Integrated soil washing and bioreactor systems for the treatment of hexachlorocyclohexane contaminated soil: A review on enhanced degradation mechanisms, and factors affecting soil washing and bioreactor performances

Full-chain analysis on emerging contaminants in soil: Source, migration and remediation

Emerging contaminants (ECs) are gaining attention due to their prevalence and potential negative impacts on the environment and human health. This paper provides a comprehensive review of the status and trends of soil pollution caused by ECs, focusing on their sources, migration pathways, and environmental implications. Significant ECs, including plastics, synthetic polymers, pharmaceuticals, personal care products, plasticizers, and flame retardants, are identified due to their widespread use and toxicity. Their presence in soil is attributed to agricultural activities, urban waste, and wastewater irrigation. The review explores both horizontal and vertical migration pathways, with factors such as soil type, organic matter content, and moisture levels influencing their distribution. Understanding the behavior of ECs in soil is critical to mitigating their long-term risks and developing effective soil remediation strategies. The paper also examines the advantages and disadvantages of in situ and ex situ treatment approaches for ECs, highlighting optimal physical, chemical, and biological treatment conditions. These findings provide a fundamental basis for addressing the challenges and governance of soil pollution induced by ECs.

Research progress on remediation of organochlorine pesticide contamination in soil

Deteriorated soil pollution has grown into a worldwide environmental concern over the years. Organochlorine pesticide (OCP) residues, featured with ubiquity, persistence and refractoriness, are one of the main pollution sources, causing soil degradation, fertility decline and nutritional imbalance, and severely impacting soil ecology. Furthermore, residual OCPs in soil may enter the human body along with food chain accumulation and pose a serious health threat. To date, many remediation technologies including physicochemical and biological ways for organochlorine pollution have been developed at home and abroad, but none of them is a panacea suitable for all occasions. Rational selection and scientific decision-making are grounded in in-depth knowledge of various restoration techniques. However, soil pollution treatment often encounters the interference of multiple factors (climate, soil properties, cost, restoration efficiency, etc.) in complex environments, and there is still a lack of systematic summary and comparative analysis of different soil OCP removal methods. Thus, to better guide the remediation of contaminated soil, this review summarized the most commonly used strategies for OCP removal, evaluated their merits and limitations and discussed the application scenarios of different methods. It will facilitate the development of efficient, inexpensive and environmentally friendly soil remediation strategies for sustainable agricultural and ecological development.

Soil properties and organochlorine compounds co-shape the microbial community structure: A case study of an obsolete site

Organochlorine compounds (OCs) such as chlorobenzenes (CB) are persistent organic pollutants that are ubiquitous in soils at organochlorine pesticides (OCP) production sites. Long-term contamination with OCs might alter the soil microbial structure and further affect soil functions. However, the effects of OCs regarding the shaping of microbial community structures in the soils of OCs-contaminated sites remain obscure, especially in the vertical soil profile where pollutants are highly concealed. Hence this paper explored the status and causes of OCs pollution (CB, hexachlorocyclohexane (HCH), and dichlorodiphenyltrichloroethane (DDT)) in an obsolete site, and its combined effects with soil properties (pH, available phosphorus (AP), dissolved organic carbon (DOC), etc) on microbial community structure. The mean total concentration of OCs in the subsoils was up to 996 times higher than that in the topsoils, with CB constituting over 90% of OCs in the subsoil. Historical causes, anthropogenic effects, soil texture, and the nature of OCs contributed to the differences in the spatial distribution of OCs. Redundancy analysis revealed that both the soil properties and OCs were important factors in shaping microbial composition and diversity. Variation partitioning analysis further indicated that soil properties had a greater impact on microbial community structure than OCs. Significant differences in microbial composition between topsoils and subsoils were observed through linear discriminant analysis effect size (LEfSe) analysis, primarily driven by different pollutant conditions. Additionally, co-occurrence network analysis indicated that heavily contaminated subsoils exhibited closer and more intricate bacterial community interactions compared to lightly contaminated topsoils. This work reveals the impact of environmental factors in co-shaping the structure of soil microbial communities. These findings advance our understanding of the intricate interplay among organochlorine pollutants, soil properties, and microbial communities, and provides valuable insights into devising effective management strategies in OCs-contaminated soils.

