Silicic acid increases dispersibility of micro-sized biochars

Fine-sized biochars have been cautioned as an emerging environmental concern because of their dispersibility, hence agrochemicals-facilitated cotransport. While inspecting factors that are responsible for the dispersibility of biochars, we likely overlook the effect of silicic acid, a common compound in nature. This work aims to address the gaps in knowledge concerning the potential influence of silicic acid on surface charge and dispersibility of biochar. The pyrolysis of rice straw at 300 and 900 °C correspondingly produced so-called ‘C-rich biochar’ and ‘Si-rich biochar’, while the ‘hybrid biochar’ was obtained from the pyrolysis at 600 °C. Having low density (i.e., 1.15 to 1.46 g cm−3) might be an initial reason for the more dispersibility and transportability of biochars. Particle size could be another initial factor, causing biochars more vulnerable to be transported. The coarser C-rich biochar showed the fastest aggregation while the finer biochars (i.e., Si-rich and hybrid biochars) presented a slower aggregation. In addition to the effect of particle size, the presence of silicic acid was also involved in regulating the colloidal dynamics of the biochars. The test-tube method used to evaluate the colloidal dynamics of the biochars revealed that adsorption of silicic acid increased negative surface charge, by which dispersion state of biochar particles was favoured. The hybrid and Si-biochars showed their higher dispersibility while the C-rich biochar represented the lowest dispersibility and highest resistance to silicic acid. Pyrolysis is, therefore, the initial factor that shapes the nature and hence the dispersibility of the biochars, while silicic acid played an additional role in enhancing the dispersibility. As biochars obtained from higher pyrolysis temperatures are more dispersive, their application to agricultural lands needs to be carefully considered.