Horizontal planetary mechanochemical method for rapid and efficient remediation of high-concentration lindane-contaminated soils in an alkaline environment

CaO assisted mechanochemical remediation of lindane-contaminated soil

In this study, the planetary ball milling with CaO addition was used to remediate lindane-contaminated soil. Based on Hertzian theory, a mathematical model was proposed to simulate the trajectory of grinding ball and the local energy transfer during a planetary operation at the disk rotation velocities of 150-250 rpm. Besides, the influence of different parameters on lindane removal in soil was investigated, whose results showed that disk rotation velocity and reagent-to-soil ratio had a positive effect, while soil moisture, initial concentration of lindane, and mass of polluted soil demonstrated a negative influence. The mechanochemical method exhibited a higher degradation performance at 3 wt% CaO addition, and a disk rotation velocity of 250 rpm. Active species generated by ball collisions in the presence of CaO, especially superoxide (·O2-) demonstrated a significant role in participating in the lindane conversion. In combination with GCMS and XPS analysis, the proposed model provides insight into mechanochemical remediation process from physical and chemical perspectives, which mainly includes four main steps: mixing, inducing, chemical reaction, and structure destruction.

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.

Graphene/MoS2-assisted alum sludge electrode induces selective oxidation for organophosphorus pesticides degradation: Co-oxidation and detoxification mechanism

Designing an electrode that can generate abundant free radicals and 1O2, which can effectively degrade and detoxify organophosphorus pesticides (OPPs) through a co-oxidation pathway, is important. In this study, we prepared a electrode GO/MoS2@AS by supporting MoS2 on alum sludge (AS) under graphene oxide (GO) nanoconfinement. The results show that the dominant role of 1O2 at the cathode and •OHads at the anode for degradation, in addition to the involvement of 1O2 in the cathodic degradation mechanism, can be attributed to the abundant precursor •O2- and H2O2. Furthermore, calculations using density functional theory and toxicity prediction of products show that the energy (∆E) requirements of •OHfree to break the C-O bond of the pyridine ring and phosphate group are higher than that required for 1O2, and this non-radical oxidation plays a key role in detoxification. In contrast, accelerating ring opening and oxidation processes are attributed to radical oxidation. Above all, the cathodic detoxification is more effective than anodic detoxification. Three prevalent OPPs, chlorpyrifos, glyphosate, and trichlorfon, were degraded in the GO/MoS2@AS system by over 90 %, with mineralization rates of 76.66 %, 85.46 %, and 82.18 %, respectively. This study provides insights into the co-oxidation degradation and detoxification mechanism mediated by 1O2 and •OHfree.

The coupling of sulfide and Fe-Mn mineral promotes the migration of lead and zinc in the redox cycle of high pH floodplain soils

In this study, we investigated the impact of fluctuating water levels on the distribution of lead (Pb) and zinc (Zn) in soil and sediments at a historical Pb-Zn smelting site along the Xiangjiang River. Despite the high pH levels (7 to 11) in the study area, which generally inhibits heavy metal solubility, we found that regular changes in water levels still affect Pb-Zn movement. Soil analysis revealed distinct redox zones within the unconfined aquifer, as shown by the variable Fe/Mn and Ce/Ce* ratios. Advanced techniques such as Mn K-edge XAFS, Mössbauer spectroscopy, and TOF-SIMS indicated persistent Fe-Mn redox cycling and highlighted the presence of Pb and Zn-rich manganese oxides near sulfur-bearing minerals. These findings suggest that acidic microzones produced by the oxidation of sulfur-bearing minerals become "refuges" for microbial and heavy metal activity. Considering that sulfur-containing minerals are widespread waste types in nonferrous metal smelting sites, these findings are instructive for a better understanding of the transformation mechanisms of heavy metal ions in nonferrous metal smelting-polluted environments and for guiding pollution remediation strategies.

Sn/Sb-assisted alum sludge electrodes for eliminating hydrophilic organic pollutants in self-produced H2O2 electro-Fenton system: Insights into the co-oxidation mediated by 1O2 and •OH(ads)

Few reports have focused on using particle electrodes with polar adsorbent properties in heterogeneous electro-Fenton (EF) system to improve the degradation of hydrophilic organic pollutants (HLOPs). In this study, a hydrophilic electrode Sn-Sb/AS was prepared by supporting metals Sn and Sb on alum sludge (AS), which can effectively degrade 91.68%, 92.54%, 89.62%, and 96.24% of the four types of HLOPs, chlorpyrifos (CPF), atrazine (ATZ), diuron (DIU), and glyphosate (PMG), respectively, within 40 min. The mineralization rates were 82.37%, 78.93%, 73.98%, and 85.65% for CPF, ATZ, DIU, and PMG, respectively. Based on the analysis of Electron Paramagnetic Resonance test, quenching test, and identified anthracene endoperoxide, the degradation at the cathode was attributed to non-radical oxidation via interaction with 1O2. In contrast, the anodic oxidation occurred via direct electron transfer at the anode and/or oxidation via interaction with adsorbed •OH (•OHads) around the particle electrodes. Furthermore, the reaction sites were calculated by Density functional theory (DFT) and Fukui function, corresponding to the electrophilic attack (fA-) of 1O2 and anodic direct oxidation, besides, the radical attack (fA0) of •OH(ads). Herein, this study proposes a targeted elimination strategy for HLOPs in wastewater treatment using particle electrodes with polar adsorbent properties in EF system.

