One hundred and fifty years of mineral extraction throughout the mountainous Ruby Creek watershed, Washington has left a legacy of historical hard rock mines and placer claims and their wastes. We conducted a watershed-scale chemical analysis of these gold-bearing tributaries, accounting for seasonal variability in streamflow, to identify spatial and temporal changes in stream chemistry and attribute them to natural processes or mining activities. We used hierarchical cluster analysis (HCA) to group chemically similar water samples based on concentrations of 23 metals, pH, and conductivity and compared the chemistry of HCA-generated clusters of water samples using pairwise comparisons to find chemical patterns. Total concentrations of As, Ba, Ca, Mg, Na, Sb, and Se, dissolved concentrations of Fe, and conductivity increased as streamflow progressed from snowmelt-influenced to baseflow. High total concentrations of Al, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, V, and Zn during spring snowmelt and after rains were attributable to acid mine drainage at historical hard rock mines and prospect sites. Smaller-scale placer mining, by way of suction dredging and motorized gold panning, was associated with high concentrations of Al, Ba, Cd, Co, Fe, Mg, Mn, Mo, and Zn downstream. Stream biota may be adversely affected by exposure to Pb, which exceeded USEPA’s Aquatic Life Criteria, and exposure to particulate metals suspended in the water column.

Every year, the leather tanning industry produces substantial quantities of residues such as chrome-tanned leather shavings (CTLS), which contain considerable amounts of Cr(III) salts. The residues have no particular value and under natural conditions can transform into toxic Cr(VI) wastes. The objective of the present work is to evaluate the transformation of these residues into carbon adsorbents at low temperatures (< 600 °C), using ZnCl₂ as an activating agent. The pyrolysis temperature and residence times were studied. The materials were characterized and qualified by Acid Black 210 (AB) adsorption. The results indicated that low amounts of chromium oxides (less than 2% of Cr), in the form of 50–200 nm particles, remained after the synthesis procedure. The deposited chromium oxides were present in (II), (III), and (IV) oxidation states. The low preparation temperatures employed prevented further chromium oxidation to Cr(VI). Maximum surface areas of 439 m²/g were obtained. The materials efficiently removed AB (maximum experimental adsorption capacity of 44.4 mg/g) by means of electrostatic interaction caused by the positively charged distribution of the carbons. The adsorption capacity was not affected by temperature, but pH had a mixed effect due to the combination of a shift in surface charge distribution and dye speciation. The results demonstrated that it is possible to obtain a value-added product, i.e., carbons modified with chromium nanoparticles for dye removal, from a hazardous residue of the tanning industry.

The adsorption properties and mechanisms of methylene blue (MB) onto novel biochars produced by the fallen leaves of Magnolia grandiflora Linn (MGL), at different pyrolysis temperatures (450 °C, 500 °C, 550 °C) were explored. Results of the adsorption experiments revealed that the fallen leaf-biochar of MGL (MGLB) pyrolyzed at 450 °C (MGLB450) had the highest adsorption capacity of MB (114.15 mg g⁻¹) and MGLB pyrolyzed at 500 °C (MGLB500) was lowest (88.13 mg g⁻¹). The characterization results showed that the BET surface area (41.784 m² g⁻¹) and total pore volume (0.043 cm³ g⁻¹) of MGLB450 were low, but the contents of oxygen-containing functional groups were highest. Oxygen-containing functional group might have a greater impact on the adsorption of MB than its physical characteristics. The adsorption capacity increased with reaction temperature, indicating that the MG adsorption onto biochars was endothermic. The higher initial concentrations of MB and pH were beneficial to adsorption. The adsorption kinetics showed that the adsorption followed pseudo-second-order kinetics model. The obtained equilibrium data were fitted better by Langmuir model rather than Freundlich model.

