Geochemistry signatures of mercury in soils of the Amazon rainforest biome.

Mercury (Hg) toxicity in soils depends on Hg species and other physical and chemical attributes, as selenium (Se) hotspots in soils, particularly relevant in Amazonian soils. The study of Hg species and their relations in representative locations of the Amazon rainforest biome is critical for assessing the potential risks of Hg in this environment. This work aimed to determine the concentration of total Hg and its species (Hg0, Hg22+ and Hg2+), and to correlate Hgtotal concentration with total elemental composition, magnetic susceptibility, and physicochemical attributes of Amazon soils. Nine sites in the Amazon rainforest biome, Brazil, were selected and analyzed for their chemical, physical, and mineralogical attributes. The clay fraction of the studied Amazon soils is dominated by kaolinite, goethite, hematite, gibbsite, and quartz. Mica was also found in soils from the States of Acre and Amazonas. Hgtotal ranged from 21.5 to 208 µg kg-1 (median = 104 µg kg-1), and the concentrations did not exceed the threshold value established for Brazilian soils (500 µg kg-1). The Hg2+ was notably the predominant species. Its occurrence and concentration were correlated with the landscape position and soil attributes. Hgtotal was moderately and positively correlated with TiO2, clay, and Se. The findings showed that geographic location, geological formation, and pedological differences influence the heterogeneity and distribution of Hgtotal in the studied soil classes. Thus, a detailed characterization and knowledgment of the soil classes is very important to clarify the complex behavior of this metal in the Amazon rainforest biome.

Steel mill waste application in soil: dynamics of potentially toxic elements in rice and health risk perspectives.

Potentially toxic elements (PTEs) are of great concern in steel mill wastes. Therefore, in order to use them as potential fertilizers in soil, risk assessments are needed. Three steel mill wastes were tested as possible amendments for soils at seven different doses (0, 0.5, 1, 2, 4, 8, 16 t ha-1): phosphate mud (PM), metallurgical press residue (MPR), and filter press mud (FPM) during rice cultivation in a pot experiment in a Haplic Gleisol. Analysis on rice tissues, including roots, shoots, husk, and grains, were conducted and contents of Cu, Cd, Ni, Zn, Mn, and Pb were assessed. Translocation and bioaccumulation factors were calculated for each element. In general, PTEs are more accumulated in roots and greater contents of Zn and Mn were found, while the lowest ones were found for Pb, probably due to its lack of functional roles during plants development. Higher translocation was observed for Mn, which is associated to the redox conditions of rice cultivation and the high mobility of this element under this condition. Application of steel mill wastes can increase PTE bioavailability and translocation factors, especially PM, but all of the wastes reveal a high hazard index.

Steel mill waste effects on rice growth: comparison of chemical extractants on lead and zinc availability.

Zinc deficiency is widespread in cultivated soils, limiting the grain crop production and the adequate human nutrition. Several wastes from metallurgical activity can be used as Zn source, but these materials generally also have other potentially toxic elements, such as Pb, that can be highly toxic for plants and humans. This study aimed to evaluate the efficiency of five chemical extractors (water, citric acid, DTPA, Mehlich 1, and USEPA 3051A) in better correlating with the bioavailable contents of Zn and Pb in soils treated with steel mill wastes (metallurgic press residue (MPR), filter press mud (FPM), and phosphate mud (PM)). Rice plants were cultivated in pots with 4 kg of a Haplic Eutrophic Gleisol and steel mill wastes were applied in soil at increasing doses (0, 0.5, 1, 2, 4, 8, and 16 t ha-1). The availability of the potentially toxic elements Zn and Pb was assessed as total contents in rice shoots, grains, husks, and roots. The results showed that the USEPA 3051A method extracted greater contents of Zn and Pb from soil compared with other extractants. Due to their greater natural Pb and Zn contents, MPR and PM promoted higher contents of these elements in soils, respectively. Doses of PM influenced Zn contents in grains. After adding 16 t ha-1 of PM, Zn content in rice grains was 0.1 mg kg-1. However, at doses 0.5, 1, 2, and 4 t ha-1, the average concentration of Zn in the grains was 40 mg kg-1. The wastes MPR and FPM at 16 t ha-1 promoted Zn concentration in grains of 42 and 45 mg kg-1, respectively. The greatest contents of Pb in grains were found after addition of FPM at doses 0.5, 1, and 2 t ha-1: 6.67, 4.96, and 0.45 mg kg-1, respectively, and above 4 t ha-1 (4, 8, and 16 t ha-1); Pb content in grains was less than 0.3 mg kg-1. The content of Pb in roots at 16 t ha-1 of PM, MPR, and FPM was 18, 25, and 155 mg kg-1, respectively, and for Zn, under the same conditions, 100, 255, and 813 mg kg-1 for MPR, FPM, and PM, respectively. USEPA 3051A can be used to assess Pb and Zn available contents, and positive correlations with bioavailable contents of these elements in roots prove its feasibility. Further studies are necessary to state the safety of using steel mill application, including the use of other crop species, but PM is a promising waste for soil Zn fertilization.

