An investigation was made into the effects of tannery sludge on soil chemical properties and microbial communities in a typical soil profile with long-term tannery sludge contamination, North China. The results showed that trivalent chromium (Cr(III)), ammonium, organic nitrogen, salinity and sulfide were the predominant contaminants in tannery sludge. Although the tannery sludge contained high chromium (Cr, 3,0970 mg/kg), the proportion of mobile Cr forms (exchangeable plus carbonate-bound fraction) only accounted for 1.32%. The X-ray diffraction and X-ray photoelectron spectroscopy results further demonstrated that the Cr existed in a stable state of oxides and iron oxides. The alkaline loam soil had a significant retardation effect on the migration of salinity, ammonium, Cr(III) and sulfide, and the accumulation of these contaminants occurred in soils (0–40 cm). A good correlation (R² = 0.959) was observed between total organic carbon (TOC) and Cr(III) in the soil profile, indicating that the dissolved organic matter from sludge leachate promoted the vertical mobility of Cr(III) via forming Cr(III)-organic complexes. The halotolerant bacteria (Halomonas and Tepidimicrobium) and organic degrading bacteria (Flavobacteriaceae, Tepidimicrobium and Balneola) became the dominant microflora in the soil profile. High contents of salinity, Cr and nitrogen were the main environmental factors affecting the abundance of indigenous microorganisms in soils.

In order to remediate ammonium contaminated groundwater, an innovative multimedia permeable reactive barrier (M-PRB) was proposed, which consisted of sequential columns combining oxygen releasing compound (ORC), zeolite, spongy iron and pine bark in the laboratory scale. Results showed that both ammonium and nitrate could be reduced to levels below the regulatory discharge limits through ion exchange and microbial degradation (nitrification and denitrification) in different compartments of the M-PRB system. The concentration of dissolved oxygen (DO) increased from 2 to above 20 mg/L after the simulated groundwater flowed through the oxygen releasing column packed with ORC, demonstrating that ORC could supply sufficient oxygen for subsequent microbial nitrification. Ammonium was efficiently removed from about 10 to below 0.5 mg N/L in the aerobic reaction column which was filled with biological zeolite. After 54 operating days, more than 70% ammonium could be removed by microbial nitrification in the aerobic reaction column, indicating that the combined use of ion exchange and nitrification by biological zeolite could ensure high and sustainable ammonium removal efficiency. To avoid the second pollution of nitrate produced by the former nitrification, spongy iron and pine bark were used to remove oxygen and supply organic carbon for heterotrophic denitrification in the oxygen removal column and anaerobic reaction column separately. The concentration of nitrate decreased from 14 to below 5 mg N/L through spongy iron-based chemical reduction and microbial denitrification.

Trivalent chromium (Cr(III)) can form stable soluble complexes with organic components, altering its adsorption properties in the water-soil environment. This increases the risk of Cr(III) migrating to deeper soils and transforming into toxic Cr(VI) due to the presence of manganese oxides in sediments. In this study, Citric Acid (CA) was selected as a representative organic ligand to prepare and characterize Cr(III)-CA complexes. The characteristics, mechanisms and environmental factors influencing the adsorption of Cr(III)-CA on porous media (silts and fine sands) were investigated in the study. The results show that Cr(III) coordinates with CA at a 1:1 molar ratio, forming stable and soluble Cr(III)-CA complexes. Compared to Cr(III) ions, the equilibrium adsorption capacity of Cr(III)-CA is an order of magnitude lower in silts and fine sands. The adsorption of Cr(III)-CA in silts and fine sands is dominated by chemical adsorption of monolayers, following the pseudo-second-order kinetic equation and the Langmuir isotherm adsorption model. Varying contents of clay minerals and iron-aluminum oxides prove to be the main causes of differences in adsorption capacity of Cr(III)-CA in silts and fine sands. Changes in solution pH affect the adsorption rate and capacity of Cr(III)-CA by altering its ionic form. The adsorption process is irreversible and only minimally influenced by ionic strength, suggesting that inner-sphere complexation serves as the dominant Cr(III)-CA adsorption mechanism.

To investigate the influence of the Hutuo River (North China) ecological water replenishment project on the hydrogeochemical processes of groundwater, 64 groundwater samples collected at different time intervals after four replenishment events, and four samples from the Middle Route of the South-to-North Water Diversion Project water, were analyzed for water chemistry. Hydrogeochemical methods such as the Piper diagram, chloride-alkalinity index, and ion correlation were employed to analyze the characteristics of groundwater chemical evolution through replenishment. The results demonstrated that the hydrochemical types of groundwater in the study area underwent significant changes during continuous replenishment in the Hutuo River region. During the initial replenishment period (October 2019), the dominant hydrochemical type of groundwater in the study area was Mg-Na-HCO₃-SO₄, whereas the dominant type in the Middle Route of the South-to-North Water Diversion Project water was Ca-Na-SO₄-HCO₃. As the replenishment continued, the hydrochemical types of groundwater in the study area evolved into Ca-Na-Mg-HCO₃-SO₄, Na-Ca-Mg-HCO₃-SO₄, and Ca-Na-Mg-SO₄-HCO₃. The groundwater experienced a dissolution of calcite, gypsum, nitratine, carbonate rocks, and gypsum, accompanied by dilution effects, resulting in reduced ion exchange as replenishment progressed. The input of the high quality Middle Route of South-to-North Water Diversion Project water effectively promoted groundwater quality improvement, leading to an overall decrease or stabilization of components other than Ca²⁺ in the groundwater. Water quality was assessed using the entropy water quality index, with indicators including Na⁺, SO₄²⁻, Cl⁻, pH, total dissolved solids, NO₃⁻-N, NO₂⁻-N, F⁻, Al, As, and Zn. The evaluation results showed that, except for one medium-quality water sample, the water quality of the other samples was suitable for drinking and domestic purposes during the early replenishment period. The Middle Route of the South-to-North Water Diversion Project exhibited excellent quality (Rank 1), and as replenishment progressed, all water samples demonstrated good quality by October 2020, with a gradual improvement.