Rare earth elements (REEs): geochemical patterns and contamination aspects in Brazilian benchmark soils.

Rare earth elements have been increasingly used in modern societies and soils are likely to be the final destination of several REE-containing (by)products. This study reports REE contents for topsoils (0-20 cm) of 175 locations in reference (n = 68) and cultivated (n = 107) areas in Brazil. Benchmark soil samples were selected accomplishing a variety of environmental conditions, aiming to: i) establishing natural background and anthropogenic concentrations for REE in soils; ii) assessing potential contamination of soils - via application of phosphate fertilizers - with REE; and, iii) predicting soil-REE contents using biomes, soil type, parent material, land use, sand content, and biomes-land use interaction as forecaster variables through generalized least squares multiple regression. Our hypotheses were that the variability of soil-REE contents is influenced by parent material, pedogenic processes, land use, and biomes, as well as that cultivated soils may have been potentially contaminated with REE via input of phosphate fertilizers. The semi-total concentrations of REE were assessed by inductively coupled plasma mass spectrometry (ICP-MS) succeeding a microwave-assisted aqua regia digestion. Analytical procedures followed a rigorous QA/QC protocol. Soil physicochemical composition and total oxides were also determined. Natural background and anthropogenic concentrations for REE were established statistically from the dataset by the median plus two median absolute deviations method. Contamination aspects were assessed by REE-normalized patterns, REE fractionation indices, and Ce and Eu anomalies ratios, as well as enrichment factors. The results indicate that differences in the amounts of REE in cultivated soils can be attributed to land use and agricultural sources (e.g., phosphate-fertilizer inputs), while those in reference soils can be attributed to parent materials, biomes, and pedogenic processes. The biomes, land use, and sand content helped to predict concentrations of light REE in Brazilian soils, with parent material being also of special relevance to predict heavy REE contents in particular.

Hydrothermally-altered feldspar as an environmentally-friendly technology to promote heavy metals immobilization: Batch studies and application in smelting-affected soils.

Hydrothermally-altered feldspar (HydroPotash, HYP) possesses, among other physicochemical properties, high pH buffering and cation exchange capacity. Therefore, it may potentially remove heavy metals from aqueous solutions and immobilize these metals in contaminated soil. This study aimed to evaluate the capabilities of two types of HydroPotash (HYP-1 and HYP-2) and a zeolite sample (a commercial adsorbent) for immobilizing cadmium (Cd), zinc (Zn), and lead (Pb) from both aqueous solution and contaminated soils from a Zn-smelting area (classified as soilhigh, soilintermediate, and soillow based on their level of soluble metal concentration). Sorption studies in natural suspension pH showed that HYPs removed 63.8-99.9% Zn, 20.6-40.7% Cd, and 68.4-99.7% Pb from aqueous solution. In the batch test with controlled pH (at pH 5.5), HYPs sorbed more Cd than zeolite. Analyses of scanning electron microscopy-energy dispersive X-ray spectroscopy after desorption showed the presence of Pb at HYP-2, indicating that this metal was effectively adsorbed. In soilhigh HYPs immobilized 99.9% of Zn, Cd, and Pb after one week of soil incubation with these products. The HYPs immobilization effect persisted up to 84 days of soil incubation with these products. The increased soil pH promoted by HYPs appears to be the main factor controlling metal sorption. In conclusion, HydroPotash can be used as an adsorbent/amendment to effectively immobilize heavy metals in both water and contaminated soils by precipitation and adsorption. Our findings indicate the high potential of this material for Cd, Zn, and Pb stabilization, which is of great relevance when recovering areas affected by mining/smelting activities with multi-element contamination.

Interactions of cadmium and zinc in high zinc tolerant native species Andropogon gayanus cultivated in hydroponics: growth endpoints, metal bioaccumulation, and ultrastructural analysis.

Cadmium (Cd) and zinc (Zn) toxicity causes physiological disorders and harms plants, interfering with the rehabilitation of areas affected by mining activities. This study evaluated how the exposure to Zn and/or Cd affects the growth of native andropogon grass (Andropogon gayanus Kunth) plants originally found in areas contaminated with Cd and/or Zn due to zinc mining activities. Plants were cultivated for 7 weeks in a nutrient solution treated with Zn (142.3-854.0 µM) or Cd (0.9-13.3 µM) separately or combined with a molar ratio of 64:1 (Zn:Cd). A control treatment was grown in a complete Hoagland and Arnon solution (without Cd). Plant height, stem diameter, internode length, dry weight, Cd and Zn concentration, and accumulation in shoots/roots, as well as ultrastructure of roots and leaves were analyzed at the end of the experiment. The root dry weight was not significantly affected by the addition of the metals. Moreover, Zn provided higher shoot dry weight (up to 160%) relative to control. Andropogon grass tolerated both metals better separately than when applied together. Transmission electron microscopy analyses showed modifications such as vesiculation and vacuolation in the ultrastructure of andropogon tissues by Cd and/or Zn. The andropogon grass was tolerant to the doses tested, evidencing that it has potential for recovering areas contaminated with Zn and/or Cd.

Land use intensification in the humid tropics increased both alpha and beta diversity of soil bacteria.

Anthropogenic pressures on tropical forests are rapidly intensifying, but our understanding of their implications for biological diversity is still very limited, especially with regard to soil biota, and in particular soil bacterial communities. Here we evaluated bacterial community composition and diversity across a gradient of land use intensity in the eastern Amazon from undisturbed primary forest, through primary forests varyingly disturbed by fire, regenerating secondary forest, pasture, and mechanized agriculture. Soil bacteria were assessed by paired-end Illumina sequencing of 16S rRNA gene fragments (V4 region). The resulting sequences were clustered into operational taxonomic units (OTU) at a 97% similarity threshold. Land use intensification increased the observed bacterial diversity (both OTU richness and community heterogeneity across space) and this effect was strongly associated with changes in soil pH. Moreover, land use intensification and subsequent changes in soil fertility, especially pH, altered the bacterial community composition, with pastures and areas of mechanized agriculture displaying the most contrasting communities in relation to undisturbed primary forest. Together, these results indicate that tropical forest conversion impacts soil bacteria not through loss of diversity, as previously thought, but mainly by imposing marked shifts on bacterial community composition, with unknown yet potentially important implications for ecological functions and services performed by these communities.