Sorption of α-, β-, γ- and δ-hexachlorocyclohexane isomers to three widely different biochars: Sorption mechanisms and application

What potential do mosses have as biomonitors of POPs? A comparative study of hexachlorocyclohexane sorption

Persistent organic pollutants (POPs) pose a significant global threat to human health and the environment, and require continuous monitoring due to their ability to migrate long distances. Active biomonitoring using cloned mosses is an inexpensive but underexplored method to assess POPs, mainly due to the poor understanding of the loading mechanisms of these pollutants in mosses. In this work, Fontinalis antipyretica (aquatic moss) and Sphagnum palustre (terrestrial moss) were evaluated as potential biomonitors of hexachlorocyclohexanes (HCHs: α-, β-, γ-, δ-HCH), crucial POPs. Moss clones, grown in photobioreactors and subsequently oven-dried, were used. Their lipid composition and distribution were characterized through molecular and histochemical studies. Adsorption experiments were carried out in the aqueous phase using the repeated additions method and in the gas phase using an active air sampling technique based on solid-phase extraction, a pioneering approach in moss research. F. antipyretica exhibited greater lipid content in the walls of most cells and higher adsorption capacity for all HCH isomers in both gaseous and liquid environments. These findings highlight the need for further investigation of POP loading mechanisms in mosses and open the door to explore other species based on their lipid content.

Digestate-derived carbonized char and activated carbon: Application perspective

The flourishment of anaerobic digestion (AD) on waste treatment emphasizes the importance of digestate valorization, which plays an essential role in determining the benefits provided by the AD process. The perception of digestate gradually shifts from waste to products to realize the concept of circular economy and maximize the benefits of digestate valorization. This review first outlined the current status of digestate valorization, focusing on thermal-chemical methods. The novel valorization methods were then summarized from the recent research, illustrating prospects for digestate valorization. Limits and perspectives are finally addressed. Methods for preparing digestate-derived activated carbon and impurity effects were elucidated. Inherent mineral content/inorganic impurity could be a niche for downstream use. High surface area and well-developed pore structure are essential for satisfying downstream use performance, but they are not the only factors. Digestate char applications other than use as an energy fuel are suggested.

Improved understanding on biochar effect in electron supplied anaerobic soil as evidenced by dechlorination and methanogenesis processes

Research interest in biochar as an environmental remediation material has rapidly increased over the past few years. However, the effect of biochar on typical environmental processes in anaerobic soil environment has been insufficiently discussed. By regulating the electron donors with sodium acetate or pyruvate, the effects and underpinning chemical-microbiological coupling mechanisms of biochar under anaerobic conditions were disclosed. Unlike the electron limited condition, the addition of electron donors alleviated the competition for electrons among various reduction processes in the soil. The effect of biochar in regulating the electron transfer processes was lessened. But more than doubled methane emissions were resulted by the exogenous substances, especially with the synergic effect of biochar. Biochar addition increased soil environmental heterogeneity. It might indirectly affect the reductive transformation of γ-HCH via increasing the bioavailability of pollutants through adsorption and promoting the metabolism of some rare microorganisms. Anaerolineaceae, Peptococcaceae and Methanosarcina had coherent phylogenetic patterns and were likely to be the enablers for the reductive dechlorination process in flooded soil. ENVIRONMENTAL IMPLICATION: Previous studies have widely reported the performance characteristics of biochar, but its effects under anaerobic environments are not systematically understood. By regulating the electron donors, the competition for electrons among various reduction processes in the soil might be alleviated, resulting in a lessened effect of biochar in regulating the electron transfer processes. The findings presented in this study highlight the role of biochar to the dynamic changes of reduction processes under anaerobic environments. The relevant soil conditions such as the electron donors and the functional microbial groups should be adequately considered for maximizing the all-around beneficial efficiency of biochar amendments.

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

Lindane is a persistent organic pollutant that has attracted worldwide attention because of its threat to human health and environmental security. A horizontal planetary mechanochemical method was developed for rapid and efficient degradation of lindane in soil in an alkaline environment. Under the condition of a very low reagent-to-soil ratio (R = 2%), ball-to-powder ratio (C_R = 6:1), rotation speed (r = 300 rpm) and high soil single treatment capacity (S_C = 60 g), the lindane in four typical soils (~ 100 mg/kg) can be degraded up to 96.30% in 10 min. This method can also remediate high-concentration lindane-contaminated soil (833 ± 26 mg/kg). The experimental results and theoretical calculations proved that the stepwise dechlorination and final carbonization of lindane in soil are mainly attributed to the combined action of mechanical energy and alkalinity. The bimolecular elimination (E2) reaction was the first step of lindane destruction. Subsequently, the unimolecular elimination (E1) reaction tended to occur with the weakening of alkalinity. Then, benzene was obtained through stepwise hydrogenolysis reaction. The last was the generation of carbon substances by fragmentation or condensation of benzene rings. This work proposes a practical remediation technology for organic contaminated soil and improves the understanding of the degradation pathways of lindane in soil in alkali-assisted mechanochemical system.

