction of low risk of mobility. The amount of Cu sorbed by HS1 was higher than that sorbed by HS2, and both humic substances showing better fit to the Freundlich than Langmuir model. Lixiviation of Cu was significantly lower in MT + HS1 (0.166 ± 0.043 mg kg^-1) and MT + HS2 (0.157 ± 0.018 mg kg^-1) than in MT (0.251 ± 0.052 mg kg^-1). Copper concentration in plants reached 185.8 ± 37.8 mg kg^-1 in the roots and 32.6 ± 7.4 mg kg^-1 in the aerial parts cultivated in MT without effect of the humic substance addition in Cu uptake nor growth. Copper concentrations in the aerial parts were adjusted to sufficient or normal levels in plant. A good management of mine tailings through phytostabilization could consider an adequate mixture of humic substances (to avoid leaching of metals) and an organic amendment that provides essential nutrients and increases biomass generation.

Chelate-assisted phytoextraction of lead using Fagopyrum esculentum: laboratory vs. field experiments

The development of more sustainable remediation techniques has been receiving greater attention, as an alternative to soil excavation plan in urban gardens. An in situ phytoextraction experiment with buckwheat (Fagopyrum esculentum) was performed with a 5 mmol kg^-1 citric acid (CA) application. Joint experiments under laboratory conditions were conducted using various cultivars of F. esculentum in two soils with a Pb contamination of either geogenic or anthropogenic origin and various chelate concentrations. Results show that a minimum dose of 50 mmol kg^-1 of CA is required to lower soil pH and raise the concentration of mobile Pb-CaCl₂ for both soils. Consequently, Pb shoot uptake is increased from 6.3 to 8.9 times depending on soil type. Phytoextraction efficiency is found to be 1.3 to 2.0 times higher in the anthropogenic contaminated soil than in the soil with geogenic Pb. A scale effect has also been identified since Pb root accumulation under laboratory conditions was 2.4 times higher than in the field experiment. Despite an increase in the Pb extraction rate with CA, buckwheat appears to lack the efficiency needed to remove Pb in moderately contaminated soils. The calculated remediation period would last 166 years to remove the mobile Pb fraction.

Combined effects of low-molecular-weight organic acids on mobilization of arsenic and lead from multi-contaminated soils

A batch experiment was conducted to examine the combined effects of three common low-molecular-weight organic acids (LMWOAs) on the mobilization of arsenic and lead in different types of multi-contaminated soils. The capacity of individual LMWOAs (at a same molar concentration) to mobilize soil-borne As and Pb varied significantly. The combination of the organic acids did not make a marked "additive" effect on the mobilization of the investigated three elements. An "antagonistic" effect on element mobilization was clear in the treatments involving oxalic acid for some soils. The acid strength of a LMWOA did not play an important role in controlling the mobilization of elements. While the mobilization of As and Pb was closely associated with the dissolution of soil-borne Fe, soil properties such as original soil pH, organic matter contents and the total amount of the element relative to the total Fe markedly complicated the mobility of that element. Aging led to continual consumption of proton introduced from addition of LMWOAs and consequently caused dramatic changes in solution-borne Fe, which in turn resulted in change in As and Pb in the soil solution though different elements behaved differently.

Phytostabilization of arsenic in soils with plants of the genus Atriplex established in situ in the Atacama Desert

