Influence of pyrolysis temperature on sludge biochar: the ecological risk assessment of heavy metals and the adsorption of Cd(II)

Biochar as a partner of plants and beneficial microorganisms to assist in-situ bioremediation of heavy metal contaminated soil

Synergistic remediation of heavy metal (HM) contaminated soil using beneficial microorganisms (BM) and plants is a common and effective in situ bioremediation method. However, the shortcomings of this approach are the low colonisation of BM under high levels of heavy metal stress (HMS) and the poor state of plant growth. Previous studies have overlooked the potential of biochar to mitigate the above problems and aid in-situ remediation. Therefore, this paper describes the characteristics and physicochemical properties of biochar. It is proposed that biochar enhances plant resistance to HMS and aids in situ bioremediation by increasing colonisation of BM and HM stability. On this basis, the paper focuses on the following possible mechanisms: specific biochar-derived organic matter regulates the transport of HMs in plants and promotes mycorrhizal colonisation via the abscisic acid signalling pathway and the karrikin signalling pathway; promotes the growth-promoting pathway of indole-3-acetic acid and increases expression of the nodule-initiating gene NIN; improvement of soil HM stability by ion exchange, electrostatic adsorption, redox and complex precipitation mechanisms. And this paper summarizes guidelines on how to use biochar-assisted remediation based on current research for reference. Finally, the paper identifies research gaps in biochar in the direction of promoting beneficial microbial symbiotic mechanisms, recognition and function of organic molecules, and factors affecting practical applications.

Pyrolysis of exhausted biochar sorbent: Fates of cadmium and generation of products

Biochar is a promising sorbent for Cd removal from water, while the disposal of the exhausted Cd-enriched biochar remains a challenge. In this study, pyrolysis was employed to treat the exhausted biochar under N2 and CO2 atmospheres at 600-900 °C, and the fate of Cd during pyrolysis and characteristics of high-valued products were determined. The results indicated that higher temperature and CO2 atmosphere favored the volatilization of Cd. Based on the toxicity characteristic leaching procedure (TCLP) results, the pyrolysis treatment under both atmospheres enhanced the stability of Cd, and the leached Cd concentration of regenerated biochar obtained at high temperatures (>800 °C) was lower than 1 mg/L. Compared with the pristine biochar, the regenerated biochar demonstrated higher carbon content and pH, whereas the contents of oxygen and hydrogen declined, and exhibited promising sorption properties (35.79 mg/g). The atmosphere played an important role in modifying biochar properties and syngas composition. The N2 atmosphere facilitated CH4 production, whereas the CO2 atmosphere increased the proportion of CO. These results implied that pyrolysis can be a valuable and environmental-friendly strategy for the treatment and reuse of exhausted biochar sorbent.

Sodium hydroxide/magnesium chloride multistage activated sludge biochar: interfacial chemical behavior and Cd(II) adsorption performance

To enhance the adsorption performance of municipal sludge biochar on Cd(II), modified sludge biochar was prepared by sodium hydroxide/magnesium chloride (NaOH/MgCl2) graded activation, and the Cd(II) adsorption performance on sludge biochar (BC), NaOH-activated sludge biochar (NBC) and NaOH/MgCl2 activated sludge biochar (NBC-Mg) was investigated. The results showed that NaOH/MgCl2 graded activation upgraded the surface structure and enhanced the graphitization of sludge biochar. The adsorption experiments indicated that the adsorption kinetic and adsorption isotherm for Cd(II) were in accordance with the pseudo second-order kinetic and Langmuir model. The adsorption capacity of NBC-Mg (143.49 mg/g) for Cd(II) was higher than that of BC (50.40 mg/g) and NBC (85.20 mg/g). The mechanism of Cd(II) adsorption included ion exchange, complexation, cation-π interaction, and mineral precipitation. After five regeneration, the removal efficiency of Cd(II) by NBC-Mg remained above 90%. This work indicated that sludge biochar prepared by multistage activation could be an effective material for Cd-containing wastewater treatment.

Hydrochar and Its Dissolved Organic Matter Aged in a 30-Month Rice-Wheat Rotation System: Do Primary Aging Factors Alter at Different Stages?

Hydrochar, recognized as a green and sustainable soil amendment, has garnered significant attention. However, information on the aging process in soil and the temporal variability of hydrochar remains limited. This study delves deeper into the interaction between hydrochar and soil, focusing on primary factors influencing hydrochar aging during a 30-month rice-wheat rotation system. The results showed that the initial aging of hydrochar (0-16 months) is accompanied by the development of specific surface area and leaching of hydrochar-derived dissolved organic matter (HDOM), resulting in a smaller particle size and reduced carbon content. The initial aging also features a mineral shield, while the later aging (16 to 30 months) involves surface oxidation. These processes collectively alter the surface charge, hydrophilicity, and composition of aged hydrochar. Furthermore, this study reveals a dynamic interaction between the HDOM and DOM derived from soil, plants, and microbes at different aging stages. Initially, there is a preference for decomposing labile carbon, whereas later stages involve the formation of components with higher aromaticity and molecular weight. These insights are crucial for understanding the soil aging effects on hydrochar and HDOM as well as evaluating the interfacial behavior of hydrochar as a sustainable soil amendment.

