Sorption, degradation and transport of veterinary antibiotics in New Zealand pastoral soils
Srinivasan, P. (2013). Sorption, degradation and transport of veterinary antibiotics in New Zealand pastoral soils (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/7390
Permanent Research Commons link: http://hdl.handle.net/10289/7390
Veterinary antibiotics are used worldwide for prevention and treatment of various diseases of livestock animals. After administration, the majority of antibiotics are excreted in urine and faeces as unchanged parent compound, and/or as their metabolites. With increases in the intensive use of antibiotics in New Zealand agriculture and direct land application of waste as manure, there is concern that excreted compounds could migrate to the receiving environment with potential impact on surface and groundwater. Antibiotic residues in soil could give rise to a variety of ecotoxicological problems and could also confer antibiotic resistance. As a first step towards assessing the risk of these compounds, it is important to investigate their fate and behaviour in the soil environment. The focus of this work was to determine the fate and transport behaviour of selected group of veterinary antibiotics in several New Zealand pastoral soils. A macrolide (TT) and three sulfonamide antibiotics (SMO, SCP and SM) were chosen for this study as they are commonly used overseas as well as in New Zealand. A simple, yet robust analytical method was developed to detect and quantify TT and the three sulfonamides using high performance liquid chromatography and ultraviolet detection. The limits of detection at signal: noise ratio of 3 were 20.0 µg L⁻¹ and 50 µg L⁻¹ for all SA’s and TT respectively. The average recoveries for all SA’s and TT in aqueous matrices ranged from 95 to 105% across the six concentrations investigated. Recoveries from the residual soils were slightly lower for SA’s and TT (~ 50 to 60%). The results of the kinetics studies showed that sorption was rapid in the first few hours of the contact time (0 to 2 h for SA’s and 0 to 4 h for TT) and thereafter an apparent equilibrium concentration was achieved slowly. Batch sorption results performed in six different pastoral soils showed that Freundlich isotherms were nonlinear (N ≠ 1) for most of the compounds. The degree of isotherm linearity (N) for SCP and SM varied between 0.87–1.11 in the six soils. SMO showed a highly non-linear pattern (N = 0.75) in just one soil (Manawatu). Isotherms of both TT and SMO were non-linear, with the degree of non-linearity for TT (N = 0.41–0.73) being greater than for SMO (0.88–1.21) in all soils. Concentration-dependent effective distribution coefficient (Kdeff ) values for the SMO, SCP and SM antibiotics in the soils ranged from 0.37 to 4.6 L kg⁻¹ and for TT Kdeff ranged from 1.2 to 500 L kg⁻¹. The sorption affinity for all soils followed an order: TT > SCP > SM > SMO. Statistical analysis of sorption data revealed a correlation between the sorption coefficients and soil properties such as % organic carbon, cation exchange capacity and clay content. Sorption of TT onto soils is mostly driven by the cation exchange capacity of the soil whereas for sulfonamides it is primarily due to hydrophobic interactions due to hydrophobic interactions. On further investigation it was found that sorption of the SMO antibiotic to soils was highly dependent on pH, ionic strength, and organic matter of the soil. Sorption of SMO decreased with the increases in pH, and increased with increasing ionic strength and organic carbon content. A hydrophobic pH-partitioning model linking sorbate speciation with species-specific sorption coefficients describing the pH dependence of the apparent sorption coefficients was successfully used to derive the fraction of each species of SMO that are likely to be present in the environment. The results from the biochar amended sorption studies showed that certain biochars can be used as a potential tool to remove residues of antibiotics. Soil amended with pine saw dust biochar was found to be the most effective in adsorbing antibiotics. Pine sawdust biochar absorbed 30 times more antibiotics than soil alone. This was attributed to its high surface area, which was four times higher than that of other biochars, or could be due to its high carbon content (91%). The results have shown that biochar applied at the rate of 0.5 to 1% by soil weight could prove to be an effective way to slow down the release of these contaminants to a manageable level thereby reducing the risk of ecotoxicity and antibiotic resistance. SMO degraded slowly in agricultural soils. The degradation times (DT₅₀) for SMO in Hamilton clay, Te Kowhai and Horotiu soils under non-sterilized conditions were 9.24, 4.3 and 13.33 days respectively. Soil dehydrogenase activities were directly correlated with degradation kinetics of SMO antibiotic. Results from degradation studies have shown that SMO is not likely to persist more than 90 days in all three soils suggesting that natural biodegradation may be sufficient for the removal of these contaminants from the soil. Both the degree of biological activity and temperature of the soil influenced overall degradation. SMO degradation in sterile soils indicated abiotic degradation and abiotic factors such as strong sorption of the antibiotic onto soil components also played a role in the degradation of SMO in soil. Four kinetic models, a single first-order model (SFO) and three biphasic kinetic models, were applied to fit the observed SMO degradation kinetics data. The results showed that the First-order double-exponential decay (FODED) and first-order two-compartment (FOTC) model was superior to the bi-exponential model (BEXP) and all three biphasic models were superior when compared to the SFO model for describing SMO degradation data. Use of the FOTC model enabled the estimation of the degradation endpoints. The results from two soil lysimeters indicate that the three sulfonamides were highly mobile and that at field collected soil pH mobility was similar to the mobility of the conservative bromide. The breakthrough curves of the three sulfonamides varied depending on soil type with Hamilton soil showing more retardation than Matawhero soil. For the three sulfonamides studied the order of breakthrough was generally observed to be: SM > SMO > SCP for both Matawhero and Hamilton soils. Residual antibiotic concentrations for SMO and SCP were detected up to depths of 18 cm. The CXTFIT model study described the peak arrival time as well as the maximum concentration of the antibiotic breakthrough curves but showed some underestimation at advanced stages of sulfonamide leaching, especially in the Hamilton soil. Results showed that the sulfonamides have weak sorption affinity and relatively high mobility in soils.
University of Waikato
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