Utilization of biochar for remediation of heavy metals in aqueous environments: A review and bibliometric analysis

Novel synthesis and application of biochar for controlling release and exposure of mercury in the farmland: From human health risk perspective

Mercury (Hg)-contaminated farmlands have received wide attention because of the adverse risks posed to food security and human health. In addition, climate change altered the mobility of Hg in the soil, limiting soil productivity and nutrient bioavailability, hence elevating health risks. To adapt to these risks, pot experiments were employed to showcase the impacts of single-pyrolytic synthesized biochar with nitrogen and phosphorus impregnation (NPBC) on the nutrient accessibility, Hg immobilization, and human health risks, compared to pristine and control groups. Results revealed that, with increased surface area and abundant function groups, impregnation amplified bulk nitrogen, phosphorus, and oxygen content from 0.47, 0.25, and 9.47 % to 3.01, 4.50, and 21.4 %, respectively. The pot experiments indicated the effectiveness of NPBC900 in immobilizing soil Hg, hence reducing Brassica rapa chinensis' Hg uptake by 88 %. Notably, NPBC transformed ∼93 % of water soluble and exchangeable Hg species to stable fractions, enhancing the residue concentration three-fold higher than the control. Additionally, NPBC700-900 showcased characteristic phosphorus and nitrogen slow-release (best at NPBC900 and NPBC500, respectively; 5 %) contributing to controlled soil available nutrients. Hg bioaccessible fraction exhibited a notably higher level (1.7 mg kg-1) in the control group measured against BC (0.8 mg kg-1) and NPBC treatments (∼0.1 mg kg-1). Through dietary and soil ingestion pathways, NPBC900 treatment demonstrated the best health risk reduction for farmers and the public by ∼93 and 69 %, respectively. With versatile capabilities, NPBC emerges as a practical, green, and sustainable alternative in Hg remedy technologies, a breakthrough for climate change adaptation.

Integrating machine learning and reliability analysis: A novel approach to predicting heavy metal removal efficiency using biochar

Soil contamination with heavy metals (HMs) presents critical environmental and public health risks due to their long-term persistence and tendency to bioaccumulate. Biochar has gained recognition as an effective amendment for HM immobilization, owing to its cost-effectiveness, environmental sustainability, and multifunctional properties. Nevertheless, consistent removal efficiency remains challenging to achieve due to the inherent variability of biochar and its interactions with complex environmental factors. This research introduces an advanced machine learning (ML) framework, utilizing deep forest (DF) algorithms, to predict and optimize the efficiency HM removal through biochar applications. The framework addresses key challenges by employing data imputation to manage missing information, data augmentation to overcome limitations of small datasets, and reliability analysis to assess predictive uncertainties, thereby improving the model's reliability and generalization capability. The findings reveal that the DF model surpasses conventional ML approaches, achieving a testing dataset coefficient of determination (R²) of 0.88. Additionally, probabilistic reliability analysis offers valuable insights into the likelihood of reaching various levels of remediation efficiency (RE). For lower RE thresholds, such as 20-30 %, the model predicts a high probability (over 80 %) of substantial HM removal, confirming biochar's effectiveness in mitigating contamination. However, as the target RE thresholds rise to moderate levels (50-70 %), the probability drops significantly (to below 5 %), highlighting the increasing difficulty of achieving higher remediation efficiencies. Furthermore, this study has developed an accessible and intuitive web-based application, enabling engineers to input relevant parameters and receive immediate predictive outputs, thus facilitating the practical application of advanced ML models in real-world scenarios.

Spent Mushroom Substrate-Derived Biochar and Its Applications in Modern Agricultural Systems: An Extensive Overview

Spent mushroom substrate (SMS), a nutrient-dense byproduct of mushroom cultivation, has emerged as a promising feedstock for biochar production, offering a sustainable solution to modern agricultural and environmental challenges. This review explores SMS properties, its conversion into biochar, and its various applications. Due to its lignocellulosic structure, high organic matter (OM), and essential nutrients, SMS is ideal for pyrolysis, a process that enhances biochar's porosity, nutrient retention, and carbon stability. These properties improve soil fertility, water retention, microbial activity, and plant growth while also contributing to climate change mitigation through carbon sequestration. SMS-derived biochar stands out for its superior benefits, including a balanced pH, a rich nutrient profile, and the ability to adsorb heavy metals, which mitigates soil and water contamination and minimizes toxic risks in the food chain. By enhancing soil structure, nutrient cycling, and moisture retention, SMS-derived biochar supports sustainable farming practices that reduce chemical fertilizer use and boost climate resilience. Beyond soil applications, SMS-derived biochar is effective in wastewater treatment, mitigating plant diseases, and improving mushroom cultivation substrates, thereby enhancing mycelial growth and productivity. Economically, it is a cost-effective alternative due to the abundant availability and inexpensive nature of SMS. Nevertheless, challenges still exist, particularly in optimizing production methods and ensuring consistency in biochar properties, influenced by variations in pyrolysis conditions and SMS types. Advances in production technology and sustainable practices are vital for scaling up SMS-derived biochar production. This paper emphasizes the transformative potential of SMS-derived biochar, advocating for its integration into circular economy frameworks and sustainable agricultural systems. Recommendations for future research and policy support are provided to maximize the ecological and economic benefits of SMS-derived biochar, fostering its widespread adoption in global agricultural and environmental strategies.