Performance and mechanism analysis of sludge-based biochar loaded with Co and Mn as photothermal catalysts for simultaneous removal of acetone and NO at low temperature

Replacing NH3 in NH3-SCR with VOCs provides a new idea for the simultaneous removal of VOCs and NOx, but the technology still has urgent problems such as high cost of catalyst preparation and unsatisfactory catalytic effect in the low-temperature region. In this study, biochar obtained from sewage sludge calcined at different temperatures was used as a carrier, and different Co and Mn injection ratios were selected. Then, a series of sludge-based biochar (SBC) catalysts were prepared by a one-step hydrothermal synthesis method for the simultaneous removal of acetone and NO in a low-temperature photothermal co-catalytic system with acetone replacing NH3. The characterization results show that heat is the main driving force of the reaction system, and the abundance of Co and Mn atoms in high valence states, surface-adsorbed oxygen, and oxygen lattice defects in the catalyst are the most important factors affecting the performance of the catalyst. The performance test results showed that the optimal pyrolysis temperature of sludge was 400 °C, the optimal dosing ratio of Co and Mn was 4:1, and the catalyst achieved 42.98% and 52.41% conversion of acetone and NO, respectively, at 240 °C with UV irradiation. Compared with the pure SBC without catalytic effect, the SBC loaded with Co and Mn gained the ability of simultaneous removal of acetone and NO through the combined effect of multiple factors. The key reaction steps for the catalytic conversion of acetone and NO on the catalyst surface were investigated according to the Mars-van Krevelen (MvK) mechanism, and a possible mechanism was proposed. This study provides a new strategy for the resource utilization of sewage sludge and the preparation of photothermal catalysts for the simultaneous removal of acetone and NO at low cost.

Biosorption Potential of Arachis hypogaea-Derived Biochar for Cd and Ni, as Evidenced through Kinetic, Isothermal, and Thermodynamics Modeling

Biochar derived from plant biomass has great potential for the decontamination of aqueous media. It is the need of the hour to test biochar derived from economical, easily available, and novel materials. In this regard, the present study provides insight into the sorption of two heavy metals, i.e., cadmium (Cd) and nickel (Ni), using native Arachis hypogaea and its biochar prepared through pyrolysis. The effect of different factors, including interaction time, initial concentration of adsorbate, and temperature, as well as sorbent dosage, was studied on the sorption of Cd and Ni through a batch experiment. Characterization of the native biowaste and prepared biochar for its surface morphology and functional group identification was executed using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Results revealed the presence of different functional groups such as -OH on the surface of the adsorbent, which plays an important role in metal attachment. SEM reveals the irregular surface morphology of the adsorbent, which makes it easy for metal attachment. Thermogravimetric analysis shows the stability of A. hypogaea biochar up to 380 °C as compared with native adsorbent. The adsorption efficacy of A. hypogaea was found to be higher than that of native A. hypogaea for both metals. The best adsorption of Cd (94.5%) on biochar was observed at a concentration of 40 ppm, an adsorbent dosage of 2 g, a contact time of 100 min, and a temperature of 50 °C. While the optimum conditions for adsorption of Ni on biochar (97.2% adsorption) were reported at a contact time of 100 min, adsorbent dosage of 2.5 g, initial concentration of 60 ppm, and temperature of 50 °C. Results revealed that biochar offers better adsorption of metal ions as compared with raw samples at low concentrations. Isothermal studies show the adsorption mechanism as physical adsorption, and the negative value of Gibb's free energy confirms the spontaneous nature of the adsorption reaction. An increase in entropy value favors the adsorption process. Results revealed that the sorbent was a decent alternative to eliminate metal ions from the solution instead of costly adsorbents.

Mitigation of Cd(II) contamination in aqueous solution and soil by multifunctional hydroxyapatite/sludge biochar composite

Biochar with well-developed pore structure is an ideal carrier for easily agglomerated hydroxyapatite (HAP). Hence, a novel multifunctional hydroxyapatite/sludge biochar composite (HAP@BC) was synthesized by chemical precipitation method and used for mitigating Cd(II) contamination form aqueous solution/soil. Compared to sludge biochar (BC), HAP@BC exhibited rougher and more porous surface. Meanwhile, the HAP was dispersed on the sludge biochar surface, which reduced the agglomeration of HAP. The adsorption performance of HAP@BC on Cd(II) was better than that of BC under the influence of different single-factor batch adsorption experiments. Moreover, the Cd(II) adsorption behavior by BC and HAP@BC was uniform monolayer adsorption, and this reaction process was endothermic and spontaneous. The Cd(II) maximum adsorption capacities of BC and HAP@BC were 79.96 and 190.72 mg/g at 298 K, respectively. Moreover, the Cd(II) adsorption mechanism on BC and HAP@BC included complexation, ion exchange, dissolution-precipitation and Cd(II)-π interaction. According to the semi-quantitative analysis, ion exchange was the main mechanism for Cd(II) removal by HAP@BC. Notably, HAP played a role in the Cd(II) removal by dissolution-precipitation and ion exchange. This result suggested that there was a synergistic effect between HAP and sludge biochar for the Cd(II) removal. HAP@BC reduced the leaching toxicity of Cd(II) in soil better than BC, indicating that the HAP@BC was able to mitigate Cd(II) contamination in soil more effectively. This work demonstrated that sludge biochar was an ideal carrier for dispersed HAP and provided an effective HAP/biochar composite for the mitigation of Cd(II) contamination in aqueous solution/soil.