Reduced attention on restricted organochlorine pesticides, whereas still noteworthy of the impact on the deep soil and groundwater: a historical site study in southern China

The use of hexachlorocyclohexanes (HCHs) in pesticides has been prohibited for decades in China. Since then, there have been urbanization and transformation of the functional areas of many sites, which were formerly involved in the HCH industry. However, it is possible that, unless properly managed, these sites may still contain HCH residues in the soil and thus pose a threat to the surrounding environment and the quality of groundwater. This study aimed to characterize soil residues in a typical site that was historically involved in HCH production in southern China, by analyzing the α-HCH, β-HCH, and γ-HCH contents of the soil. The results suggested that HCHs persist in the environment and can have long-term effects. It was found that α-HCH and β-HCH were present in many samples in concentrations that were comparable or higher than those specified by China's Class 1 screening values. The distribution of residues was significantly correlated with the historical HCH production activities in the areas. The characteristic ratios of α-HCH/γ-HCH and β-HCH/(α + γ)-HCH at different soil depths were 1.4-3.7 and 0.21-1.04, respectively, which indicated the presence of significant localized residues of HCHs. The presence of HCHs in the soil suggested a downward migration, with concentrations rapidly decreasing in the upper layer soil (0-5 m), but a gradual increase in the deeper soil (5-14 m). HCHs were detected at depths exceeding 24 m, indicating heavy penetration. The proportions of γ-HCH and β-HCH changed with increasing soil depth, which was related to their relatively volatile and stable molecular structures, respectively. The results strongly suggested that there is widespread contamination of both soil and groundwater by HCHs even after decades. The likelihood of residual HCHs in the soil should therefore be taken into full consideration during urban planning to limit risks to human and environmental health.

Optimisation of soil washing method for removal of petroleum hydrocarbons from contaminated soil around oil storage tanks using response surface methodology

Total petroleum hydrocarbons (TPHs), which are often found in soil, water, sediments, and air. These compounds are a type of pollutant that can have a serious negative impact on living things and human health. Soil washing method is a remediation technique used to remove contaminants from the soil. This process involves the use of water or other solvents to extract contaminants from the soil, followed by separation and disposal of the contaminated solution. This research engineered the effectiveness of soil washing method to remove TPHs from a genuine, sullied soil sample. After analyzing the physical and chemical properties of the soil, the Box-Benken Design (BBD) technique was used to optimize the variables that influence the process's effectiveness. A quadratic model was suggested based on the BBD design, correlation coefficients, and other factors. The minimum, maximum and mean removal of TPHs during the stages of the study were 63.5, 94.5 and 76.7%, respectively. The correlation between the variables was strong, as shown by the analysis of variance (ANOVA), F-value (1064.5) and P-value (0.0001), and the proposed model was highly significant. The most effective soil washing method (SWM) was obtained with pH 7.8, liquid to solid ratio 50:1, reaction time 52 min, surfactant concentration 7.9 mg kg-1, and three washings. A removal rate of 98.8% was accomplished for TPHs from the soil in this context. The kinetic results indicate that the kinetic of TPHs removal follows the first-order kinetics (R2 = 0.96). There was not a major difference in the process's efficiency based on temperature. The removal efficiency heightened from 0 to 150 rpm and then remained steady. Introducing air flow increased the rate of removal, and the combination of ultrasonic waves with the reaction environment increased the process efficiency and decreased the time for the process and the amount of times it needed to be washed. An analysis of the washed soil both physically and chemically revealed a substantial decrease in the concentration of other elements.

