Alkali-thermal activated persulfate treatment of tetrabromobisphenol A in soil: Parameter optimization, mechanism, degradation pathway and toxicity evaluation.

The continued accumulation of halogenated organic pollutants in soil posed a potential threat to ecosystems and human health. In this study, tetrabromobisphenol A (TBBPA) was used as a typical representative of halogenated organic pollutants in soil, for alkali-thermal activated persulfate (PS) treatment. The results of response surface methodology (RSM) showed a optimal debromination efficiency of TBBPA was 88.99 % under the optimum reaction conditions. Quenching experiments and electron paramagnetic resonance (EPR) confirmed that SO4-•, HO•, O2-• and 1O2 existed simultaneously in the oxidation process. SO4-• played a major role in the initial stage of the reaction, and O2-• played a major role in the the last stage. Based on density functional theory (DFT) and intermediate products, two degradation pathways were proposed, including debromination reaction and ß bond scission. Moreover, the basic physical and chemical properties of the soil were affected to a certain extent, while the soil surface structure, elements and functional group composition rarely changed. In addition, the T.E.S.T. analysis and biotoxicity tests proved that alkali-thermal activated PS can effectively reduce the toxicity of TBBPA-contaminated soil, which is conducive to the subsequent safe secondary utilization of soil.

A carbothermic hybrid synthesized using waste halogenated plastic in sub/supercritical CO2 and its application for lithium recovery.

Facile fabrication of porous carbon materials from waste halogenated plastic is highly attractive but frequently hampered due to potential release of halogenated organic pollutants. In this study, a novel type of carbon hybrid was tentatively synthesized from a real-world halogenated plastic as an inexpensive carbon source by sub/supercritical carbon dioxide carbonization technique. It was found that halogen-free carbon carrier was advantageously synthesized through carbonization of halogenated plastic without using catalysts due to zip depolymerization, random chain cracking and free radical reactions induced by sub/supercritical carbon dioxide technique. Exhibiting with more abundant functional groups including C-O, CO groups than pyrolytic carbon carrier, the derived carbon carrier demonstrated excellent performance in selective recovery of lithium from cathode powder with highest recovery efficiency of 93.6%. Mechanism study indicated that cathode powder was transformed into low-valence states of transition metals/metal oxides and released lithium as lithium carbonate due to collapse of oxygen framework via carbothermic reduction. This work provides an applicable and green process for synthesis of alternative carbon carrier from waste halogenated plastic and its application as carbothermic reductant in lithium recovery.

Alkali-catalyzed hydrothermal oxidation treatment of triclosan in soil: Mechanism, degradation pathway and toxicity evaluation.

The continuous accumulation of chlorinated organic pollutants in soil poses a potential threat to ecosystems and human health alike. Alkali-catalyzed hydrothermal oxidation (HTO) can successfully remove chlorinated organic pollutants from water, but it is rarely applied to soil remediation. In this work, we assessed this technique to degrade and detoxify triclosan (TCS) in soil and we determined the underlying mechanisms. The results showed a dechlorination efficiency of TCS (100 mg per kg soil) of 49.03 % after 120 min reaction (H2O2/soil ratio 25 mL·g-1, reaction temperature 180 °C in presence of 1 g·L-1 NaOH). It was found that soil organic constituents (humic acid, HA) and inorganic minerals (SiO2, Al2O3, and CaCO3) suppressed the dechlorination degradation of TCS, with HA having the strongest inhibitory effect. During alkali-catalyzed HTO, the TCS molecules were effectively destroyed and humic acid-like or fulvic acid-like organics with oxygen functional groups were generated. Fluorescence spectroscopy analysis showed that hydroxyl radicals (OH) were the dominant reactive species of TCS degradation in soil. On the basis of the Fukui function and the degradation intermediates, two degradation pathways were proposed. One started with cleavage of the ether bond between the benzene rings of TCS, followed by dechlorination and the opening of benzene via oxidation. The other pathway started with direct hydroxylation of the benzene rings of TCS, after which they were opened and dechlorinated through oxidation. Analysis of the soil structure before and after treatment revealed that the soil surface changed from rough to smooth without affecting soil surface elements. Finally, biotoxicity tests proved that alkali-catalyzed HTO effectively reduced the toxicity of TCS-contaminated soil. This study suggests that alkali-catalyzed hydrothermal oxidation provides an environmentally friendly approach for the treatment of soil contaminated with chlorinated organics such as TCS.

Activation or sequestration of heavy metals during hydrothermal process of swine manure: Interactions among metal species and particulates.

Whether the heavy metals in solid biomass is activated or sequestrated during hydrothermal process (HTP) is still debated. Herein, the speciation of light and heavy metals during HTP of swine manure (SM) was investigated to reveal the interactions among these metal species and specific particulates. With increasing temperature, most of exchangeable species and that bound to carbonates were released to liquid phase via ion exchange and acid dissolution. Dissociation of Fe-Mn oxides rarely happened in spite of anoxic atmosphere formed during HTP. Substantial decomposition of lignocelluloses hardly caused significant liberation of fraction bound to organics. Instead, a part of fraction in liquid phase was re-captured by new oxygen-containing functional groups on solid product surface to form fraction bound to organics. Donpeacorite, butschliite and iwakiite were formed as primary minerals, resulting in increase of residual fraction of all metals except for K and Mg at 250 °C. In summary, Cu, Zn and Pb species evolution was affected by speciation of K, Ca, Mg, Fe and Mn significantly. Cu, Zn, Pb, Fe, Mn and Ca were sequestrated whereas K and Mg were activated with enhancing temperature during HTP in terms of their mobility factors.