Biochars and activated carbons as adsorbents of inorganic and organic compounds from multicomponent systems - A review

Biochars are obtained by biomass pyrolysis, whereas activated carbon is a biochar that has undergone chemical or physical activation. Owing to the large surface area and easy surface modification both solids are widely applied as adsorbents. They are low-costs materials, they could be regenerated and their disposal is not troublesome. Adsorption of heavy metals, dyes, pharmaceuticals on the surface of biochars and activated carbons, from simple systems of adsorbate containing only one compound, are described extensively in the literature. The present paper provides an overview of reports on adsorption of inorganic and organic compounds onto these two types of adsorbents from the mixed adsorbate systems. The described adsorbate systems have been divided into those consisting of: two or more inorganic ions, two or more organic compounds and both of them (inorganic and organic ones). The research of this type is carried out much less frequently due to the more complicated description of interactions in the mixed adsorbate systems.

HCH Removal in a Biochar-Amended Biofilter

This study evaluated the efficiency of two biofilter systems, with and without biochar chambers installed, at degrading and removing HCH and its isomers in natural drainage water. The biochar biofilter proved to be 96% efficient at cleaning HCH and its transformation products from drainage water, a significant improvement over classic biofilter that remove, on average, 68% of HCH. Although iron- and sulfur-oxidizing bacteria, such as Gallionella and Sulfuricurvum, were dominant in the biochar bed outflows, they were absent in sediments, which were rich in Simplicispira, Rhodoluna, Rhodoferax, and Flavobacterium. The presence of functional genes involved in the biodegradation of HCH isomers and their byproducts was confirmed in both systems. The high effectiveness of the biochar biofilter displayed in this study should further encourage the use of biochar in water treatment solutions, e.g., for temporary water purification installations during the construction of other long-term wastewater treatment technologies, or even as final solutions at contaminated sites.

A critical review on the formation, fate and degradation of the persistent organic pollutant hexachlorocyclohexane in water systems and waste streams

The environmental impacts of persistent organic pollutants (POPs) is an increasingly prominent topic in the scientific community. POPs are stable chemicals that are accumulated in living beings and can act as endocrine disruptors or carcinogens on prolonged exposure. Although efforts have been taken to minimize or ban the use of certain POPs, their use is still widespread due to their importance in several industries. As a result, it is imperative that POPs in the ecosystem are degraded efficiently and safely in order to avoid long-lasting environmental damage. This review focuses on the degradation techniques of hexachlorocyclohexane (HCH), a pollutant that has strong adverse effects on a variety of organisms. Different technologies such as adsorption, bioremediation and advanced oxidation process have been critically analyzed in this study. All 3 techniques have exhibited near complete removal of HCH under ideal conditions, and the median removal efficiency values for adsorption, bioremediation and advanced oxidation process were found to be 80%, 93% and 82% respectively. However, it must be noted that there is no ideal HCH removal technique and the selection of removal method depends on several factors. Furthermore, the fates of HCH in the environment and challenges faced by HCH degradation have also been explained in this study. The future scope for research in this field has also received attention.

Role of nanocatalyst in the treatment of organochlorine compounds - A review

Since a few centuries ago, organochlorine compounds (OCs) become one of the threatened contaminants in the world. Due to the lipophilic and hydrophobic properties, OCs always discover in fat or lipid layers through bioaccumulation and biomagnification. The OCs are able to retain in soil, sediment and water for long time as it is volatile, OCs will evaporate from soil and condense in water easily and frequently, which pollute the shelter of aquatic life and it affects the function of organs and damage system in human body. Photocatalysis that employs the usage of semiconductor nanophotocatalyst and solar energy can be the possible alternative for current conventional water remediation technologies. With the benefits of utilizing renewable energy, no production of harmful by-products and easy operation, degradation of organic pollutants in rural water bodies can be established. Besides, nanophotocatalyst that is synthesized with nanotechnology outnumbered conventional catalyst with larger surface area to volume ratio, thus higher photocatalytic activity is observed. In contrast, disadvantages particularly no residual effect in water distribution network, requirement of post-treatment and easily affected by various factors accompanied with photocatalysis method cannot be ignored. These various factors constrained the photocatalytic efficiency via nanocatalysts which causes the full capacity of solar photocatalysis has yet to be put into practice. Therefore, further modifications and research are still required in nanophotocatalysts' synthesis to overcome limitations such as large band gaps and photodecontamination.

