Mechanochemical immobilization of heavy metals in contaminated soils: A novel mathematical modeling of experimental outcomes

Mechanochemical remediation of contaminated soil: A review

Mechanochemical techniques have been garnering growing attention in remediation of contaminated soil. This paper summarizes the performance, mechanism, influential factors, and environmental impacts of mechanochemical remediation (MCR) for persistent organic pollutants (POPs) contaminated soil and heavy metal(loid) s (HMs) contaminated soil. Firstly, in contrast to other technologies, MCR can achieve desirable treatment of POPs, HMs, and co-contaminated soil, especially with high-concentration pollutants. Secondly, POPs undergo mineralization via interaction with mechanically activated substances, where aromatic and aliphatic pollutants in soil may go through varied degradation routes; inorganic pollutants can be firmly combined with soil particles by fragmentation and agglomeration induced by mechanical power, during which additives may enhance the combination but their contact with anionic metal(loid)s may be partially suppressed. Thirdly, the effect of MCR primarily hinges on types of milling systems, the accumulation of mechanical energy, and the use of reagents, which is basically regulated through operating parameters: rotation speed, ball-to-powder ratio, reagent-to-soil ratio, milling time, and soil treatment capacity; minerals like clay, metal oxides, and sand in soil itself are feasible reagents for remediation, and alien additives play a crucial role in synergist and detoxification; additionally, various physicochemical properties of soil might influence the mechanochemical effect to varying degrees, yet the key influential performance and mechanism remain unclear and require further investigation. Concerning the assessment of soil after treatment, attention needs to be paid to soil properties, toxicity of POPs' intermediates and leaching HMs, and long-term appraisement, particularly with the introduction of aggressive additives into the system. Finally, proposals for current issues and forthcoming advancements in this domain are enumerated in items. This review provides valuable insight into mechanochemical approaches for performing more effective and eco-friendly remediation on contaminated soil.

A critical review on the migration and transformation processes of heavy metal contamination in lead-zinc tailings of China

The health risks of lead-zinc (Pb-Zn) tailings from heavy metal (HMs) contamination have been gaining increasing public concern. The dispersal of HMs from tailings poses a substantial threat to ecosystems. Therefore, studying the mechanisms of migration and transformation of HMs in Pb-Zn tailings has significant ecological and environmental significance. Initially, this study encapsulated the distribution and contamination status of Pb-Zn tailings in China. Subsequently, we comprehensively scrutinized the mechanisms governing the migration and transformation of HMs in the Pb-Zn tailings from a geochemical perspective. This examination reveals the intricate interplay between various biotic and abiotic constituents, including environmental factors (EFs), characteristic minerals, organic flotation reagents (OFRs), and microorganisms within Pb-Zn tailings interact through a series of physical, chemical, and biological processes, leading to the formation of complexes, chelates, and aggregates involving HMs and OFRs. These interactions ultimately influence the migration and transformation of HMs. Finally, we provide an overview of contaminant migration prediction and ecological remediation in Pb-Zn tailings. In this systematic review, we identify several forthcoming research imperatives and methodologies. Specifically, understanding the dynamic mechanisms underlying the migration and transformation of HMs is challenging. These challenges encompass an exploration of the weathering processes of characteristic minerals and their interactions with HMs, the complex interplay between HMs and OFRs in Pb-Zn tailings, the effects of microbial community succession during the storage and remediation of Pb-Zn tailings, and the importance of utilizing process-based models in predicting the fate of HMs, and the potential for microbial remediation of tailings.

Remediation of petroleum-contaminated soil by ball milling and reuse as heavy metal adsorbent

A simple mechanochemical (MC) method is used to treat petroleum-contaminated soil and prepare a heavy metal adsorbent in one step. XRD, Raman, FT-IR, VSM, BET, and XPS were used to characterize the adsorbent. After MC treatment, the dissolved total petroleum hydrocarbons of the adsorbent is less than 1 mg·L^-1, and a porous structure and carbonization phenomenon are evident. The specific surface area and cumulative void volume increase, and the adsorption pore size decreases. On the surface of soil, the percentages of iron oxides, carbonates, CO, -C-O-H, -COOH, and π unsaturated bonds increase. The Langmuir model shows that the maximum adsorption capacity of Pb²⁺, Cu²⁺, Cd²⁺, and Zn²⁺ are 338.58, 51.61, 32.34, and 25.05 mg·g^-1, respectively. The pseudo-second-order kinetic model fits the Pb adsorption process, indicating the domination of chemical adsorption. GC-MS shows that petroleum hydrocarbons are completely degraded. The Tessier continuous extraction result shows that heavy metals are bound to carbonate, iron manganese oxide, and organic matter. The MC treatment achieves deep cleanup and resource utilization of petroleum-contaminated soil through the formation of amorphous carbon, carbonates, and iron oxides on the surface of soil particles. The material is magnetic and can be recycled when used in wastewater treatment.

