Contrasting microcystin-LR sorption and desorption capability of different farmland soils amended with biochar: Effects of biochar dose and aging time

Mechanisms and influencing factors of yttrium sorption on paddy soil: Experiments and modeling

High-technology rare earth elements (REEs) as emerging contaminants have potentially hazardous risks for human health and the environment. Investigating the sorption of REEs on soils is crucial for understanding their migration and transformation. This study evaluated the sorption mechanisms and influencing factors of the rare earth element yttrium (Y) on paddy soil via integrated batch sorption experiments and theoretical modeling analysis. Site energy distribution theory (SEDT) combined with kinetics, thermodynamics, and isotherm sorption models were applied to illustrate the sorption mechanism. In addition, the effects of phosphorus (P), solution pH, particle size of soil microaggregates, and initial Y content on the sorption processes were evaluated by self-organizing map (SOM) and Boruta algorithm. The sorption kinetic behavior of Y on paddy soil was more consistent with the pseudo-second-order model. Thermodynamic results showed that the Y sorption was a spontaneous endothermic reaction. The generalized Langmuir model well described the isotherm data of Y sorption on heterogeneous paddy soil and soil microaggregates surface. The maximum sorption capacity of Y decreased with increasing soil particle size, which may be related to the number of sorption sites for Y on paddy soil and soil microaggregates, as confirmed by SEDT. The heterogeneity of sorption site energy for Y was the highest in the original paddy soil compared with the separated soil microaggregates. The SOM technique and Boruta algorithm highlighted that the initial concentration of Y and coexisting phosphorus played essential roles in the sorption process of Y, indicating that the addition of phosphate fertilizer may be an effective way to reduce the Y bioavailability in paddy soil in practice. These results can provide a scientific basis for the sustainable management of soil REEs and a theoretical foundation for the remediation of REEs-contaminated soils.

Effect of varying pH and co-existing microcystin-LR on time- and concentration-dependent cadmium sorption by goethite-modified biochar derived from distillers' grains

Cadmium (Cd) is one dangerous and widespread heavy metal that of great environmental concern. To cost-efficiently adsorb aqueous Cd under influence of various factors, this study succeeded in fabricating goethite-modified biochar (GBC) derived from distillers' grains (DGs) for Cd sorption of different concentrations (10-100 mg L^-1) at pH of 3, 6 and 8 with and without microcystin-LR (MC-LR). Sorption kinetics and isotherms data revealed that Cd sorption capacity of GBC and unmodified BC increased as pH elevated from 3 to 6 but stabilized when pH further elevated to 8. Pseudo-second-order and Langmuir models more accurately fitted to sorption data for both BCs, implying monolayer chemisorption of Cd onto BCs. GBC exhibited more robust sorption for each Cd concentration than unmodified BC, with the maximum sorption capacity of around 28 mg g^-1 at neutral and weak alkaline pH. Notably, goethite-modification obviously increased bulk polarity, specific surface area, porosity and surface oxygenic group abundance of BC, thus strongly enhancing Cd sorption by creating more sorption sites mainly via pore-filling, electrostatic attraction, and also via complexation and cation exchange. Co-existing MC-LR of 100 μg L^-1 did not obviously affect Cd sorption by both BCs for most Cd levels at each pH, mostly because sorption mechanisms diverged between MC-LR and Cd to largely avoid their competition for sorption sties. Thus, goethite could modify DG-BC as promising and cost-efficient sorbent for Cd even with co-existing MC-LR, especially at neutral and weak alkaline pH that common in the nature. This study was greatly implicated in modifying and applying DG-BC for Cd immobilization in MC-LR laden waters with various pH circumstances.

A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

In recent years, the application of biochar in the remediation of heavy metals (HMs) contaminated soil has received tremendous attention globally. We reviewed the latest research on the immobilization of soil HMs by biochar almost in the last 5 years (until 2021). The methods, effects and mechanisms of biochar and modified biochar on the immobilization of typical HMs in soil have been systematically summarized. In general, the HMs contaminating the soil can be categorized into two groups, the oxy-anionic HMs (As and Cr) and the cationic HMs (Pb, Cd, etc.). Reduction and precipitation of oxy-anionic HMs by biochar/modified biochar are the dominant mechanism for reducing HMs toxicity. Pristine biochar can effectively immobilize cationic HMs. The commonly applied modification method is to add substances that can precipitate HMs to the biochar. In addition, we assessed the risks of biochar applications. For instance, biochar may cause the leaching of certain HMs; biochar aging; co-transportation of biochar nanoparticles with HMs. Future work should focus on the artificial/intelligent design of biochar to make it suitable for remediation of multiple HMs contaminated soil.