In the ChiuChiu village (Atacama Desert, Chile), there is a high concentration of arsenic (As) in the soil due to natural causes related to the presence of volcanoes and geothermal activity. To compare the levels of As and the growth parameters among plants of the same genus, three species of plants were established in situ: Atriplex atacamensis (native of Chile), Atriplex halimus, and Atriplex nummularia. These soils have an As concentration of 131.2 ± 10.4 mg kg(-1), a pH of 8.6 ± 0.1, and an electrical conductivity of 7.06 ± 2.37 dS m(-1). Cuttings of Atriplex were transplanted and maintained for 5 months with periodic irrigation and without the addition of fertilizers. The sequential extraction of As indicated that the metalloid in these soils has a high bioavailability (38 %), which is attributed to the alkaline pH, low organic matter and Fe oxide content, and sandy texture. At day 90 of the assay, the As concentrations in the leaves of A. halimus (4.53 ± 1.14 mg kg(-1)) and A. nummularia (3.85 ± 0.64 mg kg(-1)) were significantly higher than that in A. atacamensis (2.46 ± 1.82 mg kg(-1)). However, the three species accumulated higher levels of As in their roots, indicating a phytostabilization capacity. At the end of the assay, A. halimus and A. nummularia generated 30 % more biomass than A. atacamensis without significant differences in the As levels in the leaves. Despite the difficult conditions in these soils, the establishment of plants of the genus Atriplex is a recommended strategy to generate a vegetative cover that prevents the metalloid from spreading in this arid area through the soil or by wind.

Effects of elemental sulphur on heavy metal uptake by plants growing on municipal sewage sludge

In this study experiment was carried out to determine the phytoextraction potential of six plant species (Conium maculatum, Brassica oleraceae var. oleraceae, Brassica juncea, Datura stramonium, Pelargonium hortorum and Conyza canadensis) grown in a sewage sludge medium amended with metal uptake promoters. The solubility of Cu, Cd and Pb was significantly increased with the application of elemental S due to decrease of pH. Faecal coliform number was markedly decreased by addition of elemental sulphur. The extraction of Cu, Cr and Pb from sewage sludge by using B. juncea plant was observed as 65%, 65% and 54% respectively that is statistically similar to EDTA as sulphur. The bioaccumulation factors were found higher (>1) in the plants tested for Cu and Pb like B. juncea. Translocation index (TI) calculated values for Cd and Pb were greater than one (>1) in both C. maculatum and B. oleraceae var. oleraceae. The results cleared that the amendment of sludge with elemental sulphur showed potential to solubilize heavy metals in phytoremediation as much as EDTA.

Atriplex atacamensis and Atriplex halimus resist As contamination in Pre-Andean soils (northern Chile)

The Pre-Andean area of Chile exhibits saline soils of volcanic origin naturally contaminated with arsenic (As), and we hypothesise that revegetation with resistant species may be a valid alternative for soil management in this area. Thus, the xerophytic and halophytic shrubs Atriplex halimus and Atriplex atacamensis were cultivated in containers for 90 days in Pre-Andean soil, As-soil, (111±19 mg As kg(-1), pH8.4±0.1) or control soil (12.7±1.1 mg As kg(-1), pH7.8±0.1) to evaluate As accumulation and resistance using stress bioindicators (chlorophylls, malondialdehyde (MDA) and total thiols). Sequential extraction of As-soil indicated that 52.3% of As was found in the most available fraction. The As distribution was significantly different between the species: A. halimus translocated the As to leaves, whilst A. atacamensis retained the As in roots. At 30 and 90 days, A. halimus showed similar As concentrations in the leaves (approximately 5.5 mg As kg(-1)), and As increased in stems and roots (up to 4.73 and 16.3 mg As kg(-1), respectively). In A. atacamensis, As concentration was lower (2.6 in leaves; 3.2 in stems and 6.9 in roots in mg As kg(-1)). Both species exhibited a high concentration of B in leaves (362-389 mg kg(-1)). If the plants are used for animal feed, it should be considered that A. halimus accumulates higher concentration of As and B in the leaves than A. atacamensis. Neither plant growth nor stress bioindicators were negatively affected by the high levels of available As, with the exception of MDA in the leaves of A. halimus. The results indicate that these plants resist contamination by arsenic, accumulating mainly the metalloid in the roots and can be recommended to generate plant cover in As-contaminated soils in the Pre-Andean region, under saline conditions controlled, preventing the dispersion of this metalloid via wind and leaching.