Adsorption is a commonly used method to treat groundwater contaminated with trace elements (TE). A lot of low-cost adsorbents from agricultural waste, industrial by-products and natural and modified materials are being developed and applied to contaminated water. However, seldom spent sorbent is safely handled afterwards and often end up in landfill, thus creating new problems and posing new risks to humans and environment. To overcome this issue, pre-treatment of spent sorbents prior final deposit might be necessary. As an alternative to traditional thermal methods, e.g. combustion or pyrolysis, hydrothermal carbonization (HTC) opens possibility to treat wet waste streams, without the need for an energy-intensive drying before or during the process.
Hydrothermal carbonization is a wet, relatively low temperature (180-350 °C) process where, under autogenous pressures, biomass undergo a chain of reactions, including hydrolysis, dehydration, decarboxylation, condensation polymerization, and aromatization. Recently, HTC has received attention as a method for producing low-cost carbon-based nano-compounds used for fertilization, soil amelioration, and carbon sequestration in soil. However, application of HTC for complex waste streams, such as spent sorbents, is little explored. Basic published trends show that major part of inorganics remain with the hydrochar, but in general, behaviour of compounds, like TE, during the HTC is largely unknown.
The aim of the study was to determine the feasibility of using HTC as a treatment method for spent iron-peat sorbents, highly enriched with TE. Physical, chemical and thermal properties of the hydrochar were evaluated after the HTC treatment.
Spent iron-peat sorbent with high loads of arsenic (As) (1 g kg-1), chromium (Cr), copper (Cu) and zinc (Zn) (up to 4 g kg-1) was hydrolysed at 230 °C (420 PSI) for three hours. Determination of the major elements, calorific value, and sequential extraction were performed with the obtained hydrochar. While dissolved carbon, density, and total elemental composition were analysed for the aqueous phase.
The calorific value of the hydrochars increased by at least 30% as compared to HTC-untreated spent sorbent. Obtained hydrochar showed similar calorific value of coal. More than 99% of total adsorbed Cr, Cu and Zn remained in the hydrochar but nearly 30% of As was transferred to aqueous phase. Dissolved organic content substantially increased in the aqueous phase because of a high content of degradation products such as sugars, organic acids, and aromatics.
Hydrothermal carbonization as a treatment method for spent sorbents with high TE loads is promising. However, environmental and practical implications associated with the evolved gases and composition of the aqueous phase needs further investigation.