Heavy-metal pollution of soils has become a major environmental concern around the world presently. Soil washing provides an effective measure of removing contaminants from soil permanently, of which washing reagent plays a vital role in the process. This article reviews the current knowledge acquired on the main aspects concerning washing reagents of soil washing for remediation of heavy-metal-contaminated soil based on more than 150 published studies over a period from 1990 to 2021. The review identifies and discusses the types of washing reagents with their associated characteristics. Based on the factors influencing washing remediation, multi-criteria decision-analysis, together with an integrated four-step procedure, is put forward to manage the selection and prioritizing of washing reagents. It crops out from the literature survey that the selection of the washing reagents is a balanced process by considering washing effect, environmental impact, and cost-effectiveness. On the basis of such observation and evaluation, it is recommended that further study should be focused on developing new washing reagents or compound washing reagents that possess the advantage over heavy-metal removal ability, eco-friendliness, and cost-effectiveness as well. Selecting washing agents that are capable of being recovered from waste such as dissolved organic matter would be a promising trend in washing remediation. Artificial intelligence is expected to assist in the selection of washing reagents.
The Baishan Mo deposit in the eastern Tianshan Mountains is representative of the local metallogenic Triassic Mo belt. In this study, the geological characteristics, petrography, and geochemistry of the granite porphyry and the Mo deposit were examined using zircon U-Pb dating, molybdenite Re-Os isotope dating, whole-rock geochemistry, and fluid inclusions. Results show that the crystallisation age of the Baishan granite porphyry is 229.8 ± 1.4 Ma and the Re-Os age of molybdenite in the ore body quartz vein is 225.0 ± 1.2 Ma. Therefore, the ore formed basically at the same time as diagenesis in the Late Triassic. The Baishan granite porphyry has high levels of Si, Al, and alkalis (K 2 O + Na 2 O = 5.72-8.88%) and low levels of Mg.The differentiation index is high (81.68-91.20), while the aluminium saturation index is low, indicating that the pluton belongs to metaluminous-weak peraluminous highly fractionated I-type granite. The rock is also enriched in light rare earth elements (LREE) and large-ion lithophile elements but depleted in heavy rare earth elements (HREE) and high-field-strength elements, with a distinct differentiation between LREEs and HREEs. δEu is weakly negative. The high Sr/Y and La/Yb ratios indicate geochemical characteristics similar to adakite rocks. The formation of the deposit is closely related to Mo-rich, low-salinity (1.23-to 9.73-wt.% NaCl eqv.), and mediumto low-temperature (91-280°C) magmatic fluid in the plate extension setting. The magma was derived from partial melting of thickened lower crust and possibly mixed with intrusive mantle-derived magma. The ore-forming fluids were uplifted, transported, mixed with meteoric water, and precipitated to form the Mo ore.
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