Nanosized spent coffee grounds can increase soil clay dispersibility

Abstract

The application of spent coffee grounds (SCGs) to soil has been widely encouraged because of the agronomic benefits of SCGs. However, the negative impacts of this practice are not fully understood. This study aims to present the effect of nanosized SCGs (nSCGs) on the dispersibility/transportability of soil clay. An nSCG sample with an average hydrodynamic size of ∼260 nm was extracted from SCG waste, and its effects on the dispersibility of clay were tracked over a pH range of ∼3 to ∼9 and an ionic strength range of 0.001–0.1 N by using dynamic light scattering techniques and a test tube-based method. nSCGs were found to carry negative charges (zeta potential, ζ, varying from −2 to −30 mV), and the presence of nSCGs in clay suspensions resulted in decreases in the surface charge (towards more negative charge) of the clay suspensions, particularly at pH < 6. By providing more negative charges to the clay system, nSCGs stimulated and favoured clay dispersal. The edge surface of clays and Fe oxides may play a central role in promoting soil particles association, but this effect was likely weakened in the presence of nSCGs. In general, it can be inferred that the utilization of SCGs may have adverse impacts such as soil/clay losses or soil structural changes, and these impacts need to be considered.