Valorization of anaerobic digestion digestate: A prospect review

Anaerobic digestion is recognized as promising technology for bioenergy production from biowaste, with huge quantity of digestate being produced as the residual waste. The digestate contains substantial amounts of organic and inorganic matters that be considered highly risky contaminants to the receiving environments if not properly treated, but also potential renewable resources if are adequately recovered. This prospect review summarized the current research efforts on digestate valorization, including aspects of resource recovery and the proposed applications, particularly on the conversion techniques and economic feasibility. The prospects for digestate valorization were highlighted at the end of this review.

Biochars as media for air pollution control systems: Contaminant removal, applications and future research directions

Biochars are low-cost and renewable biomaterials with several applications, including soil amendment, mitigation of greenhouse gas emissions, and removal of both inorganic and organic contaminants in aqueous systems. An increasing body of recent evidence indicates that biochars can also remove gaseous chemical contaminants, such as those occurring in industrial flue gases. However, unlike other applications such as in agroecosystems, soil amendments, and aquatic systems, comprehensive reviews on biochar applications in the field of air pollution control are still lacking. The current paper examined existing evidence to understand the nature of contaminants, particularly the gaseous ones, potential applications, constraints, and future research needs pertaining to biochar applications in air pollution control. The preparation of biochars and their functionalized derivatives, and the properties influencing their capacity to remove gaseous contaminants are summarized. The removal capacity and mechanisms of various organic and inorganic gaseous contaminants by biochars are discussed. Evidence shows that biochars effectively remove metal vapours, particularly elemental mercury (Hg⁰), acidic gases (H₂S, SO₂, CO₂), ozone, nitrogen oxides (NO_x), and organic contaminants including aromatic compounds, volatile organic compounds, and odorous substances. The mechanisms for the removal of gaseous contaminants, including; adsorption, precipitation, and size exclusion were presented. Potential industrial application domains include remediation of gaseous emissions from incinerators, waste-to-energy systems, kilns, biomass and coal-fired boilers/cookers, cremation, smelters, wastewater treatment, and agricultural production systems including livestock husbandry. These industrial applications, coupled with the renewable, low-cost and sustainable nature of biochars, point to opportunities to further develop and scale up the biochar technology in the air pollution control industry. However, the biochar-based air filter technology still faces several challenges, largely stemming from constraints and several knowledge gaps, which were highlighted. Hence, further research is required to address these constraints and knowledge gaps before the benefits of the biochar-based air filters are realized.

Nanoscale zero-valent iron reduction coupled with anaerobic dechlorination to degrade hexachlorocyclohexane isomers in historically contaminated soil

Hexachlorocyclohexane (HCH) isomers pose potential threats to the environment and to public health due to their persistence and high toxicity. In this study, nanoscale zero-valent iron (nZVI) coupled with microbial degradation by indigenous microorganisms with and without biostimulation was employed to remediate soils highly polluted with HCH. The degradation efficiency of total HCHs in both the "nZVI-only" and "Non-amendment" treatments was approximately 50 %, while in the treatment amended with nZVI and acetate, 85 % of total HCHs was removed. Addition of nZVI and acetate resulted in enrichment of anaerobic microorganisms. The results of quantitative PCR (qPCR) and 16S rRNA gene amplicon sequencing revealed that Desulfotomaculum, Dehalobacter, Geobacter, and Desulfuromonas likely contributed to the depletion of HCH isomers. Moreover, some abiotic factors also favored this removal process, including pH, and the generation of iron sulfides as revealed by the result of Mössbauer spectrometer analysis. Our research provides an improved remediation strategy for soils polluted with HCH isomers and an understanding of the synergistic effect of nZVI and indigenous microorganisms.