Comparison of bioaccessibility methods in spiked and field Hg-contaminated soils.

Estimating bioaccessible content of mercury in soils is essential in evaluating risks that contaminated soils pose. In this study, soil samples spiked with HgCl2 through adsorption were used to test the effects of liming, soil organic matter, soil depth, and Hg concentration on the following bioaccessibility tests: dilute nitric acid at room temperature, dilute nitric acid at body temperature, Simplified Bioaccessibility Extraction Test (SBET) method, and gastric phase of the In vitro Gastrointestinal (IVG) protocol. Soil and sediment samples from Descoberto, Minas Gerais (Brazil), a city with a well-known record of Hg contamination from artisanal mining, were subjected to these bioaccessibility tests for the first time, and the different methods of estimating bioaccessible content were compared. Bioaccessible fractions in spiked samples ranged from 10% to 60%, and this high bioaccessibility was due to the highly soluble species of Hg and the short time under adsorption. In general, clay and organic matter decreased bioaccessible content. Although the soil in Descoberto is undoubtedly polluted, mercury bioaccessibility in that area is low. In general, dilute nitric acid estimated higher bioaccessible content in soil samples, whereas the SBET method estimated higher bioaccessible content in sediment samples. In multivariate analysis, two groups of bioaccessibility tests arise: one with the two nitric acid tests, and the other with SBET and the gastric phase of the IVG protocol. The addition of pepsin and glycine in the last two tests suggests a more reliable test for assessing mercury bioaccessibility.

Mercury fractionation in tropical soils: A critical point of view.

Problems related to specificity and re-precipitation of metals in sequential chemical extractions can impair their routine use. In order to test the efficiency of a sequential chemical procedure, model compounds composed by soil components commonly found in tropical soils such as goethite, Al-goethite, ferrihydrite, hematite, bauxite, and humic acid were incubated with either Hg(NO3)2 or HgSO4 and submitted to chemical extraction. The procedure aims to assess: (i) water soluble Hg; (ii) bioaccessible Hg at pH near human stomach; (iii) Hg associated with organic matter; (iv) reduced Hg; (v) Hg associated with Fe, Al, and Mn oxides; and, (vi) residual Hg. This procedure was also tested via single and sequential extractions using the surface and subsurface samples of two tropical soils, i.e., a Rhodic Acrudox and a Typic Hapludox, with and without lime application. Soil samples were submitted to an adsorption experiment with HgCl2 and a high adsorption percentage was observed. The majority of Hg at both single and sequential procedure was extracted by an acetic acid solution (pH = 2). Liming, soil depth, and soil type were not determinative on Hg extractability. The sequential extraction applied showed a lack specificity of Hg fractions, confirmed by the model components.

Toxic effects of lead in plants grown in brazilian soils.

Lead (Pb) in soils can be transferred to plants, animals, and even humans. The toxicity of Pb is worrisome and therefore environmental quality criteria, established by laws to support the management of contaminated sites, have been developed to prevent its deleterious effects in a wide range of soils, uses, and occupations. In Brazil, the CONAMA Resolution 420/2009 established that Brazilian states may define their prevention values (PV) for metals in soils. However, the established values should be well studied, since a wide variation of sensitivity of species exposed to Pb is reported and several have a high tolerance. We aimed to evaluate Pb toxicity to validate the suitability of the current Brazilian Pb-prevention value. A trial was carried with two plant species (sorghum and soybean) grown in two tropical soils (Typic Hapludox and Rhodic Acrudox), following ISO 11.269-2 protocols (ISO 2012). The tested soils were contaminated with Pb-acetate at the following concentrations: 0, 200, 400, 800, 1200, 1600, 2200, 2800, and 3200 mg kg-1 of dry soil. Differences regarding species sensitivity were observed and sorghum seemed to be less sensitive to Pb concentration in soils. Soil characteristics as higher clay and organic matter content were responsible for decreasing the overall availability of Pb for plants. Using data from this study and from the literature, we constructed a species sensitivity distribution curve and calculated the HC5 (hazardous concentration to 5% of variables evaluated). The HC5 was 132.5 mg kg-1, which suggests that the PV currently used in Brazil (72 mg kg-1) is sufficiently protective for Brazilian soils.