To explore the possibility of using flue gas desulfurization gypsum (FGDG) for inhibiting phosphorus (P) loss due to agricultural runoff, a 3-year study was performed in the farmlands of Chongming Dongtan between 2012 and 2015. Five different quantities of FGDG were used to treat the soil, and the effects of different treatments on the characteristics of soil P and crop growth were investigated. The results showed that 2 years after application of FGDG, the soil density at a depth of 0–10 cm decreased by 4.35–7.97%, the porosity increased by 1.77–11.0%, and the topsoil permeability increased by 0.87–3.81 times. Although the use of FGDG did not change the total P concentration in the soil, it decreased the concentration of sodium bicarbonate extractable P in the soil. Compared to the control, the average extractable P concentration at depths of 0–10 cm, 10–20 cm, and 20–30 cm decreased by 22.0–46.1%, 26.9–40.5%, and 22.8–34.8%, respectively. The inorganic P in the soil increased as the amount of FGDG increased, and the increase was mainly as Ca–P in the forms Ca₂–P and Ca₁₀–P. The decrease in bicarbonate extractable P and increase in inorganic P in the soil did not affect the growth of the crops, and the biomass and output of the crops increased compared to the control. Therefore, FGDG can enhance soil P immobilization, thus reducing soluble P runoff from farm fields, and improving water quality in receiving lakes and rivers while maintaining P nutrition to the crops.

The composition of SrCuₓO mixed metal oxides (MMOs) was engineered via varying the amount of copper relative to strontium. As-synthesized SrCuₓO were highly active for degrading methyl orange (MO) pollutant at dark ambient conditions without the aid of other reagents. The catalytic activity of SrCuₓO demonstrated a reverse-volcano relationship with copper content. Copper-rich MMOs (SrCu₂O) exhibited the highest degradation activity for MO by far and degraded ca. 96% MO within 25 min. MO degradation over SrCu₂O was a surface-catalytic reaction and fitted pseudo-first-order reaction kinetics. The contact between MO molecules and catalyst surface initiated the reaction via the catalytic-active phase (Cu⁺/Cu²⁺ redox pair), which serves as an electron-transfer shuttle ([Formula: see text]) from MO to dissolved O₂, inducing the consecutive generation of reactive oxygen species, which resulted in MO degradation as evidenced by radical trapping experiment. XPS and XRD analysis revealed that active phases in SrCu₂O materials underwent irreversible transformation after reaction, contributing to the observed deactivation in the cycling experiment. The observations in this study demonstrate the significance of chemical composition tailoring in catalyst synthesis for environmental remediation under dark ambient conditions. Graphical abstract

The efficient removal of heavy metals from aqueous environment is imperative and challenging. A novel ternary composite constructed of diaminopyridine polymers, graphene oxide, and ferrite magnetic nanoparticles was designed by a facile in situ polymerization strategy for the removal of Pb(II) from aqueous solution. Detailed characterization of morphological, chemical, and magnetic properties was employed systematically to confirm the formation of the composite material. Batch adsorption experiment studies suggested that the composite was an excellent adsorbent for Pb(II) which was easily collected after use via exposure to an external magnetic field for 30 s. The effects of different parameters such as solution pH, adsorbent dosage, contact time, initial Pb(II) concentration, temperature, and co-existing ions were examined. The maximum adsorption capacity at pH = 5 was estimated to be 387.2 mg g⁻¹ at 298 K by the Langmuir isotherm model, accompanied by favorable adsorption recyclability according to the investigation of regeneration experiments. Thermodynamic studies revealed that the Pb(II) adsorption via our ternary composite was endothermic and spontaneous. The corresponding removal performance for effluent containing Pb(II) from the battery industry was successfully examined. The present results indicated that our designed adsorbent is beneficial to the practical Pb(II) removal in wastewater purification.