Biochar made from Luffa cylindrica and applied as a bifunctional electrocatalyst in Zn-air batteries

Biochar has been prepared by pyrolysis of Luffa cylindrica (the vegetable sponge produced by Luffa aegiptiaca) and activated by mixing the pyrolyzed powder with KOH and pyrolyzed again. Non-activated and activated biochar have both been structurally and then electrochemically characterized to record their differences and assess their suitability as bifunctional oxygen reduction and oxygen evolution reaction electrocatalysts in Zn-air batteries. Non activated biochar carries several functional groups; however, the activation procedure led to a material with mainly O and Mg groups. Biochar activation improved its electrocatalytic properties, but both activated and non-activated luffa biochar were functional as bifunctional electrocatalysts to a satisfactory degree. This is justified by the fact that both carried a large percentage of carbon and graphitic carbon. The advantage of the non-activated biochar versus the activated biochar was its variety of functional groups while that of the activated biochar was its large specific surface area.

Heavy metals remediation through lactic acid bacteria: Current status and future prospects

With the development of industrialization and urbanization, heavy metal (HM) pollution has become an urgent problem in many countries. The use of microorganisms to control HM pollution has attracted the attention of many scholars due to its advantages of mild conditions, low process cost, and no secondary pollution. In this context, this review aimed to compile recent advances on the potential of lactic acid bacteria (LAB) as HMs biosorbents. As a food-safe class of probiotic, LAB can not only be used for HM remediation in soil and wastewater, but most importantly, can be used for metal removal in food. The extracellular adsorption and intracellular accumulation are the main mechanisms of HM removal by LAB. Lactic acid (LA) fermentation is also one of the removal mechanisms, especially in the food industry. The pH, temperature, biomass, ion concentration and adsorption time are the essential parameters to be considered during the bioremediation. Although the LAB remediation is feasible in theory and lab-scale experiments, it is limited in practical applications due to its low efficiency. Therefore, the commonly used methods to improve the adsorption efficiency of LAB, including pretreatment and mixed-cultivation, are also summarized in this review. Finally, based on the review of literature, this paper presents the emerging strategies to overcome the low adsorption capacity of LAB. This review proposes the future investigations required for this field, and provides theoretical support for the practical application of LAB bioremediation of HMs.

Exploring acid mine drainage treatment through adsorption: a bibliometric analysis

Discharge of acidic wastewater from mining activities (acid mine drainage (AMD)) is a major global environmental and public health issue. Although several approaches, including chemical precipitation and membrane technology, have been developed to treat AMD, adsorption has emerged as the most promising technology due to its cost-effectiveness and efficacy. Despite the wide adoption of adsorption in treating AMD, the evolution of research in this area remains poorly understood. To address this gap, a bibliometric analysis of the most recent literature involving the application of adsorption in AMD remediation was conducted by merging datasets of articles from Scopus (1127) and the Web of Science Core Collection (1422), over the past decade (2013-2022). This analysis revealed a yearly increase of 11% in research publications, primarily contributed by China, the United States, and South Africa. Keyword analysis revealed that natural schwertmannites and their transformations, activated carbon, zeolites, and clay minerals, are the most extensively employed adsorbents for the removal of common metals (arsenic, chromium, iron, manganese, among others). The findings underscore the need for future focuses on recovering rare earth elements, using nanoparticles and modified materials, pursuing low-cost, sustainable solutions, integrating hybrid technologies, pilot-scale studies, exploring circular economic applications of AMD sludges, and inter-continental collaborations. These insights hold significant future implications, serving as a valuable reference to stakeholders in the mining industry.

Leveraging plant-based remediation technologies against chromite mining toxicity

The release of hazardous hexavalent chromium from chromite mining seriously threatens habitats and human health by contaminating water, air, and soil. Vulnerability to hexavalent chromium can result in significant health risks, viz, respiratory issues, gastrointestinal illnesses, skin problems in humans, and a plethora of toxic effects in animals. Moreover, Cr(VI) toxicity can adversely affect plant physiology by inhibiting seed germination, nutrient uptake, cell division, and root development, ultimately impairing growth and vitality. Fortunately, innovative techniques such as phytoremediation and nanotechnology have been developed to address heavy metal contamination, offering a promising solution, mainly through the use of hyperaccumulating plants. Biochar derived from plant waste is widely used and is emerging as a sustainable strategy for remediating Cr(VI) contamination. Biochar is rich in carbon and highly influential in removing Cr(VI) from contaminated soils. This approach addresses immediate challenges while providing a sustainable pathway for environmental rehabilitation in chromium mining. Integrating innovative technologies with nature-based solutions offers a holistic approach to reducing the harmful effects of chromium mining, thus protecting both human well-being and ecosystems. This review highlights the impact of Cr(VI) on different living biotas and further emphasizes the use of plants and plant-based materials for the sustainable remediation of chromite mining regions.