Evaluation of mercury phytoavailability in Oxisols.

Mercury is a metal which is potentially toxic for the environment. Many factors control its retention in the soil, such as cation exchange capacity, pH, clay content, organic matter, and redox potential. It is important to know the phytotoxic effects of soil Hg to prevent environmental contamination and its entry into the food chain. Several analytical methods are used to measure metal phytoavailability in soils, but none has been reported for Hg in Oxisols, the most common soil class in Brazil and a very important soil class throughout the tropics. The aim of this study was to select the chemical extractor that best correlated the Hg levels in plants and the Oxisols. The soils used were classified as Dystrophic Red-Yellow Oxisol (LVAd) and Dystroferric Red Oxisol (LVdf), which were collected in the 0-0.2-m soil layer. The species selected for cultivation were a monocotyledon, oat (Avena sativa L. cv. São Carlos) and a eudicotyledon, common bean (Phaseolus vulgaris L. cv. Madrepérola). Each test plot was composed of a 500 cm3 pot filled with soil samples contaminated with HgCl2. Treatments were arranged in a completely randomized design, with four replications. The experiment was conducted for 30 days. Mercury contents were separately extracted with the following extractors: USEPA 3051A, Mehlich-1, Mehlich-3, DTPA, and water. Mercury was determined by hydride generation atomic absorption spectroscopy. The extracted contents were correlated with the contents in the tissues of the plants' aerial part by the Pearson correlation. Although it is not considered a standard procedure to evaluate metal phytoavailable contents, the method that presented the best correlations between soil Hg and plant Hg was USEPA 3051A (r = 0.75*). As expected, the worst correlation was with water (r = 0.57* for common bean and r = 0,05ns for oat).

Effectiveness of Ferric, Ferrous, and Aluminum (Hydr)Oxide Coprecipitation to Treat Water Contaminated with Arsenate.

Coprecipitation of Fe and Al (hydr)oxides has been considered a low-cost process to remove As from wastewater. Arsenate is the most stable form of As in aerobic environments such as surface water, soils, and sediments and can be removed from water through methods based on this process. Iron/aluminum molar ratios of 100:0, 80:20, and 60:40 were used to treat water contaminated with As at concentrations of 50 and 500 mg L. Aluminum, ferrous, and ferric sulfates were used to coprecipitate Al and Fe (hydr)oxides at high pH. Maghemite, magnetite, lepidocrocite, and goethite were detected in precipitates from Fe(II), whereas hematite and ferrihydrite were identified in Fe(III) treatments. Segregation of Al (hydr)oxides as gibbsite and bayerite as well as the Al isomorphic substitution in Fe (hydr)oxides were detected in the presence of Al. The precipitates were classified as nonhazardous according to the leaching test based on Brazilian Technical Standard NBR 10005. The presence of Al increased the stability of the sludge from Fe(II) treatments but did not affect the stability of precipitates from Fe(III) treatments. High efficiencies for As removal from water were obtained for all treatments, but concentrations of soluble As were, in general, lower for Fe(III) treatments especially, in the absence of Al. Treatments were efficient in reaching the threshold to effluent discharge (0.5 mg L), but only treatments with initially 50 mg L of As reached the threshold for drinking water (10 µg L).

Arsenite removal from contaminated water by precipitation of aluminum, ferrous and ferric (hydr)oxides.

Several methods to remove arsenic from water have been considered, including co-precipitation with Fe and Al (hydr)oxides. Such compounds are considered very effective to remove As from contaminated water due to strong bindings between them. Three Fe:Al molar ratios (100:0, 80:20, and 60:40) were used to synthesize aluminum, ferrous, and ferric (hydr)oxides by precipitation in water highly contaminated with arsenite (50 and 500 mg L-1). The method was very efficient for all treatments (> 93%) at the beginning of the incubation period, excepted the one with 60:40 Fe(II):Al molar ratio at the higher As concentration (500 mg L-1) in which gibbsite was identified in precipitated phases. In spite of the high efficiency, however, the threshold for drinking water was not attained, mainly to the higher As concentration, even 84 days after precipitation. At this high concentration of arsenite, even the required threshold for effluent discharge was not attained in some treatments. The sludge resulting from treatments with higher As concentration were considered hazardous according to results from leaching test and corroborated by BCR extractions. Arsenic associated with Al and adsorbed phases were also assessed by extractions with NH4F and KH2PO4, respectively. In general, the presence of Al increased the efficiency as well as the stability of the sludge resulting from Fe (II) treatments, but did not affect Fe (III) treatments, which were more efficient for As removal.