Effect of surfactants on the sorption-desorption, degradation, and transport of chlorothalonil and hydroxy-chlorothalonil in agricultural soils

The fungicide chlorothalonil (CTL) and its metabolite hydroxy chlorothalonil (OH-CTL) constitute a risk of soil and water contamination, highlighting the need to find suitable soil remediation methods for these compounds. Surfactants can promote the bioavailability of organic compounds for enhanced microbial degradation, but the performance depends on soil and surfactant properties, sorption-desorption equilibria of contaminants and surfactants, and possible adverse effects of surfactants on microorganisms. This study investigated the influence of five surfactants [e.g., Triton X-100 (TX-100), sodium dodecyl sulphate (SDS), hexadecyltrimethylammonium bromide (HDTMA), Aerosol 22 and Tween 80] on the sorption-desorption, degradation, and mobility of CTL and OH-CTL in two volcanic and one non-volcanic soil. Sorption and desorption of fungicides depended on the sorption of surfactants on soils, surfactants' capacity to neutralize the net negative charge of soils, surfactants' critical micellar concentration, and pH of soils. HDTMA was strongly adsorbed on soils, which shifted the fungicide sorption equilibria by increasing the distribution coefficient (K_d) values. Contrarily, SDS and TX-100 lowered CTL and OH-CTL sorption on soils by decreasing the K_d values, which resulted in an efficient extraction of the fungicide compounds from soil. SDS increased the degradation of CTL, especially in the non-volcanic soil (DT₅₀ values were 14 and 7 days in natural and amended soils, with final residues <7% of the initial dose), whereas TX-100 enabled an early start and sustenance of OH-CTL degradation in all soils. CTL and OH-CTL stimulated soil microbial activities without noticeable deleterious effects of the surfactants. SDS and TX-100 also reduced the vertical transport of OH-CTL in soils. Results of this study could be extended to soils in other regions of the world because the tested soils represent widely different physical, chemical, and biological properties.

Polyaromatic hydrocarbons (PAHs) in the water environment: A review on toxicity, microbial biodegradation, systematic biological advancements, and environmental fate

Polycyclic aromatic hydrocarbons (PAHs) are considered a major class of organic contaminants or pollutants, which are poisonous, mutagenic, genotoxic, and/or carcinogenic. Due to their ubiquitous occurrence and recalcitrance, PAHs-related pollution possesses significant public health and environmental concerns. Increasing the understanding of PAHs' negative impacts on ecosystems and human health has encouraged more researchers to focus on eliminating these pollutants from the environment. Nutrients available in the aqueous phase, the amount and type of microbes in the culture, and the PAHs' nature and molecular characteristics are the common factors influencing the microbial breakdown of PAHs. In recent decades, microbial community analyses, biochemical pathways, enzyme systems, gene organization, and genetic regulation related to PAH degradation have been intensively researched. Although xenobiotic-degrading microbes have a lot of potential for restoring the damaged ecosystems in a cost-effective and efficient manner, their role and strength to eliminate the refractory PAH compounds using innovative technologies are still to be explored. Recent analytical biochemistry and genetically engineered technologies have aided in improving the effectiveness of PAHs' breakdown by microorganisms, creating and developing advanced bioremediation techniques. Optimizing the key characteristics like the adsorption, bioavailability, and mass transfer of PAH boosts the microorganisms' bioremediation performance, especially in the natural aquatic water bodies. This review's primary goal is to provide an understanding of recent information about how PAHs are degraded and/or transformed in the aquatic environment by halophilic archaea, bacteria, algae, and fungi. Furthermore, the removal mechanisms of PAH in the marine/aquatic environment are discussed in terms of the recent systemic advancements in microbial degradation methodologies. The review outputs would assist in facilitating the development of new insights into PAH bioremediation.

Recent Advances in the Biocatalytic Mitigation of Emerging Pollutants: A Comprehensive Review

The issue of environmental pollution has been worsened by the emergence of new contaminants whose morphology is yet to be fully understood. Several techniques have been adopted to mitigate the pollution effects of these emerging contaminants, and bioremediation involving plants, microbes, or enzymes has stood out as a cost-effective and eco-friendly approach. Enzyme-mediated bioremediation is a very promising technology as it exhibits better pollutant degradation activity and generates less waste. However, this technology is subject to challenges like temperature, pH, and storage stability, in addition to recycling difficulty as it is arduous to isolate them from the reaction media. To address these challenges, the immobilization of enzymes has been successfully applied to ameliorate the activity, stability, and reusability of enzymes. Although this has significantly increased the uses of enzymes over a wide range of environmental conditions and facilitated the use of smaller bioreactors thereby saving cost, it still comes with additional costs for carriers and immobilization. Additionally, the existing immobilization methods have their individual limitations. This review provides state-of-the-art information to readers focusing on bioremediation using enzymes. Different parameters such as: the sustainability of biocatalysts, the ecotoxicological evaluation of transformation contaminants, and enzyme groups used were reviewed. The efficacy of free and immobilized enzymes, materials and methods for immobilization, bioreactors used, challenges to large-scale implementation, and future research needs were thoroughly discussed.

