The alkaline mineral amendment is a practical means of alleviating Cd concentration in rice grain (CdR) in the short-term; however, the long-term remediation effect of mineral amendment on the CdR and the eco-environmental controls remains unknown. Here a mineral (Si–Ca–Mg) amendment, calcined primarily from molybdenum tailings and dolomite, was applied biannually over 6 years (12 seasons) to acidic and moderately Cd-contaminated double-rice cropping ecosystems. This study investigated the inter-annual variation of Cd in the rice-soil ecosystem and the eco-environmental controls in subtropical rice ecosystems. CdR was reduced by 50%–86% following mineral amendment. The within-year reduction in CdR was similar between early rice (50%–86%, mean of 68%) and late rice (68%–85%, mean of 74%), leading to CdR in all early rice and in 83% of late rice samples below the upper limit (0.2 mg kg⁻¹) of the China National Food Safety Standards. In contrast, the inter-annual reduction in CdR was moderately variable, showing a greater CdR reduction in the later 3 years (73%–86%) than in the former 3 years (54%–79%). Three years continuous mineral amendment was required to guarantee the safety rice production. The concentrations of DTPA-extractable and exchangeable Cd fractions in soil were reduced, while the concentration of oxides-bound Cd was increased. In addition, the soil pH, concentrations of Olsen-P and exchangeable Ca and Mg were elevated. These imply a lower apparent phytoavailability of Cd in the soil following mineral amendment. An empirical model of the 3-variable using soil DTPA-Cd, soil Olsen-P, and a climatic factor (precipitation) effectively predicted temporal changes in CdR. Our study demonstrates that Cd phytoavailability in soil (indexed by DTPA-extractable Cd) and climatic factors (e.g., temperature and precipitation) may directly/indirectly control the inter-annual reduction in CdR following mineral amendment in slightly and moderately Cd-contaminated paddy ecosystems.

Large quantities of lead/zinc (Pb/Zn) mine tailings were deposited at tailings impoundments without proper management, which have posed considerable risks to the local ecosystem and residents in mining areas worldwide. Therefore, the geochemical behaviors of potentially toxic elements (PTEs) in tailings were in–depth investigated in this study by a coupled use of batch kinetic tests, statistical analysis and mineralogical characterization. The results indicated that among these studied PTEs, Cd concentration fluctuated within a wide range of 0.83–6.91 mg/kg, and showed the highest spatial heterogeneity. The mean Cd concentrations generally increased with depth. Cd were mainly partitioned in the exchangeable and carbonate fractions. The release potential of PTEs from tailings was ranged as: Cd > Mn > Zn > Pb > As, Cd > Pb > Zn > Mn > As and Cd > Pb > Mn > Zn > As, respectively, under the assumed environmental scenarios, i.e. acid rain, vegetation restoration, human gastrointestinal digestion. The results from mineralogical characterization indicated that quartz, sericite, calcite and pyrite were typical minerals, cumulatively accounting for over 80% of the tailings. Sulfides (arsenopyrite, galena, and sphalerite), carbonates (calcite, dolomite, cerussite and kutnahorite), oxides (limonite) were identified as the most relevant PTEs–bearing phases, which significantly contributed to PTEs release from tailings. A combined result of statistical, geochemical and mineralogical approaches would be provided valuable information for the alteration characteristics and contaminant release of Pb/Zn mine tailings.