Mechanochemical degradation performance of lindane in different types of soils: The effects of soil properties and elemental components

Although mechanochemical remediation of organic-contaminated soil has received substantial attention in recent years, the effects of soil properties on soil remediation performance are not clear. In this work, the properties and elemental components of 16 soils were tested, and the mechanochemical degradation performance of lindane in these soils was investigated through experiments. Most importantly, the relationships between soil variables and the mechanochemical degradation rates of lindane in the additive-free and CaO systems were elucidated. The results showed that the mechanochemical degradation efficiencies of lindane in the 16 soils were significantly different without additives, with a range of 31.0 %-97.2 % after 4 h. The mechanochemical degradation rates of lindane in the 16 soils varied from 0.7 h-1 to 15 h-1 after the addition of 9 % CaO. Correlation analysis, redundancy analysis and the partial least squares path modeling results clearly showed that the main factors affecting the reaction rate (k1) without additives were organic matter (-) > clay (+) > bound water (-) > Si (+). After the addition of 9 % CaO, the order in which the main factors affected the reaction rate (k2) was organic matter (-) > bound water (-) > Ti/Fe/Al (-) > pH (+) > clay (+). The established and corrected multiple nonlinear regression equations can be used to accurately predict the mechanochemical degradation performance of hexachlorocyclohexanes in actual soils with and without additives.

Enhanced piezoelectric catalysis of BaTiO3 by ZVAl for mechanochemical defluorination of PFOA: Promotion of electron transfer

Mechanochemical (MC) destruction of pollutants is effective; however, the emerging electron transfer mechanism is ambiguous owing to a lack of systematic evaluation. Therefore, this study aims to evaluate the contribution of electrons to perfluorooctanoic acid (PFOA) defluorination during MC process. A synergistic effect was obtained by activating BaTiO3 to generate piezoelectrons and applying zero-valence aluminum (ZVAl) to facilitate electron transfer, with 95.66% PFOA defluorination and reaction time decreasing from 6 h to 3 h. The mechanism of piezoelectric catalysis of the BaTiO3/ZVAl system was further investigated through kinetic analyses and intersystem comparisons. The major contribution of piezo-excited electrons was revealed through probe detection and quantitative determination. A positive correlation between electron generation and PFOA defluorination was ascertained, and the calculation of the electron utilization ratio revealed an electron transfer mechanism. The detached fluorides were confirmed to be bonded directly to the additives. Furthermore, PFOA decomposition was identified as a cyclical process with constant dissociation of the CF2 groups.

Peroxymonosulfate assisted mechanochemical remediation of high concentration DDTs contaminated soil

DDTs (DDT and its metabolites) contaminated sites urgently need to be treated efficiently and greenly. In this study, a horizontal planetary mechanochemical method with co-milling additives was developed aiming at efficiently degrading high-concentration DDTs in historical contaminated soil (∼7500 mg/kg). Peroxymonosulfate (PMS) was firstly used to the mechanochemical degradation of DDTs in historical contaminated soil, with a degradation efficiency of over 95% after 1 h of milling under the optimal milling conditions (CR = 30:1, r = 500 rpm, R = 1:4). Mechanism study indicated that DDTs in soil were partially dechlorinated and mineralized. The main products formed might be chlorinated aliphatic hydrocarbons, which need further treatment by ball milling or other methods. Under the action of mechanical energy, PMS could oxidize DDTs in soil through non-radical way rather than common radical way. Then, a comprehensive assessment of this remediation method was conducted by analyzing the changes in soil properties and acute biotoxicity after ball milling. Although PMS had a great performance on the degradation of DDTs, especially p, p'-DDE, it would cause the acidification and salinization of soil. Therefore, further pH adjustment and desalination treatment were suggested to reduce the negative impacts. This work successfully presents a practical approach to mechanochemical remediation of DDTs contaminated sites.

Mechanochemical remediation of lindane-contaminated soils assisted by CaO: Performance, mechanism and overall assessment

Soil contamination caused by persistent organic pollutants (POPs) has been a worldwide concern for decades. With lindane-contaminated soil as the target, a mechanochemical method assisted by CaO was comprehensively evaluated in terms of its remediation performance, degradation mechanism and overall assessment. The mechanochemical degradation performance of lindane in cinnamon soil or kaolin was determined under different additives, lindane concentrations and milling conditions. 2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazinyl free radical (DPPH^•) and electron spin resonance (ESR) tests evidenced that the degradation of lindane in soil was caused mainly by the mechanical activation of CaO to produce free electrons (e^-) and the alkalinity of the generated Ca(OH)₂. Dehydrochlorination or dechlorination by elimination, alkaline hydrolysis, hydrogenolysis and the subsequent carbonization were the main degradation pathways of lindane in soil. The main final products included monochlorobenzene, carbon substances and methane. The mechanochemical method with CaO was proved to also efficiently degrade lindane in three other soils and other hexachlorocyclohexane isomers and POPs in soil. The soil properties and soil toxicity after remediation were assessed. This work presents a relatively clear discussion of various aspects of the mechanochemical remediation of lindane-contaminated soil assisted by CaO.