Analysis of the Degradation of OCPs Contaminated Soil by the BC/nZVI Combined with Indigenous Microorganisms

Organochlorine pesticides (OCPs) were typical persistent organic pollutants that posed great hazards and high risks in soil. In this study, a peanut shell biochar-loaded nano zero-valent iron (BC/nZVI) material was prepared in combination with soil indigenous microorganisms to enhance the degradation of α-hexachlorocyclohexane(α-HCH) and γ-hexachlorocyclohexane(γ-HCH) in water and soil. The effects of BC/nZVI on indigenous microorganisms in soil were investigated based on the changes in redox potential and dehydrogenase activity in the soil. The results showed as follows: (1) The specific surface area of peanut shell biochar loaded with nano-zero-valent iron was large, and the nano-zero-valent iron particles were evenly distributed on the peanut shell biochar; (2) peanut shell BC/nZVI had a good degradation effect on α-HCH and γ-HCH in water, with degradation rates of 64.18% for α-HCH and 91.87% for γ-HCH in 24 h; (3) peanut shell BC/nZVI also had a good degradation effect on α-HCH and γ-HCH in soil, and the degradation rates of α-HCH and γ-HCH in the 1% BC/nZVI reached 55.2% and 85.4%, second only to 1% zero-valent iron. The degradation rate was the fastest from 0 to 7 days, while the soil oxidation-reduction potential (ORP) increased sharply. (4) The addition of BC/nZVI to the soil resulted in a significant increase in dehydrogenase activity, which further promoted the degradation of HCHs; the amount of HCHs degradation was significantly negatively correlated with dehydrogenase activity. This study provides a remediation strategy for HCH-contaminated sites, reducing the human health risk of HCHs in the soil while helping to improve the soil and increase the activity of soil microorganisms.

Surfactant recovery and efficient separation of PAHs from surfactant solutions by low-cost waste activated sludge and two-stage design optimization

The treatment and surfactant recovery of soil washing/flushing effluent containing high levels of surfactants and organic pollutants are critical for the surfactant-assisted remediation of soils and waste management due to their complexity and high-potential risks. Combination of waste activated sludge material (WASM) and a kinetic-based two-stage system design was introduced in this study as a novel strategy for the separation of phenanthrene and pyrene from Tween 80 solutions. The results showed that WASM can effectively sorb phenanthrene and pyrene with high affinities (K_d) of 2325.5 L·kg^-1 and 9911.2 L·kg^-1, respectively. This allowed a high-level recovery of Tween 80 of 90.47 ± 1.86%, with selectivity of up to 69.7. In addition, a two-stage design was constructed, and the results showed an improved reaction time (approximately 5% of equilibrium time in conventional single-stage process) and increased the separation levels of phenanthrene or pyrene from Tween 80 solutions. For instance, the minimal total operating time for the sorption of 99% pyrene from 1.0 g·L^-1 Tween 80 was only 23.0 min in the two-stage process compared to that of 480 min with a 71.9% removal level in the single-stage system. Results indicated that the combination of low-cost waste WASH and two-stage design was a high-efficiency and time-saving way to recover surfactants from soil washing effluents.

A review for recent advances on soil washing remediation technologies

Contaminated soils have caused serious harm to human health and the ecological environment due to the high toxicity of organic and inorganic pollutants, which has attracted extensive attention in recent years. Because of its low cost, simple operation and high efficiency, soil washing technology is widely used to permanently remove various pollutants in contaminated soils and is considered to be the most promising remediation technology. This review summarized the recent developments in the field of soil washing technology and discusses the application of conventional washing agents, advanced emerging washing agents, the recycling of washing effluents and the combination of soil washing and other remediation technologies. Overall, the findings provide a comprehensive understanding of soil washing technology and suggest some potential improvements from a scientific and practical point of view.