Influence of soil physicochemical properties,particle size fractions and mineralogy on the leaching potentials of arsenic and antimony in abandoned mine soils

At abandoned mine sites,arsenic(As)-and antimony(Sb)-enriched soils are often disposed of through onsite burial or capping.In highly weathered mine sites,the mobility of As and Sb is typically controlled by iron(Fe)(III)/Fe(II)phases;thus,the suitability of such disposal methods and appropriate testing techniques are questionable.In the present study,leaching potentials of As and Sb were examined using the toxicity characteristic leaching procedure(TCLP),waste extraction test(WET),and WET-extended procedure(WET-EXT)at three abandoned mine site soils in Australia.The leached concentration of As regularly exceeded USEPA criteria(5 mg L^(-1)).The highest leached concentrations of As and Sb were observed in the finest particle size fraction(<0.053 mm)by WET-EXT(1040 mg L^(-1)for As and 21.10 mg L^(-1)for Sb)followed by WET(800 mg L^(-1)for As and 20.90 mg L^(-1)for Sb).The TCLP method resulted in the lowest concentrations of leached As(0.0009 mg L^(-1))and Sb(0.0003 mg L^(-1)).Crystalline and amorphous As-bearing Fe oxides were the main phases in the soils studied.However,the best correlations of leached As determined by TCLP(0.832),WET(0.944),and WET-EXT(0.961)were found with the non-specifically sorbed(NS1)As fraction.The mineralogical and sequential extraction data clearly indicate the dominant role of Fe geochemistry in controlling leachability of As and Sb.The TCLP method was unlikely to be suitable for assessing leachability,as it exhibited no relationship with leachable Fe and substantially lower leached As and Sb than the other two methods.Given the high to extremely high leachable As and Sb concentrations,most of the soil samples would not be recommended for placement in capping works,old shafts,or reduction systems(e.g.,collection in drainage basins).

Pollution of hazardous substances in industrial construction and demolition wastes and their multi-path risk within an abandoned pesticide manufacturing plant

Exploration of heavy metals and organic pollutants, their leaching capacity along with health and environmental risks in contaminated industrial construction and demolition waste （ICDW） within a pesticide manufacturing plant were investigated. A maximum content of 90.8 mg·kg-1 Cd was found present in the wastes, which might originate from phosphorus rocks and industrial sulfuric acid used in pesticide production processes. An average concentration of 979.8 mg·kg-1 dichlorovos and other 11 organophosphorus pesticide were also detected. Relatively high leaching rates of around 4.14‰ were obtained from laboratory simulated ICDW using both glacial acetic acidsodium hydroxide and deionized water. Pesticide pollutants had the strongest tendency to retaining on dry bricks （leaching rate 1.68 9‰） compared to mortar-coatings, etc. due to their different physical characteristics and octanol-water partioning coefficient. Mobility of pesticide from on-site ICDW by water was spatially correlated to waste types, process sections and human activities, with a flux of leaching rate between 5.9‰ to 27.4%. Risk-based corrective action （RBCA） model was used to simulate the risk of contaminated ICDW debris randomly scattered. Oral and demlal ingestion amount by local workers was 9.8 × 10-3 and 1.9 × 10-2 mg.（kg-d）-1, respectively. Potential leaching risk to aquatic systems exceeded the limit for nearly 75% waste. Environmental and health risk exceedance was found in most ICDW, while the risk value of the most severely contaminated brick waste was 660 times beyond critical level. Implications for waste management involving construction and deconstruction work, waste transferring and regulation supplying were also provided.