Groundwater chemistry can be affected by and related to earthquakes, thus it is crucial to understand the hydrochemical changes and associated processes caused by earthquakes for post-seismic groundwater utilization. Here we reported the major ion concentrations changes of the Ganze Spring in response to the May 30, 2014 Ms 6.1 Yingjiang earthquake, southwest China based on the daily time series (from 1st January 2012 to 20th July 2014) of Ca²⁺, Mg²⁺ and HCO₃⁻ concentrations, as well as data of bulk strain and Peak Ground Velocity (PGV) recorded at a nearby station. The results showed that the entire hydrochemical response process can be divided into two stages after the earthquake occurred: 1). decline stage which was characterized by an increasingly decline of the three ion concentrations, indicating a gradually significant dilution effect. At first, the relationship of molar concentrations of ions showed no obvious changes; but later as the rate of decrease in ion concentrations increased, the relationship between Ca²⁺ and HCO₃⁻ reversed from Ca²⁺ excess to HCO₃⁻ excess, probably resulting from a relatively decreased Ca²⁺ contribution from dissolution of gypsum and dolomite due to dilution in mixing water. 2). recover stage when the ion concentrations recovered gradually with relatively lower values than that at pre-earthquake, revealing the reduction of dilute water inflow. In combination with the bulk strain and PGV data, the study suggested that major ion concentrations changes are attributed to dilution effect due to new fracture creation or unclogging/clogging of fractures triggered by the earthquake. The results could enhance the understanding of earthquake induced water chemistry changes and could have implications for water resources management and security in tectonically active areas.

Hydrogeochemistry and isotope hydrology were carried out to investigate the spatial distribution of fluoride (F−) and the mechanisms responsible for its enrichment in the western region of the Ordos basin, northwestern China. Sixty-two groundwater samples from the unconfined aquifer and fifty-six from confined aquifer were collected during the pre-monsoon (June 2016). Over 77% of groundwater samples from the unconfined aquifer (F− concentration up to 13.30 mg/L) and approximately 66% from confined aquifer (with a maximum F− concentration of 3.90 mg/L) exhibit F− concentrations higher than the Chinese safe drinking limit (1.0 mg/L). High-F− groundwater presents a distinctive hydrochemical characteristic: a high pH value and HCO3− concentration with Ca-poor and Na-rich. Mineral dissolution (e.g., feldspar, calcite, dolomite, fluorite), cation exchange and evaporation in the aquifers predominate the formation of groundwater chemistry, which are also important for F− enrichment in groundwater. Mixing with unconfined groundwater is a significant mechanism resulting in the occurrence of high-F− groundwater in confined aquifer. These findings indicate that physicochemical processes play crucial roles in driving F− enrichment and that may be useful for studying F− occurrence in groundwater in arid and semi-arid areas.

Stable isotopes ratios (‰) of Hydrogen (δ2H) and Oxygen (δ1⁸O) were used to trace the groundwater recharge mechanism and geochemistry of arsenic (As) contamination in groundwater from four selected sites (Larkana, Naudero, Ghari Khuda Buksh and Dokri) of Larkana district. The stable isotope values of δ2H and δ1⁸O range from 70.78‰ to −56.01‰ and from −10.92‰ to −7.35‰, relative to Vienna Standard for Mean Ocean Water (VSMOW) respectively, in all groundwater samples, thus indicating the recharge source of groundwater from high-salinity older water. The concentrations of As in all groundwater samples were ranged from 2 μg/L to 318 μg/L, with 67% of samples exhibited As levels exceeding than that of World Health Organization (WHO) permissible limit 10 μg/L and 42% of samples expressed the As level exceeding than that of the National Environmental Quality Standard (NEQS) 50 μg/L. The leaching and vertical mixing with return irrigation water are probably the main processes controlling the enrichment of As in groundwater of Larkana, Naudero, Ghari Khuda Buksh and Dokri. The weathering of minerals mostly controlled the overall groundwater chemistry; rock-water interactions and silicate weathering generated yielded solutions that were saturated in calcite and dolomite in two areas while halite dissolution is prominent with high As area.

