Maize straw application reduced cadmium and increased arsenic uptake in wheat and enhanced the rhizospheric bacterial communities in alkaline-contaminated soil

Many fields where wheat is grown in northern China are co-polluted by arsenic (As) and cadmium (Cd). Thus, remediation of As and Cd-contaminated alkaline soils is crucial for safe wheat production. In this study, a pot experiment was carried out to investigate the impact of 1% and 2% maize straw (MS) incorporation on As and Cd bioavailability, binding forms, uptake by winter wheat (Triticum aestivum L.), and bacterial communities in smelter (SS) and irrigation (IS) alkaline contaminated soils. The results indicated that 2% MS incorporation significantly (p < 0.05) increased bioavailable-As by 37% (SS) and 39% (IS) with no significant change in the bioavailable-Cd in SS2% (31.95%) from 31.95% (SSCK) and IS2% (33.33%) from 32.82% (ISCK). Incorporation of 2% MS increased the grain As concentration from 0.22 mg kg-1 (SSCK) to 0.51 mg kg-1 (SS2%) and from 0.59 mg kg-1 (ISCK) to 0.84 mg kg-1 (IS2%) which is above the acceptable standard of 0.5 mg kg-1 (GB2726-2017). In contrast, the Cd content in grains was maintained at 0.09 (SS1%), 0.04 (SS2%) and 0.03 (IS1%), 0.02 (IS2%) below the acceptable standard of 0.10 mg kg-1 (GB2762-2017). The amendment through dissolved organic carbon mediated As desorption enhanced As transfer to wheat grain, decreasing DTPA-Cd in the soils and its consequent translocation to wheat leaves and grain. The 2% MS incorporation increased the active As fractions, reduced mobile Cd into immobile fractions, and promoted the abundance of Actinobacteria, Bacteroidetes, and Firmicutes in the two soils. These attributes of MS in decreasing the accumulation of Cd in wheat leaves and grains signified its potential as a suitable ingredient for Cd sequestration and food safety in Cd-contaminated soils.

Arsenic contamination in abandoned and active gold mine spoils in Ghana: Geochemical fractionation, speciation, and assessment of the potential human health risk

This work aims to study the pseudo-total content, geochemical fractions, and species of arsenic (As) in the bulk soil and in the coarse and fine particles of top soil and soil profiles collected from active and abandoned gold mine spoils in Ghana. The human health risk for adults (male and female) and children has been assessed. To achieve our aims, we collected 51 samples, characterized them, determined the total As content, and sequentially extracted the geochemical fractions of As including water-soluble and un-specifically bound As (FI); specific-sorbed/exchangeable As (FII); poorly (FIII)- and well-crystalline (IV) Fe oxide; and residual/sulphide fraction (FV). In selected samples, As species were determined using synchrotron-based X-ray absorption near edge structure (XANES). Pseudo-total As contents varied from 1807 to 8400 mg kg^-1, with the extremes occurring at the abandoned mine spoil. Arsenic was almost 10-fold higher in the fine particles (<0.63 μm) than in the coarse particles. Arsenic was mainly associated with FIII and FV, indicating that the distribution of As in these spoils is governed by their contents of amorphous Fe oxides, sulphides and As bearing minerals. The XANES results indicated that scorodite (FeAsSO₄ = 65-76%) and arsenopyrite (FeAsS = 24-35%) are the two major As-containing minerals in the spoils. The potential mobility (PMF = ∑FI-FIV) of As in the fine particles of the top soil was higher (48-61%) than in the coarse particles (25-44%). The mobile fraction (MF) (FI+FII) and PMF of As in the coarse particles of the profiles increased with depth while it decreased in the fine particles. The median hazard index values indicated an elevated human health risk, especially for children. The high contamination degree and potential mobility of As at the studied mine spoils indicate high potential risk for human and environmental health.

Trace metal behavior during in-situ iron removal tests in Leuven, Belgium

Subsurface iron removal (SIR) is an in-situ technique to lower the iron content of extracted groundwater. Through cyclic injection of oxygenated water ferrous iron oxidises and precipitates as iron hydroxide in a zone surrounding the extraction well, enhancing the sorptive capacity of the aquifer. During subsequent pumping phases, groundwater traverses the oxidation zone and ferrous iron sorbs to available and newly formed exchange and sorption sites, thereby retarding the breakthrough of dissolved iron. The process is well-understood in regards to the retardation of iron. Less well understood, however, is the behavior of a number of trace metals and metalloids during SIR operations, foremost arsenic (As). In this study, we analyse major and minor ion and trace metal concentrations from a number of SIR tests in a sand aquifer near Leuven, Belgium. We use reactive transport modelling to evaluate conceptual models of trace metal release and arrest. The test data, underpinned by model results, show that metal release, namely arsenic and barium (Ba), occurs through the oxidation of trace amounts of sulphide minerals during the injection phase. Sorption through cation exchange retards Ba while complexation lowers dissolved As concentrations. Arsenic is mobilized again during the pumping phase through varying phosphate concentrations in the native groundwater, despite available sorption surfaces, while Ba remains adsorbed. Concentrations, however, remain below WHO guideline values for As and Ba (10 μg/l and 0.7 mg/l), respectively. The developed conceptual model of As fate reveals a high propensity for As mobility during SIR due to desorption reactions and delivers an explanation as to why many SIR operations fail to show substantial As removal, despite efficient iron removal. Other monitored trace elements showed no mobilisation, including Zn, Al, Cd, Cr, Cu, F, Hg, Ni, Pb, Sb and Se.