Enhanced cadmium removal by biochar and iron oxides composite: Material interactions and pore structure

Adsorption of heavy metals by biochar in aqueous solution: A review

Heavy metal pollution (e.g., Cd, Hg, Pb, Cu, Ni, Zn, As and Cr) has become a crucial issue worldwide. Among various remediation strategies, adsorption is widely recognized for its environmental sustainability, cost-effectiveness, and operational simplicity. In this context, biochar has gained significant attention due to its promising adsorption performance. To systematically support adsorption studies, this review compiled essential models for adsorption experiments, including commonly used adsorption kinetics models, isotherm models, and thermodynamic analysis methods. Moreover, we systematically analyzed key factors affecting heavy metal adsorption by biochar, such as its physicochemical properties, environmental pH, temperature, initial concentration, dosage, and the presence of coexisting ions, to identify the conditions that govern adsorption capacity. In addition, the adsorption performance of biochar toward eight significant heavy metals is reviewed in detail, with a focus on elucidating the underlying mechanisms, including complexation, ion exchange, cation-π bonding, electrostatic interactions, and precipitation. Finally, based on identified research gaps and critical challenges, we discuss emerging research tools, including machine learning and advanced surface modifications, to guide the targeted design of biochar materials for enhanced adsorption capacity.

Effect of isomeric polysaccharides on heteroaggregation of nanoplastics in high ionic strength conditions: Synergies of morphology and molecular conformation

Polysaccharides with various molecular structures and morphology may influence the aggregation kinetics of nanoplastics. This study used various characterization methods to elucidate the heteroaggregation mechanism of polystyrene nanoplastics (PSNPs) in the presence of polysaccharides (ionic strength (IS) 1-800 mM NaCl and 0.01-60 mM CaCl2). The results showed that under high IS, cellulose (CL) accelerated the heteroaggregation of PSNPs, and the aggregation rate of PSNPs increased by approximately 62.05 %, while amylose (AM) had little effect (10.38 %). Compared with AM (43.2 nm), the morphology of the CL (78.4 nm) gully had improved surface roughness, leading to its decisive role in the heteroaggregation of PSNPs. Quantum chemistry calculations indicated that van der Waals forces of PSNPs-CL systems (-217.28 kJ mol-1) were stronger than those of PSNPs-AM systems (-184.62 kJ mol-1) based on the subtle molecular conformation differences between CL and AM (opposite and same sides of OH groups in CL and AM, respectively). The morphology and molecular conformation of polysaccharides collaboratively controlled the heteroaggregation of PSNPs. Because the morphology of polysaccharides was based on their molecular conformation, the latter is the most critical factor. These findings provide new insights into the effects of PSNPs stability in the environment.

Nanoconfinement of MgO in nitrogen pre-doped biochar for enhanced phosphate adsorption: Performance and mechanism

Advanced metal-doped biochar with superior phosphate (P) adsorption capacity plays a crucial role in combating eutrophication, depending on the rational design of the biochar structure for uniform and nanoscale dispersion of metal oxides. Herein, the nanoconfinement of magnesium oxide (MgO) was successfully attained in nitrogen pre-doped biochar (Mg/N-BC). The well-dispersed MgO was confined within nanoscale structure of Mg/N-BC, delivering P adsorption capacity of 108.41 mg g-1 and adsorption rate of 18.01 mg g-1h-1. More importantly, its adsorption performance at equilibrium 0.5 mg P/L was 17.70 times higher. Results suggested the decrease in pore size was positively correlated with the increase of N, confirming the role of N pre-doping in structure shaping and MgO confinement. The enhanced P adsorption was attributed to the well-dispersed MgO nanoparticles within the biochar. This study introduced a facile synthesis approach for biochar-incorporated nanoscale MgO, offering a new strategy for enhanced P removal.

Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces: CD-MUSIC model and DFT computation

Calcium (Ca2+) and phosphorous (PO43-) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) are still lacking. This study employed batch adsorption experiments, charge distribution and multi-site complexation (CD-MUSIC) models and density functional theory (DFT) calculations to elucidate the mechanism by which Ca2+ and PO43- affect the adsorption of NH4+-N and NO3--N on the goethite (GT) surface. The results showed that the adsorption of NH4+-N on the GT exhibited an initial increase followed by a decrease as pH increased, peaking at a pH of 8.5. Conversely, the adsorption of NO3--N decreased with rising pH. According to the CD-MUSIC model, Ca2+ minimally affected the NH4+-N adsorption on the GT but enhanced NO3--N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. Similarly, PO43- inhibited the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. However, PO43- boosted NH4+-N adsorption by facilitating the formation of ≡Fe3O-NH4+ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH4+-N than monodentate phosphate (SP), SP-NH4+ was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH4+-N. The results of CD-MUSIC model and DFT calculation were verified mutually, and provide novel insights into the mechanisms underlying N fixation and migration in soil.

Iron and silicon modified biochar for enhancing cadmium removal from water: Unveiling the crucial role of iron-induced silicon dissolution

The interaction mechanisms of silicon (Si) and active ingredient iron (Fe) on cadmium (Cd) removal are still unknown. Herein, the Fe/Si modified biochar (Fe/Si-BC) was synthesized to enhance Cd removal by pre-immersion of Fe and ball milling loading of Si. Detailed characterizations indicated that Fe and Si were successfully introduced into Fe/Si-BC, resulting in the formation of a new metallic silicate (Ca2.87Fe0.13(SiO3)2). The maximum Cd adsorption capacity of Fe/Si-BC (31.66 mg g-1) was 3.6 times and 2.5 times higher than that of Fe-BC (8.89 mg g-1) and Si-BC (11.03 mg g-1), respectively, deriving from an enhancement of Si dissolution induced by Fe introduction. The dissolved Si could capture and combine Cd to form CdSiO3 precipitation, which was strongly supported by the random forest regression and correlation between dissolved Si content and Cd adsorption capacity. This study advances the mechanistic insights into synergistic functions of Si and Fe in Cd removal.