Biochar preparation and evaluation of its effect in composting mechanism: A review

This article provides an overview of biochar application for organic waste co-composting and its biochemical transformation mechanism. As a composting amendment, biochar work in the adsorption of nutrients, the retention of oxygen and water, and the promotion of electron transfer. These functions serve the micro-organisms (physical support of niche) and determine changes in community structure beyond the succession of composing primary microorganisms. Biochar mediates resistance genes, mobile gene elements, and biochemical metabolic activities of organic matter degrading. The participation of biochar enriched the α-diversity of microbial communities at all stages of composting, and ultimately reflects the high γ-diversity. Finally, easy and convincing biochar preparation methods and characteristic need to be explored, in turn, the mechanism of biochar on composting microbes at the microscopic level can be studied in depth.

The Fate of Heavy Metals and Risk Assessment of Heavy Metal in Pyrolysis Coupling with Acid Washing Treatment for Sewage Sludge

Pyrolysis is an emerging and effective means for sludge disposal. Biochar derived from sludge has broad application prospects, however, is limited by heavy metals. In this study, the fate of heavy metals (HMs) in pyrolysis coupling with acid washing treatment for sewage sludge was comprehensively investigated for the first time. Most of the HMs redistributed in the pyrolyzed residues (biochar) after pyrolysis, and the enrichment order of the HMs was: Zn > Cu > Ni > Cr. Compared with various washing agents, phosphoric acid presented a superior washing effect on most heavy metals (Cu, Zn, and Cr) in biochars derived at low pyrolysis temperature and Ni in biochars derived at high pyrolysis temperature. The optimal washing conditions for heavy metals (including Cu, Zn, Cr, and Ni) removal by H₃PO₄ were obtained by batch washing experiments and the response surface methodology (RSM). The total maximum HM removal efficiency was 95.05% under the optimal washing specifications by H₃PO₄ (acid concentration of 2.47 mol/L, L/S of 9.85 mL/g, and a washing temperature of 71.18 °C). Kinetic results indicated that the washing process of heavy metals in sludge and biochars was controlled by a mixture of diffusion and surface chemical reactions. After phosphoric acid washing, the leaching concentrations of HMs in the solid residue were further reduced compared with that of biochar, which were below the USEPA limit value (5 mg/L). The solid residue after pyrolysis coupling with acid washing resulted in a low environmental risk for resource utilization (the values of the potential ecological risk index were lower than 20). This work provides an environmentally friendly alternative of pyrolysis coupling with acid washing treatment for sewage sludge from the viewpoint of the utilization of solid waste.

Na4P2O7-Modified Biochar Derived from Sewage Sludge: Effective Cu(II)-Adsorption Removal from Aqueous Solution

With the rapid development of industrialization, the amount of copper-containing wastewater is increasing, thereby posing a threat to the aquatic ecological environment and human health. Sludge biochar has received extensive concern in recent years due to its advantages of low cost and sustainability for the treatment of heavy-metal-containing wastewater. However, the heavy-metal-adsorption capacity of sludge biochar is limited. This study prepared a sodium pyrophosphate- (Na4P2O7-) modified municipal sludge-based biochar (SP-SBC) and evaluated its adsorption performance for Cu(II). Results showed that SP-SBC had higher yield, ash content, pH, Na and P content, and surface roughness than original sewage sludge biochar (SBC). The Cu(II)-adsorption capacity of SP-SBC was 4.55 times than that of SBC at room temperature. For Cu(II) adsorption by SP-SBC, the kinetics and isotherms conformed to the pseudo-second-order model and the Langmuir–Freundlich model, respectively. The maximum adsorption capacity of SP-SBC was 38.49 mg·g−1 at 35°C. Cu(II) adsorption by SP-SBC primarily involved ion exchange, electrostatic attraction, and precipitation. The desired adsorption performance for Cu(II) in the fixed-bed column experiment indicated that SP-SBC can be reused and had good application potential to treat copper-containing wastewater. Overall, this study provided a desirable sorbent (SP-SBC) for Cu(II) removal, as well as a new simple chemical-modification method for SBC to enhance Cu(II)-adsorption capacity.