Abstract
Hydrothermal carbonisation (HTC) is a wet and relatively low-temperature process where, under autogenous pressures, biomass undergoes a chain of reactions leading to the defragmentation of organic matter. As well as its other uses (e.g. for producing low-cost carbon-based nano-compounds), HTC is utilised for the treatment of wet wastes, such as manure and biosludge. This study aimed to determine if hydrothermal carbonisation is a feasible treatment method for spent sorbents that are highly enriched with arsenic, chromium, copper, and zinc. The chemical properties of hydrochar and process liquid were evaluated after HTC treatment, where peat-based spent sorbents were carbonised at 230 °C for 3 h. Analysis of Fourier transform-infrared spectra revealed that during HTC, the oxygenated bonds of ethers, esters, and carboxylic groups were cleaved, and low-molecular-weight organic fragments were dissolved in the process liquid. A large fraction of arsenic (up to 62%), copper (up to 25%), and zinc (up to 36%) were transferred from the solids into the process water. Leaching of these elements from the hydrochars increased significantly in comparison with the spent sorbents.
Highlights
Contamination of groundwater and surface waterbodies due to natural and anthropogenic activities has been reported worldwide
This was an expected outcome because due to increasing temperature as well as pressure, biomass is more accessible to water, which leads to the degradation of the physical biomass structure (Allen et al 2001)
A slightly acidic pH, as determined in the process liquids, was a likely result of lowmolecular-weight organic acids that are usually produced during biomass hydrolysis (Berge et al 2011)
Summary
Contamination of groundwater and surface waterbodies due to natural and anthropogenic activities has been reported worldwide. By their nature, contaminants may be organic or inorganic. Inorganic contaminants include metals, such as chromium (Cr), copper (Cu), and zinc (Zn), and metalloids, such as arsenic (As). These metal(loid)s are commonly associated with pollution and toxicity problems (Lafa et al 2015; Küpper and Andresen 2016). A common method to remove metal(loid)s from contaminated water is to adsorb them onto a reactive media and separate them from the solution.
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