The present study aims to investigate the spatial distribution and associated various geochemical mechanisms responsible for fluoride (F⁻) contamination in groundwater of unconfined aquifer system along major rivers in Sindh and Punjab, Pakistan. The concentration of F⁻ in groundwater samples ranged from 0.1 to 3.9 mg/L (mean = 1.0 mg/L) in Sindh and 0.1–10.3 mg/L (mean = 1.0 mg/L) in Punjab, respectively with 28.9% and 26.6% of samples exhibited F⁻ contamination beyond WHO permissible limit value (1.5 mg/L). The geochemical processes regulated F⁻ concentration in unconfined aquifer mainly in Sindh and Punjab were categorized as follows: 1) minerals weathering that observed as the key process to control groundwater chemistry in the study areas, 2) the strong correlation between F⁻ and alkaline pH, which provided favorable environmental conditions to promote F⁻ leaching through desperation or by ion exchange process, 3) the 72.6% of samples from Sindh and Punjab were dominated by Na⁺- Cl⁻ type of water, confirmed that the halite dissolution process was the major contributor for F⁻ enrichment in groundwater, 4) dolomite dissolution was main process frequently observed in Sindh, compared with Punjab, 5) the arid climatic conditions promote evaporation process or dissolution of evaporites or both were contributing to the formation of saline groundwater in the study area, 6) the positive correlation observed between elevated F⁻ and fluorite also suggested that the fluorite dissolution also played significant role for leaching of F⁻ in groundwater from sediments, and 7) calcite controlled Ca2⁺ level and enhanced the dissolution of F-bearing minerals and drive F⁻ concentration in groundwater. In a nut shell, this study revealed the worst scenarios of F⁻ contamination via various possible geochemical mechanisms in groundwater along major rivers in Sindh and Punjab, Pakistan, which need immediate attention of regulatory authorities to avoid future hazardous implications.

A massive and dense textured layer (ca. 35–50 cm thick) of hardpan was uncovered at the top layer, which capped the unweathered sulfidic Cu-Pb-Zn tailings in depth and physically supported gravelly soil root zones sustaining native vegetation for more than a decade. For the purpose of understanding functional roles of the hardpan layer in the cover profile, the present study has characterized the microstructures of the hardpan profile at different depth compared with the tailings underneath the hardpans. A suit of microspectroscopic technologies was deployed to examine the hardpan samples, including field emission-scanning electron microscopy coupled with energy dispersive spectroscopy (FE-SEM-EDS), X-ray diffraction (XRD) and synchrotron-based X-ray absorption fine structure spectroscopy (XAFS). The XRD and Fe K-edge XAFS analysis revealed that pyrite in the tailings had been largely oxidised, while goethite and ferrihydrite had extensively accumulated in the hardpan. The percentage of Fe-phyllosilicates (e.g., biotite and illite) decreased within the hardpan profile compared to the unweathered tailings beneath the hardpan. The FE-SEM-EDS analysis showed that the fine-grained Ca-sulfate (possibly gypsum) evaporites appeared as platelet-shaped that deposited around pyrite, dolomite, and crystalline gypsum particles, while Fe-Si gels exhibited a needle-like texture that aggregated minerals together and produced contiguous coating on pyrite surfaces. These microstructural findings suggest that the weathering of pyrite and Fe-phyllosilicates coupled with dolomite dissolution may have contributed to the formation of Ca-sulfate/gypsum evaporites and Fe-Si gels. These findings have among the first to uncover the microstructure of hardpan formed at the top layer of sulfidic Cu-Pb-Zn tailings, which physically capped the unweathered tailings in depth and supported root zones and native vegetation under semi-arid climatic conditions.