Immobilization of lead in anthropogenic contaminated soils using phosphates with/without oxalic acid

Understanding the effects of oxalic acid（OA） on the immobilization of Pb（Ⅱ） in contaminated soils by phosphate materials, has considerable benefits for risk assessment and remediation strategies for the soil. A series of phosphate amendments with/without oxalic acid were applied to two anthropogenic contaminated soils. We investigated the immobilization of Pb（Ⅱ） by KH2PO4, phosphate rock（PR）, activated phosphate rock（APR） and synthetic hydroxyapatite（HAP） at different phosphate：Pb（P：Pb） molar ratios（0, 0.6, 2.0 and 4.0） in the presence/absence of 50 mmol oxalic acid/kg soil, respectively. The effects of treatments were evaluated using single extraction with deionized water or Ca Cl2, Community Bureau of Reference（BCR） sequential extraction and toxicity characteristic leaching procedure（TCLP）methods. Our results showed that the concentration of water extractable, exchangeable and TCLP-Pb all decreased with incubation time. The concentration of water-extractable Pb after120 days was reduced by 100% when soils were amended with APR, HAP and HAP ＋ OA, and the TCLP-Pb was 〈5 mg/L for the red soil at P：Pb molar ratio 4.0. Water-soluble Pb could not be detected and the TCLP-Pb was 〈5 mg/L at all treatments applied to the yellow-brown soil. BCR results indicated that APR was most effective, although a slight enhancement of water-soluble phosphate was detected at the P：Pb molar ratio 4.0 at the beginning of incubation. Oxalic acid activated phosphates, and so mixing insoluble phosphates with oxalic acid may be a useful strategy to improve their effectiveness in reducing Pb bioavailability.

Adsorption ability of rare earth elements on clay minerals and its practical performance

The adsorption behaviors of rare earth elements on clay minerals would have great influence on the mineralization process and the leaching process of the ion-adsorption type rare earths ore.In this work,the adsorption thermodynamics of REEs on kaolin were investigated thoroughly and systematically.The experimental results showed that the adsorption characteristics of La,Nd,Y on kaolin did fit well with the Langmuir isotherm model and their saturated adsorption capacities were 1.731,1.587 and 0.971 mg/g,respectively.The free energy change（ΔG）values were –16.91 kJ/mol（La）,–16.05 kJ/mol（Nd）and –15.58 kJ/mol（Y）,respectively.The negative values of ΔG demonstrated that the adsorption of rare earth on kaolin was a spontaneously physisorption process.The deposit characteristic of the volcanic ion-adsorption type rare earths ore and the behavior of the rare earth in the column leaching process were also developed here.With the increase of the ore body depth,the distribution of the LREEs decreased and the HREEs increased.And the slight differences in the adsorption ability of REEs on clay minerals led to the fractionation effect in the column leaching process.These developed more evidences and better understanding of metallogenic regularity,and provided a theoretical basis and scientific approach to separation of the HREEs and LREEs in the leaching process.

Effective alleviation of Cd stress to microbial communities in mining reclamation soils by thiourea-modified biochar amendment

Reclaimed soils in mining area usually display low fertility and present Cd stress.The amendment of modified biochar effectively fixes Cd in soils,enhances soil fertility,and reduces Cd stress in soil microorganisms.However,the effect of thiourea-modified biochar(TBC)on microbial adaptability to Cd stress in mining reclamation soils is still unclear.The present work studied the Cd immobilization and microbial community changes in a mining reclamation soil displaying extreme Cd contamination under TBC amendment.The results indicated that the amendment of TBC significantly enhanced(P<0.05)soil pH,the content of available phosphorus(AP),and the activities of urease and polyphenol oxidase by 1.3%,463.4%,54.4%,and 84.0%,respectively,compared to the control without amendment.The amount of toxicity characteristic leaching procedure-extracable Cd decreased(P<0.05)by 68.0%in the TBC-amended soil compared with the unamended soil.The structure of soil microbiota was reorganized and the alpha diversity index was increased in the TBC treatment.The TBC amendment increased the relative abundances of Proteobacteria,Bacteroidota,and Zoopagomycota,which were strongly associated(P<0.01)with higher soil pH and AP.Structural equation model results demonstrated that Cd immobilization was directly influenced by soil pH,AP,and urease,and indirectly affected by bacterial structure in the TBC treatment.The TBC amendment can effectively improve the structural composition of soil bacteria under Cd stress and enhance the pathways of decreasing soil Cd availability as well.The results might facilitate the development of in-situ remediation programs in Cd-contaminated soils in the future.