Transport and retention of hydrochar-diatomite nanoaggregates in water-saturated porous sand: Effect of montmorillonite and phosphate at different ionic strengths and solution pH

Abstract Hydrochar, a solid hydrate with a high energy density, is produced by hydrothermal carbonization of lignocellulosic biomass and is widely applied in agriculture as a soil amendment. The fate and transport of hydrochar when applied to soil need to be investigated. The major components of soil, clay and phosphate, likely interact with hydrochar in the subsurface. This study investigated the cotransport behavior of hydrochar and diatomite (D) through water-saturated quartz sand in the presence of montmorillonite (M) and/or phosphate in NaCl (1–50 mM) solutions at pH 6.0 and 9.0. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images and zeta potential (ZP) results showed that hydrochar-D nanoaggregates formed preferentially due to surface charge heterogeneity. M inhibited the transport of hydrochar-D in sand columns regardless of the solution pH mainly because the organo-mineral clusters of hydrochar-D with M were prone to filling the pores of the sand medium. Moreover, fine M particles preferentially attached to sand could decrease the ZP of the sand surface and subsequently decrease the repulsive forces between hydrochar-D and sand. The copresence of M and phosphate slightly facilitated hydrochar-D transport at pH 6.0 due to phosphate adsorption, whereas a negligible effect on transport occurred at pH 9.0. Thus, phosphate played a predominant role in the transport of hydrochar when clays were also present. A two-site kinetic retention model suggested that k1d/k1 and k2 are responsible for hydrochar-clay aggregate deposition in sand. Our findings relate to the potential risks posed by hydrochar in subsurface soils and aquifers where clay and phosphate ubiquitously co-occur.