title: Influence of biochar properties on sorption and remobilization of organic contaminants – Implications for environmental remediation
Creators: name: Schreiter, Inga Johanna
Publisher: name: UNSPECIFIED
Year: 2022

description: Environmental pollution is increasingly recognized as a global concern. Therefore, the development of cost-effective remediation technologies continues to be a growing topic in the scientific community. A material receiving increased attention in the field of soil and water remediation in the last decade is biochar – a carbon-rich product obtained through pyrolysis of organic materials. Biochar can be applied as a sorbent in environmental remediation for a variety of different contaminants. In this thesis, the sorption and remobilization behavior of two model organic contaminants, trichloroethylene (TCE) and tetrachloroethylene (PCE), on different biochars was investigated to gain a better understanding of biochar-pollutant interaction mechanisms. To this end, a set of laboratory experiments was conducted that are presented in the three core chapters of this thesis. Three biochars produced from cattle manure, grain husk, and wood chips at 450°C were used in all experiments. For selected tests, an activated carbon was included to serve as a fully carbonized reference. Single- and bi-solute batch adsorption experiments were conducted to study the effects of biochar feedstock on sorption behavior, to explore competitive and concentration-dependent effects in bi-solute systems, and to determine how sorbent properties influence partitioning and adsorption in both systems. In single-solute experiments, all biochars showed stronger sorption for TCE compared to PCE, which was attributed to steric effects. Plant-derived, carbon-rich biochars with high specific surface area and microporosity predominantly sorbed via pore-filling. Biochar produced from manure, with higher ash content and polarity, and smaller total pore volume, showed significant contribution of partitioning. In bi-solute systems, TCE and PCE showed different competition behavior depending on biochar properties. Plant-based biochars are pore-filling-dominated and show strong competition, whereby manure-derived biochar with high polarity and lower total pore volume showed significant partitioning and less competition. When biochar is applied in remediation technologies, its properties and sorption behavior can be naturally altered by, e.g., dissolution of minerals and mobilization of leachable organic carbon (LOC). This was investigated by artificially leaching the three biochars, characterizing their leachates, determining changes in the biochars’ chemical and structural properties, and relating these changes to specific differences in sorption mechanisms. The manure-derived biochar mobilized significantly more LOC and total ions, compared to the two plant-based biochars. Leaching increased external surface area, mesopore volume, and hydrophobicity of the manure-derived biochar, and decreased its polarity. This enhanced sorption via partitioning. In the plant-based biochars, micropore volume and pore size distribution were altered, most likely through the un-blocking of pores, causing increased sorption via pore-filling for both TCE and PCE. When biochar is applied in remediation technologies, it is not only important to understand the adsorption process, but it is also vital to know if pollutants are sequestered for the long-term or if they are readily released again into the environment. To investigate the influence of biochar properties on the remobilization of organic contaminants, the three biochars were pre-loaded with defined amounts of TCE and PCE and sequentially extracted with different solvents to analyze their remobilization behavior (water, methanol, toluene, and n-hexane). Water was only able to mobilize a comparably low fraction of the actually extractable mass. A significantly higher mobilized fraction of TCE and PCE was water-extractable from the polar, mesoporous manure-derived biochar. The more hydrophobic sorbents (grain husk biochar, wood chips biochar, and activated carbon), containing significant micropore volume, showed a lower water-extractable fraction. For all biochars, methanol contributed the highest share to the total extracted mass of all solvents at the two lowest pre-loading levels, whereby mass fractions released by toluene and n-hexane were comparably low. The non-extractable fraction appeared to be higher for PCE compared to TCE for the two plant-based biochars. Further, the fraction remaining unextracted was slightly lower at the higher pre-loading level. Overall, the results indicated that contaminant trapping in narrow micropores, especially at low concentrations, significantly influenced the release behavior of sorbed compounds. This supports the well-established hypothesis of contaminant trapping caused by swelling and deformation of the pore structure as a cause for irreversible sorption in the investigated biochars. In addition, overall matrix and pore accessibility governed by sorbent bulk hydrophobicity likely also have an impact. The results from this thesis highlight that biochar is an excellent sorption material for organic pollutants with great potential for a variety of different applications in environmental remediation. Thereby, feedstock-governed biochar properties show a strong influence on the overall fate of pollutants, indicating that sorbents ultimately could be tailored by feedstock selection.