Efficacy of activated bentonite, activated carbon, and natural phosphate under experimental conditions was tested as low-cost adsorbents for spiramycin antibiotic removal from aqueous solution. Equilibrium kinetic and isotherm adsorption process are well described by pseudo-second order and Langmuir isotherm models for activated bentonite and activated carbon, while natural phosphate follows pseudo-first order and Freundlich models, respectively. Obtained results revealed that activated bentonite has the highest adsorption capacity (260.3 mg/g) as compared to activated carbon (80.3 mg/g) and natural phosphate (1.7 mg/g). The adsorption capacity decreases for all adsorbents in the presence of NaCl. The adsorption processes are facilitated in the alkaline pH range for activated bentonite and activated carbon, whereas, for natural phosphate, the acidic pH range is favorable. They are involving ion exchange and hydrogen bond mechanisms as well as Van der Waals forces and also π interactions for activated carbon. Thermodynamic calculation shows that spiramycin adsorption is endothermic and spontaneous on all adsorbents. The activated bentonite reusability is more efficient by more than 95% in two-step desorption using NaOH and HCl eluents compared to activated carbon. Thus, activated bentonite is a promising adsorbent for spiramycin removal from aqueous solution.

Biological reduction is an effective method for removal of perchlorate (ClO₄⁻), where perchlorate is transformed into chloride by perchlorate-reducing bacteria (PRB). An external electron donor is required for autotrophic and heterotrophic reduction of perchlorate. Therefore, plenty of suitable electron donors including organic (e.g., acetate, ethanol, carbohydrate, glycerol, methane) and inorganic (e.g., hydrogen, zero-valent iron, element sulfur, anthrahydroquinone) as well as the cathode have been used in biological reduction of perchlorate. This paper reviews the application of various electron donors in biological perchlorate reduction and their influences on treatment efficiency of perchlorate and biological activity of PRB. We discussed the criteria for selection of appropriate electron donor to provide a flexible strategy of electron donor choice for the bioremediation of perchlorate-contaminated water.

In the present study, the Taguchi design (TD) and the Box-Behnken design (BBD) were used to determine the effect of surfactant concentration, flushing velocity, and dense non-aqueous-phase liquid–trichloroethylene (DNAPL TCE) mass on the time of remediation for the removal of DNAPL zones, which is one of the main persistent sources of groundwater pollution. In the Taguchi approach, the performance of the response variable is measured based on the signal-to-noise (S/N) ratio whereas estimation of the full quadratic model of the parameters is allowed in the Box-Behnken design. The mean experimental values ranged between 3.97–8.31 and 4.01–9.70 for BBD and TD, respectively. Surfactant concentration was identified as the most significant parameter contributing to remediation efficiency in both design techniques. Minimum remediation effort was determined as 5.99 at obtained optimal conditions of surfactant concentration (2.5%), flushing rate (6 cm/h), and DNAPL TCE mass (365 mg) using BBD. In the case of TD, the optimal conditions were determined at a surfactant concentration of 10%, 2 cm/h flushing rate, and 365 mg DNAPL TCE mass. Analysis of variance (ANOVA) revealed a good relationship between the predicted and experimental values with 1.96% and 0.31% of the total variation that was not explained by the model using TD and BBD, respectively. Consequently, from this comparative study, it was concluded that BBD was a more suitable alternative to TD for the evaluation of remediation of DNAPL-contaminated sites.

Lotus seed shell was employed using one-step method combining carbonization with ZnCl₂ activation to synthesize activated carbons because of its inexpensiveness and local accessibility. The lotus seed shell–activated carbons (LSSACs) with the highest surface area (2450.8 m²/g) and mesoporosity (98.6%) and the largest pore volume (1.514 cm³/g) were tailored under optimum conditions as follows: impregnation ratio = 2:1, carbonization temperature = 600 °C, and time = 1.0 h. The surface Zn(II), abundant hydroxyl, and carboxyl functional groups from the activation process could result in rapid Pb(II) adsorption onto the LSSAC surface through surface complexation, ion exchange, or precipitation. The maximum monolayer adsorption capacity (qₘ) for Pb(II) of 247.7 mg/g at 25 °C could be fitted from the Langmuir isotherm. The Gibbs free energy (△G) and positive enthalpy (△H) indicated that the adsorption process was spontaneous and endothermic, and to some extent, it was explained by the intra-particle diffusion mechanism. Our results may provide a promising way to produce activated carbons with high adsorption capacity using solid waste, which will eventually promote the environmental sustainability.