Sorption And Desorption Of SMX On Chars Derived From Sediments With High Organic Matter Content

Sediment samples with high organic carbon contents (22.04%and8.46%) were collected and thermally treated using a method analogous to biochar production. The resulting char samples showed a much higher degree of carbon capture in comparison to biochar derived from common biomass, indicating potential use of this type of charred samples in soil amendment and carbon sequestration.Sorption of char with organic contaminants was investigated using sulfamethoxazole (SMX) as a model sorbate. SMX sorption increased greatly with increased pyrolytic temperature. The reasons could be listed as follows. Firstly, there is electrostatic repulsion between SMX and sediments, for sediment colloids took negatively charged as usual and SMX mainly existed in molecular forms and negatively charges at our experimental pH (6-8) as well, that coactions is not take advantage of adsorption. Some hydrophilic groups such as hydroxyl or carboxyl on surface could be removed by pyrolysis. The elimination of hydrophilic groups improved hydrophobicity and increased adsorption of hydrophobic organic pollutants by thermally-treated sediments. Secondly, adsorption sites amount increased due to increased aromaticity of sediments and sequentially increased the surface area (SA) and total pore volume (TPV) with the pyrolytic temperature. Adsorption mechanisms were summarized as electrostatic interaction, π-π electron-donor-acceptor interactions, hydrogen bonding interaction, cation bridging and hydrophobic interaction.Desorption of original sediments is stronger than that of thermally-treated sediments due to the process of partition. Desorption of SMX on original sediments from Waihai is low because the Waihai sediments contained more mineral materials than those from Caohai, and the sorption affinity of SMX to mineral materials is stronger than partition interaction which could account for the stronger absorption of SMX on original sediments from Caihai. The thermal treatment removes the hydration and electrostatic shell and thus more inner high-energy sorption sites easily available to SMX molecules. Then E*(sorption energy) increases with pyrolytic temperature, also suggesting high energy sites are generated during the pyrolysis. Therefore, higher pyrolytic temperature resulted in lower RR for weaker desorption of SMX from high energy sites. For the same reason, SMX molecules occupy all sorption sites with higher energy firstly and then, as increased SMX concentration gradient, more SMX spreads to sorption sites with lower energy, which resulted in RR increases with increasing SMX sorption (Qe).The thermodynamic analysis showed:the absolute magnitude of standard Gibbs free energy change (△G) can reflect the degree of sorption interaction. The negative values of△G for sorption on sediments and thermally-treated sediments indicate that the sorption processes are all thermodynamically favorable and spontaneous. It is known that the change in free energy for physisorption is between-20and0kJ/mol. Thus the sorption of SMX in all the samples occurred through physical processes. The less negative the AG, the weaker the driving force of sorption. We found that△G became less negative (absolute value of△G increased) as solid-phase concentrations increased suggesting decreased sorption. And it became more negative as temperature increased at the same sorbate loading suggesting that the driving force of sorption increased with increasing temperature. Positive enthalpy change (△H) indicates that most reactions are endothermic processes. That means elevating temperature is favorable for adsorption. This conclusion is also documented by the experiments results Kd:35℃>25℃>5℃tested under5、25、35℃equilibrium temperature. For both original sediments, the△Hs are always much higher than the chars. Therefore, more energy is needed for sediment sorption. Another thermodynamic parameter, entropy (△S), is also used to determine the process of a reaction. The higher entropy increase (positive△S) is well related with the decreased desorption ratio with increased solid-phase concentration for the original sediments. The fate-controlling effect of contaminants in char application for soil amendment should be evaluated through combining sorption/desorption and sorption thermodynamic studies.