Biochar-based adsorption for heavy metal removal in water: a sustainable and cost-effective approach

The increasing contamination of aquatic bodies by heavy metals poses a significant threat to environment and human health, necessitates innovative, sustainable and cost-effective remediation strategies. Due to their persistence and toxicity, heavy metals like copper (Cu), lead (Pb), mercury (Hg), and cadmium (Cd) pose severe threats, even in trace amounts. Traditional removal methods of these heavy metals, like chemical precipitation, oxidation/reduction, filtration, ion exchange, membrane separation, and adsorption, are costly, inefficient, and have drawbacks. As an efficient and low-cost adsorbent, biochar has the potential for heavy metal remediation from water. Biochar is a versatile carbonaceous material produced through pyrolysis of organic wastes, emerged as a powerful adsorbent for heavy metal removal from contaminated water. The unique property of biochar makes it an effective medium immobilizing and capturing of heavy metals like Pb, Cd, As and Hg. Various factors affect its adsorption potential and capacity. Feedstocks type, composition, activation methods, and production processes including the pyrolysis temperature, temperature rate and residence time significantly impact the efficacy of biochar. Therefore, this review has assessed, compared, and contrasted different forms of biochar along with their production methods, modification techniques and mechanisms for their potential use as an adsorbent for heavy metal removal from the contaminated water. Modified biochar offers an environmentally friendly and cost-effective solution for water purification and remediation of toxic heavy metals from water. This review highlights the biochar potential as a crucial component for future research projects focusing on water treatment technologies, providing avenues for safer and cleaner water resources.

Biochar imparted constructed wetlands (CWs) for enhanced biodegradation of organic and inorganic pollutants along with its limitation

The remediation of polluted soil and water stands as a paramount task in safeguarding environmental sustainability and ensuring a dependable water source. Biochar, celebrated for its capacity to enhance soil quality, stimulate plant growth, and adsorb a wide spectrum of contaminants, including organic and inorganic pollutants, within constructed wetlands, emerges as a promising solution. This review article is dedicated to examining the effects of biochar amendments on the efficiency of wastewater purification within constructed wetlands. This comprehensive review entails an extensive investigation of biochar's feedstock selection, production processes, characterization methods, and its application within constructed wetlands. It also encompasses an exploration of the design criteria necessary for the integration of biochar into constructed wetland systems. Moreover, a comprehensive analysis of recent research findings pertains to the role of biochar-based wetlands in the removal of both organic and inorganic pollutants. The principal objectives of this review are to provide novel and thorough perspectives on the conceptualization and implementation of biochar-based constructed wetlands for the treatment of organic and inorganic pollutants. Additionally, it seeks to identify potential directions for future research and application while addressing prevailing gaps in knowledge and limitations. Furthermore, the study delves into the potential limitations and risks associated with employing biochar in environmental remediation. Nevertheless, it is crucial to highlight that there is a significant paucity of data regarding the influence of biochar on the efficiency of wastewater treatment in constructed wetlands, with particular regard to its impact on the removal of both organic and inorganic pollutants.

Adsorption of Toxic Metals Using Hydrous Ferric Oxide Nanoparticles Embedded in Hybrid Ion-Exchange Resins

Acid mine drainage (AMD) is a major environmental problem caused by the release of acidic, toxic, and sulfate-rich water from mining sites. This study aimed to develop novel adsorbents for the removal of chromium (Cr(VI)), cadmium (Cd(II)), and lead (Pb(II)) from simulated and actual AMD using hybrid ion-exchange resins embedded with hydrous ferric oxide (HFO). Two types of resins were synthesized: anionic exchange resin (HAIX-HFO) for Cr(VI) removal and cationic exchange resin (HCIX-HFO) for Cd(II) and Pb(II) removal. The resins were characterized using scanning electron microscopy and Raman spectroscopy, which confirmed the presence of HFO particles. Batch adsorption experiments were conducted under acidic and sulfate-enhanced conditions to evaluate the adsorption capacity and kinetics of the resins. It was found that both resins exhibited high adsorption efficiencies and fast adsorption rates for their respective metal ions. To explore the potential adsorption on actual AMD, HCIX-HFO demonstrated significant removal of some metal ions. The saturated HCIX-HFO resin was regenerated using NaCl, and a high amount of the adsorbed Cd(II) and Pb(II) was recovered. This study demonstrates that HFO-embedded hybrid ion-exchange resins are promising adsorbents for treating AMD contaminated with heavy metals.