MgFe-LDH@biochars for removing ammonia nitrogen and phosphorus from biogas slurry: Synthesis routes, composite performance, and adsorption mechanisms

Rapid humification of cotton stalk catalyzed by coal fly ash and its excellent cadmium passivation performance

Due to industrialization, soil heavy metal pollution is a growing concern, with humic substances (HS) playing a pivotal role in soil passivation. To address the long duration of the compost humification problem, coal fly ash (CFA) in situ catalyzes the rapid pyrolysis of the cotton stalk (CS) to produce HS to address Cd passivation. Results indicate that the highest yield of humic acid (HA) (8.42%) and fulvic acid (FA) (1.36%) is obtained when the CS to CFA mass ratio is 1:0.5, at 275 ℃ for 120 min. Further study reveals that CFA catalysis CS humification, through the creation of alkaline pyrolysis conditions, Fe2O3 can stimulate the protein and the decomposition of hemicellulose in CS, and then, through the Maillard and Sugar-amine condensation reaction synthesis HA and FA. Applying HS-CS&CFA in Cd-contaminated soil demonstrates a 26.69% reduction in exchangeable Cd within 30 days by chemical complexation. Excellent maize growth effects and environmental benefits of HS products are the prerequisites for subsequent engineering applications. Similar industrial solid wastes, such as steel slag and red mud, rich in Fe2O3, can be explored to identify their catalytic humification effect. It could provide a novel and effective way for industrial solid wastes to be recycled for biomass humification and widely applied in remediating Cd-contaminated agricultural soil.

Enhanced adsorption of phosphate by rice straw-based biochar prepared via metal impregnation and bio-template technology

Phosphate removal from water through green, highly efficient technologies has received much attention. Biochar is an effective adsorbent for phosphate removal. However, adsorption capacity of phosphate on pristine rice straw-based biochar was not optimistic due to low anion exchange capacity. In this study, Fe-modified, Mg-modified and MgFe-modified rice straw-based biochar (Fe-BC, Mg-BC and MgFe-BC) were prepared by combining metal impregnation and biological template methods to improve the adsorption capacity of phosphate. The surface characteristics of biochar and the adsorption behavior of phosphate on biochar were investigated. The modified biochar had the specific surface area of 17.910-39.336 m2/g, and their surfaces were rich in a large number of functional groups and metal oxides. Phosphate release was observed on pristine rice straw-based biochar without metal impregnation. The maximum adsorption capacities of phosphate on MgFe-BC, Mg-BC and Fe-BC at 298 K were 6.93, 5.75 and 0.23 mg/g, respectively. Adsorption was a spontaneous endothermic process, while chemical adsorption dominated and electrostatic attraction and pores filling existed simultaneously. Based on the site energy distribution theory study, the standard deviation of MgFe-BC decreased from 6.96 to 4.64 kJ/mol with temperature increasing, which proved that the higher the temperature would cause the lower heterogeneity. Moreover, the effects of pH, humic acid, co-existing ions and ionic strength on phosphate adsorption of MgFe-BC were also discussed. MgFe-BC with fine pores and efficient adsorption sites is an ideal adsorbent for phosphate removal from water.

Efficient Adsorption of Nitrogen and Phosphorus in Wastewater by Biochar

Nitrogen and phosphorus play essential roles in ecosystems and organisms. However, with the development of industry and agriculture in recent years, excessive N and P have flowed into water bodies, leading to eutrophication, algal proliferation, and red tides, which are harmful to aquatic organisms. Biochar has a high specific surface area, abundant functional groups, and porous structure, which can effectively adsorb nitrogen and phosphorus in water, thus reducing environmental pollution, achieving the reusability of elements. This article provides an overview of the preparation of biochar, modification methods of biochar, advancements in the adsorption of nitrogen and phosphorus by biochar, factors influencing the adsorption of nitrogen and phosphorus in water by biochar, as well as reusability and adsorption mechanisms. Furthermore, the difficulties encountered and future research directions regarding the adsorption of nitrogen and phosphorus by biochar were proposed, providing references for the future application of biochar in nitrogen and phosphorus adsorption.

Layered double hydroxide loaded pinecone biochar as adsorbent for heavy metals and phosphate ion removal from water

This study modified pinecone biochar with a novel layered double hydroxide (LDH) to enhance its capacity to adsorb heavy metal and phosphate ions from water. The unmodified pinecone biochar demonstrated moderate adsorption capacities for Cu2+ (25.0 mg/g), Co2+ (24 mg/g), Pb2+ (22.9 mg/g), and phosphate (36.0 mg/g). However, after LDH modification, the LDH-biochar showed significantly improved adsorption capacities for Pb2+ (135.9 mg/g) and phosphate (160.8 mg/g) and a slight increase for Cu2+ (30.6 mg/g) and Co2+ (28.0 mg/g). The LDH layer enhances the adsorption of Pb2+ through ion exchange, phosphate through surface precipitation, and Cu2+ and Co2+ through surface complexation. The integration of LDH creates a multifaceted layer that utilizes various mechanisms to boost the biochar's adsorption capacity.

Phosphate adsorption on LDHs-biochar composite: Double-layer model for quantifying the contribution of ion exchange and ligand exchange

The adsorption performance of layered double hydroxides (LDHs) is limited owing to self-aggregation. To avoid this and effectively control the eutrophication of water bodies, biochar (BC) was synthesized, herein, by pyrolyzing waste sheep manure at 500°C, and Ca-Al-LDHs were loaded on the surface via a coprecipitation method to obtain Ca-Al-LDHs-BC(CA) composites with varying LDH contents. The fitted maximum adsorption capacities of the CA-5%, CA-10%, CA-15%, and CA-20% samples (corresponding to samples with 5%, 10%, 15%, and 20% LDHs, respectively) were 10.21, 16.14, 22.40, and 28.47 mg g-1, which were (when converted into metal proportions) 1.48, 1.23, 1.15, and 1.13 times of that of single hydrotalcite, respectively. The double-layer model was fitted using the Levenberg-Marquardt iterative algorithm, which when combined with the characterization results, confirmed that the adsorption of phosphate ions by CA-BC occurred via the double-layer adsorption mechanism. Two types of direct adsorption were observed: ion exchange, which resulted in first-layer adsorption, and ligand exchange, which resulted in second-layer adsorption, with first-layer adsorption accounting for a higher proportion. This double-layer adsorption mechanism showed that LDHs-BC could achieve higher ligand exchange performance compared to that achieved using only LDHs.