Improved fractal kinetic model to predict mechanochemical destruction rate of organic pollutants

Mechanochemical destruction of organic pollutants by high energy milling with inorganic reagents is considered a promising non-thermal technology to detoxify hazardous waste. However, due to complex nature of the physicochemical phenomena involved, pollutant destruction kinetics heavily depends on the used reagents and operating parameters, thus varying case by case. In the present work, a fractal model was validated as flexible tool to interpolate pollutant mechanochemical destruction data satisfactorily. In addition, such model was expanded to estimate the contributions of the inorganic reagent and the pollutant to the overall reaction rate. Specifically, the kinetic constant associated to mechanical activation of the co-milling reagent and that related to pollutant destruction reaction were calculated. Their values resulted to depend only on the specific compound, hence, the tabulated data could be used to predict the pollutant mechanochemical degradation rate for any kind of mixture.

Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Biochar is widely used for environmental remediation. However, the effects of aging on the bioavailability of trace metals in biochar-amended soil remain largely unknown, especially for the biochars with various concentrations of endogenous metals. In this study, three biochars marked as BB, MB, and HB were produced from the straws of Pennisetum sinese grown in background soil, moderately-, and highly-polluted soils by trace metals, respectively. We distinguished the effects of dry-wet (DW) and freeze-thaw (FT) aging on the bioavailability of copper (Cu) and cadmium (Cd) from soil particles, the biochar interior, and the surface of biochar. The adsorption capacities of Cu²⁺ and Cd²⁺ followed the order of BB > MB > HB. DW and FT aging both increased the adsorption capacity of Cu²⁺, but decreased that of Cd²⁺ in the three biochars, resulting in a reduction in Cu bioavailability and increase in Cd bioavailability in the biochars after the saturated adsorption of Cu²⁺ and Cd²⁺. The incorporation of the three biochars decreased Cu bioavailability compared to the control after incubation for 30d, while the addition of MB increased Cd bioavailability. DW and FT aging decreased Cu bioavailability in biochar-amended soil by decreasing the bioavailability of Cu adsorbed on the biochar surface and immobilized by soil particles. Meanwhile, aging decreased Cd bioavailability by decreasing the bioavailability of Cd immobilized by soil particles. Overall, environmental risk would be increased by the application of biochars with high endogenous Cd. The major implications are that biochar dosage and environmental risk should be carefully assessed before large-scale, continuous application, especially for biochars containing high contents of endogenous trace metals.

Experiments and modeling of mine soil inertization through mechano-chemical processing: from bench to pilot scale using attritor and impact mills

Mechano-chemical treatment has been recognized to be a promising technology for the immobilization of heavy metals (HMs) in contaminated soils without the use of additional reagents. Despite this, very few studies aiming to investigate the applicability of this technology at full scale have been published so far. In this study, a quantitative approach was developed to provide process design information to scale-up from laboratory- into pilot-scale mechano-chemical reactors for immobilizing heavy metals in contaminated mining soil. In fact, after preliminary experiments with laboratory-scale ball mills, experiments have been carried out by taking advantage of milling devices suited for pilot-scale applications. The experimental data of this work, along with literature ones, have been quantitatively interpreted by means of a mathematical model allowing to describe the effect of milling dynamics on the HM immobilization kinetics for applications at different scales. The results suggest that the mechanical process can trigger specific physico-chemical phenomena leading to a significant reduction of HMs leached from mining soils. Specifically, after suitably prolonged processing time, HM concentration in the leachate is lowered below the corresponding threshold limits. The observed behavior is well captured by the proposed model for different HMs and operating conditions. Therefore, the model might be exploited to infer design parameters for the implementation of this technique at the pilot and full scale. Moreover, it represents a valuable tool for designing and controlling mechano-chemical reactors at productive scale.