Biochar efficacy for reducing heavy metals uptake by Cilantro (Coriandrum sativum) and spinach (Spinaccia oleracea) to minimize human health risk

Environmentally friendly and cost-effective techniques are required to reclaim land degraded during mining activities. Bioaccumulation of heavy metals (HMs) in vegetables grown on contaminated soils can increase human health risks. The potential effects of hardwood biochar (HWB) was assessed for chromium (Cr), zinc (Zn), copper (Cu), manganese (Mn) and lead (Pb) bioavailability in mine-contaminated soils and their subsequently bioaccumulation in crops and associated health risk. HWB was applied to chromium-manganese mine contaminated soils at the rate of 3% to investigate the efficiency of HWB for the second crop in crop rotation technique. Cilantro (Coriandrum sativum) and spinach (Spinaccia oleracea) were grown as second crop in the same pots which were already used for rice cultivation as first crop (without adding further amendments). Application of HWB decreased the concentrations of Cr, Zn, Cu, Mn, and Pb in cilantro by 25.5%, 37.1%, 42.5%, 34.3%, and 36.2%, respectively as compared to control. In spinach, the reduction in concentrations of Cr was 75.0%, Zn 24.1%, Cu 70.1%, Mn 78.0%, and Pb 50.5% as compared to control. HWB significantly (P < 0.01) reduced the HMs uptake in spinach cultivated in the amended soils as compared to the spinach in control. Bioaccumulation factor results also indicate that HWB decreased the bioaccumulation of selected HMs in cilantro and spinach, thus reducing health risks. Results of the study clearly demonstrate that the use of HWB can significantly reduce HMs in vegetables, associated health risk and improve food quality, therefore can be used as soil amendment for reclamation of mine-degraded soils.

A Review of Non-Soil Biochar Applications

Biochar is the solid residue that is recovered after the thermal cracking of biomasses in an oxygen-free atmosphere. Biochar has been used for many years as a soil amendment and in general soil applications. Nonetheless, biochar is far more than a mere soil amendment. In this review, we report all the non-soil applications of biochar including environmental remediation, energy storage, composites, and catalyst production. We provide a general overview of the recent uses of biochar in material science, thus presenting this cheap and waste-derived material as a high value-added and carbonaceous source.

Partitioning of chlorinated organic compounds from dense non-aqueous phase liquids and contaminated soils from lindane production wastes to the aqueous phase

Hexachlorocyclohexane (HCH) and mainly the γ-HCH isomer, namely lindane, were extensively produced and used as pesticides. Huge amounts of wastes, solids and liquids, were disposed of in the surroundings of the production sites. The liquid residuum was a complex mixture of chlorinated organic compounds, COCs, from chlorobenzene to heptachlorocyclohexane. This Dense Non-Aqueous Phase Liquid, DNAPL, migrated by density through the subsurface to greater depths, being trapped or adsorbed into the soil in this movement posing a significant risk to the groundwater. Knowledge of the partitioning in water of COCs in DNAPL is a key issue to determine its fate in the environment. However, there are no data in literature for the partitioning and/or solubility of many of the COCs in this DNAPL, such as pentachlorocyclohexene, hexachlorocyclohexene and heptachlorocyclohexane despite them constitute about 13-30% of the mole fraction of the DNAPLs. In this work, the partitioning to water of COCs in free and those adsorbed onto soil has been studied. In addition, measured and predicted aqueous concentrations of each COC in the DNAPL mixture have been compared. To do this, the solubility of a compound that is a solid crystal when pure at P = 298 K and P = 1 atm has been evaluated considering the approach of sub-cooled liquid state of solid organochlorines. Samples were obtained at Sabiñanigo landfills and soils used had several grain sizes. Transformation in alkaline media of COCs had a positive environmental impact.

Chemical oxidation and reduction of hexachlorocyclohexanes: A review

Lindane (γ-hexachlorocyclohexane) and its isomers (HCH) are some of the most common and most easily detected organochlorine pesticides in the environment. The widespread distribution of lindane is due to its use as an insecticide, accompanied by its persistence and bioaccumulation, whereas HCH were disposed of as waste in unmanaged landfills. Unfortunately, certain HCH (especially the most reactive ones: γ- and α-HCH) are harmful to the central nervous system and to reproductive and endocrine systems, therefore development of suitable remediation methods is needed to remove them from contaminated soil and water. This paper provides a short history of the use of lindane and a description of the properties of HCH, as well as their determination methods. The main focus of the paper, however, is a review of oxidative and reductive treatment methods. Although these methods of HCH remediation are popular, there are no review papers summarising their principles, history, advantages and disadvantages. Furthermore, recent advances in the chemical treatment of HCH are discussed and risks concerning these processes are given.