As an efficient and low-cost environmental remediation material, biochar has been widely used in many ways, such as removing contaminants, improving soil properties and increasing crop yields.

Professor Zhang Qianru's research group from the Innovation Team of Soil-Plant Interactions of Agricultural Resources and Regional Planning (IARRP) of the Chinese Academy of Agricultural Sciences (CAAS) recently cooperated with the research group of Professor Shang Jianying from the College of Land Science and Technology, China Agricultural University (CAU), making new progress in understanding the stability of biochar colloids.

The research results were published in the journal Biochar under the title "Effects of low molecular weight organic acids on aggregation behavior of biochar colloids at acid and neutral conditions".

According to Prof. Zhang, low molecular weight organic acids (LMWOAs) are widely present in soil, especially in the soil rhizosphere. LMWOAs can be continuously produced by plant roots and participate in soil carbon cycles as active soluble carbon although the concentration is very low. Biochar colloids are the important components of dissolved organic carbon in soil, groundwater and marine ecosystems. The stability of biochar colloids is of great significance for the long-term effective and safe application of biochar and environmental risk assessment. With the application of biochar in farmland, biochar colloids are inevitably released into groundwater and the soil environment, where they interact with soil root exudates (i.e., LMWOAs) during soil pore migration. LMWOAs may influence the environmental behavior of biochar colloids in soil as active components of the rhizosphere carbon cycle.

The pine-wood and wheat-straw biochar (named PB and WB, respectively) colloids were used as model biochar colloids in the study. The effects of LMWOAs (oxalic acid, citric acid, and malic acid) on the aggregation behavior of biochar colloids with the typical acidic soil pH values (pH 4 and 6) were investigated. The experimental results demonstrated that LMWOAs decreased the critical flocculation concentrations (CCCs) of PB and WB colloids. The most significant effect occurred in pH 4 due to more LWMOAs sorption on the surface of the biochar colloids, which greatly reduces the stability of biochar colloids and inhibits the biochar colloids in soil root migration. At the same time, it was also found that the different molecular weight and chemical structure of LMWOAs caused various CCCs changes of biochar colloids.

The study shows that the chemical structure (functional group), molecular weight of LMWOAs and the electrophoretic mobility (EPM) of biochar co-influence the interactions between biochar colloids and LMWOAs, thus affecting the stability of biochar colloids. The presence of LMWOAs accelerated the aggregation of colloidal biochar by increasing the interaction of surface bridging bonds (hydrogen bonding) and decreasing the repulsive force between colloidal biochar particles.  

The study provides new evidence for the stability of biochar colloids in soil solutions, especially in the soil rhizosphere.

The research was co-funded by the International Science and Technology Innovation Program of the Chinese Academy of Agricultural Sciences and the National Natural Science Foundation of China.

Paper link: https://doi.org/10.1007/s42773-022-00142-5

The schematic diagram of the LWMOAs effect on the aggregation of biochar colloids under pH 4 and 6 conditions

Principal component analysis (PCA) of the changes of CCC values (∆CCC), the structure and molecular weight of LMWOAs, solution pH, and the absolute EPM values of biochar colloids w/wo LMWOAs.