Sorption of cadmium in columns of sand-supported hydrothermally carbonized particles

Sanitation in urban slums, especially in countries in Sub-Saharan Africa, is a challenge. One of the solutions to sanitation is to valorize waste, and to convert bio-waste present in the slum in a cheap and affordable way into lignite via hydrothermal carbonization (HTC). HTC is simple, cheap, converts all carbon (100%), eliminates pathogens completely, and requires wet starting products/biomass, thereby avoiding complicated drying schemes. In this research, we investigated the effectiveness of removing a divalent metal-ion, cadmium, using equilibrium batch experiments and columns of sand-supported hydrothermally carbonized colloidal lignite (HTCCL) derived from sugar, maize, and grass. Our results indicated that equilibrium sorption could be best described by a Langmuir isotherm. The uptake capacity varied from 0.11 to 0.21 mg Cd/g HTC, dependent on the type of HTC used. These values were relatively low compared to other carbonaceous sorbents. However, removal efficiencies in column experiments were remarkably high: 70–100% during 20–24 pore volumes or bed volumes of flushing. We concluded that HTCCL is a promising sorbent that can be used to treat heavily polluted water and/or wastewater.