In acidic medium, hazardous heavy metals of lead-zinc tailing (LZT) are easily leachable and mobilizable. Thus, the hazard, amount, form, and complexity of the leached heavy metals under acidic precipitation become a major environmental concern. This work investigates the gangue minerals, toxicity, speciation, leaching characteristics of heavy metals in LZT under simulated acid rain, as well as immobilization effects and mechanisms using a sustainable binder. In LZT, dolomite, quartz, calcite, and muscovite are the main gangue minerals, tiny hazardous metallic minerals were absorbed in the surface. The results revealed that Pb, Zn, Cr, and Cd were the predominant harmful elements, particularly Pb and Zn. Zn is leached completely and is the concerned hazardous element under simulated acid rain. In the acid rain neutralization ability test, the amount of leachable Pb, Cr, Ca, and Si maintained in equilibrium, leached Zn, Cd, Al, and Mg depended on the addition of acid. Pb and Ca were sedimented in residues. Immobilization of Pb, Zn, Cr, and Cd depended on the stability of Ca(OH)₂/C–S–H of hydrates, and 70% LZTHP after curing 7 days can be used for some practical engineering projects. This work opens up deeply understandings for the leached heavy metals under acidic precipitation and improves the sustainable and safe in the field of immobilization of heavy metals.

Contaminant vulnerability in the critical zones like groundwater (GW)-seawater (SW) continuum along the entire Gujarat coast was investigated for the first time through an extensive water monitoring survey. The prime focus of the study was to evaluate whether or not: i) seawater intrusion induced metal load translates to toxicity; ii) in the coastal groundwater, metal distribution follows the pattern of other geogenic and anthropogenic contaminants like NO₃- and F-; and iii) what future lies ahead pertaining to metal fate in association with saturation conditions of the coastal aquifers. The spatial distribution of contaminants depicts that the Gulf of Khambhat area is highly contaminated. Ecological risk assessment (ERA) indicates that the Gujarat coast is experiencing a high ecological risk compared to the southeast coast of India. Investigation results revealed that metals, pH, NO₃, and CO₃ are more vulnerable at the SW-GW mixing interface. An increase in pH is reflected in fewer ionic species of metals in the GW. Salinity ingress due to seawater intrusion (SWI) reduces the toxicities of all trace metals except Cu, attributed to the increase of Ca in GW, leading to dissociation of CuCO₃. Reactive species are dominant for Zn and Cd; and M-CO₃ ligands are dominant for Cu and Pb owing to the undersaturation of dolomite and calcite in the aquifer system. SWI tends to increase the metal load but the toxicity of metals varies with the density of industries, anthropogenic activities, changes in the mixing-induced saturation conditions, and intensive salt production across the coast. Multivariate analysis confirmed that the hydrogeochemical processes change due to GW-SW mixing and dictates over natural weathering. The ecological risk index (ERI) for the Arabian sea is experiencing moderate (300 ≥ ERI>150) to high ecological risk (ERI >600). Children population is likely to encounter a high health risk through ingestion and dermal exposure than adults. Overall, the study emphasizes the complexity of toxicity-related health impacts on coastal communities and suggests the dire need for frequent water monitoring along the coastal areas for quick realization of sustainable development goals.

Biofuel production via pyrolysis has received increasing interest as a promising solution for utilization of now wasted food residue. In this study, the fast pyrolysis of mixed food waste (MFW) was performed in a bubbling fluidized-bed reactor. This was done under different operating conditions (reaction temperatures and carrier gas flow rate) that influence product distribution and bio-oil composition. The highest liquid yield (49.05 wt%) was observed at a pyrolysis temperature of 475 °C. It was also found that the quality of pyrolysis bio-oils (POs) could be improved using catalysts. The catalytic fast pyrolysis of MFW was studied to upgrade the pyrolysis vapor, using dolomite, red mud, and HZSM-5. The higher heating values (HHVs) of the catalytic pyrolysis bio-oils (CPOs) ranged between 30.47 and 35.69 MJ/kg, which are higher than the HHVs of non-catalytic pyrolysis bio-oils (27.69–31.58 MJ/kg). The major components of the bio-oils were fatty acids, N-containing compounds, and derivatives of phenol. The selectivity for bio-oil components varied depending on the catalysts. In the presence of the catalysts, the oxygen was removed from oxygenates via moisture, CO₂, and CO. The CPOs contained aliphatic hydrocarbons, polycyclic aromatic compounds (such as naphthalene), pyridine derivatives, and light oxygenates (cyclic alkenes and ketones).