Application of sequential extraction analysis to Pb(II) recovery by zerovalent iron-based particles.

Zerovalent iron (ZVI) is an environmental-friendly reactive reagent for recovering heavy metals. However, the detailed recovery mechanism remains unclear due to a lack of quantitative analysis of recovery products. Herein, microscale ZVI, nanoscale ZVI and Ni/Fe nanoparticles were used to recover Pb(II) in aqueous solution and a sequential extraction procedure (SEP) was applied to determine the formed lead species quantitatively. At high initial Pb(II) concentration (500 mg L-1), more than 99.5% of Pb(II) was immobilized by Ni/Fe and n-ZVI, whereas m-ZVI caused inferior recovery efficiency (<25%). XRD and XPS results revealed that Pb(II) was reduced to Pb0 prior to the formation of metal hydroxides as the external shell of ZVI. SEP results showed that the fraction bound to carbonates (PbO), fraction bound to iron oxides and exchangeable fraction were the main lead species conducted by Ni/Fe, n-ZVI and m-ZVI, respectively. Consequently, (co-)precipitation and specific adsorption dominated Pb(II) recovery by Ni/Fe and n-ZVI, whereas m-ZVI conducted Pb(II) recovery mainly via weak adsorption. The reactivity of ZVI toward Pb(II) followed the increasing order of m-ZVI << n-ZVI ≤ Ni/Fe. The detailed mechanisms of Pb(II) recovery conducted by different ZVI were proposed.

Electrokinetic removal of Cu and Zn in anaerobic digestate: interrelation between metal speciation and electrokinetic treatments.

In recent years, a potential controversy has arisen that whether the metal speciation in solid matrix determined its electrokinetic (EK) removal efficiency or by contrast. In present study, Cu and Zn in anaerobic digestate were selected as candidates to investigate the relation between the species of metal and EK treatment. The obtained results show that the removal efficiency for each fraction decreased in the order as follows: exchangeable ≥ bound to carbonates > bound to Fe-Mn oxides>bound to organic matters >> residual. For both Cu and Zn, their total removal performance was dependent on their dominant fraction in the digestate. A constant pH maintenance around the digestate via circulation of acid electrolyte is an optional operation because a strong acid atmosphere (pH < 2) around the digestate can be formed automatically as EK time elapses. Despite that many reactions occurred during EK process, the species distribution of Cu and Zn in the digestate determined their total EK removal efficiency essentially.

Nutrition potential of biogas residues as organic fertilizer regarding the speciation and leachability of inorganic metal elements.

Biogas residues (BRs) are prospective organic fertilizer sources for agricultural cultivation. Besides N and P, however, other inorganic metal elements, such as K, Fe, Cu, Zn and so on, also affect the nutritional level of BRs significantly. In this study, a sequential extraction procedure (SEP) combined with a toxicity characteristic leaching procedure (TCLP) was conducted to investigate the speciation and leachability of metal components in BRs. The results showed that element K was the most effective nutrient component due to its largest available fraction and highest mobility factor (MF) of 78.4, whereas phytotoxic Al was the most stable and inert element in terms of its 96.68% residual fraction. Ca and Mg could be viewed as potential nutrient sources because their MFs exceeded 60. TCLP results revealed that these BRs could be classed as non-toxic organic waste but Cu and Zn should be paid more attention in that their total contents were beyond the permissible values. Meanwhile, more concerns should be given to Ni and Pb due to their large TCLP extractable fraction. In conclusion, these BRs can be used as a prospective nutrient pool for agricultural cultivation. SEP combined with TCLP can be effectively applied for assessing the nutrient level of the BRs as organic fertilizer for agricultural use.

Sequential extraction of anaerobic digestate sludge for the determination of partitioning of heavy metals.

In China, agricultural use of anaerobic digestate sludge is considered a concern due to high heavy metal content of the sludge. In this study, sequential extraction procedure (SEP) was conducted to determine metal speciation which affects release and mobility of metal significantly. The results of SEP showed that each heavy metal possessed different distribution characteristics. Cu mainly reacted with carboxyl functional group to form the fraction bound to organic matter. Zn and Mn were dominated in the fraction bound to Fe-Mn oxides and carbonates, respectively. Pb, Ni, Cr, Cd and As were present as the residual fraction. Examination of mobility factors (MFs) indicated that Zn, Pb, Ni, Mn and Cd were more mobile whereas Cr and As were immobilized in anaerobic digestate. Based on the results, it can be stated that Cu, Zn, Mn, Ni and Cd may be grouped as toxic and active components in sludge and should be regarded as the priority pollutants for elimination. Pb should be monitored in terms of its high mobility factors (MF). Cr and As, nevertheless, were the most stable components in sludge.