| As an organic contaminants(OCs), Sulfamethoxazole(SMX) can enter into farmland ecosystems of the West Sichuan Plain via the sewage irrigation, manure farmland, abandoned drug abandon, and so on. However, the migration and diffusion of SMX occurs with a variety of transform processes among the environmental media. Eventually, SMX can be integrated into the biogeochemical cycle of the West Sichuan Plain. It poses potential risk to the human health and ecological environment.Dissolved organic matter(DOM) is very critical in the global carbon cycle.DOM derived from crop straw, to a certain extent, has controlling effects on environmental geochemical behavior of SMX in farmland soils. Previous reports neglected the environmental effects of DOM derived from the original straws,and lacked discussion on the mechanism of the interaction between DOM-SMX.In this paper we have traced closely the six representative paddy soils collected from the West Sichuan Plain. According to the batch experiment, the sorption and desorption kinetics of SMX on paddy soils, and the effects of paddy soil properties and external conditions were analyzed. The dynamic processes of the interaction in the DOM-SMX were tracked. It clarifies the dominant mechanism of the interaction between DOM and SMX-paddy soils. It enhances the deeper understanding of environmental behavior of SMX on paddy soils.Results of this study will provide further information for environmental health risk assessment of SMX in the agro-ecosystem. The main conclusions of this work are drawn.(1) Adsorption kinetics and desorption kinetics of SMX on paddy soil was studied. The adsorption capacity of SMX on different paddy soils increased with the increase of the adsorption time. For the first 12 h, the adsorption process was fast until the balance at 120 h. However, the adsorption rate was gradually decreased. The results of our investigation indicated that, the kinetic curves could be formed with the first order two-compartment model(r2adj: 0.97 ~ 1.00,mean: 0.99), the first-order kinetics model(r2adj: 0.72 ~ 0.97, mean: 0.84) and the second-order kinetics model(r2adj: 0.80 ~ 0.98, mean: 0.91). At this moment, the adsorption experimental data was better fit for the first order two-compartmentmodel. For the first 12 h, the adsorption process proved to be quick before it reached balance in 120 h, and the first order two-compartment model was consisted of the fast and slow sorption compartment. The amount of desorption of SMX on paddy soils increased with the increase of the interaction time until the balance at 140 h. The time consumed by the desorption balance was longer than that of the adsorption balance. The average desorption rate of SMX in 12 kinds of paddy soils was 49%.(2) The effect of factors on the adsorption of SMX on paddy soil was studied. The adsorption ability of cohesive paddy soils to SMX was stronger than that of the sandy soils. The adsorption capacity of SMX on paddy soils were positively correlated with surface area(r=0.946). However, the adsorption capacity on the paddy soils with high values of O/C*, to a certain degree, had the variability. The adsorption capacity of SMX was increased with the decrease of particle size of paddy soils. However, the adsorption capacity on the paddy soils with high values of(N+O)/C*, to a certain degree, had the variability. The adsorption capacity of SMX on the paddy soil with the p H condition(less than 7)was higher than that on the media(acid-base degree>7). However, the adsorption capacity on the paddy soils with high values of(O + N + S)%, to a certain degree,had the variability.(3) The simple system of DOM-SMX was discussed. By extracting and freeze-drying, DOM of rape straw remained to be a brown solid material. DOM could reduce the content of SMX in solution effectively. DOM could interact with SMX, and interfere with the migration of free SMX to a certain extent. The removal rate(RR) of SMX showed a positive correlation(r=0.996) with DOM concentration(CD). With the increase of CD, the removal efficiency of SMX increased, and the correlation between RR and CD became weaker(r=0.978). For the first 20 h, the removal rate of SMX was positively correlated with the reaction time(r=0.920). The correlation of them increased gradually with the time going by(r=0.949).(4) The system of DOM- paddy soils- SMX was discussed. DOM(2 mg ?m L-1) had a retarding effect on the adsorption process of SMX on paddy soils.The effect of DOM on the adsorption of SMX on paddy soils was consisted of competitive adsorption, coadsorption, cumulative adsorption and complexation.With addition of DOM, the adsorption amount of SMX on a small number of paddy soils increased(T1, T2). By contrast, most of them decreased(L1, L2, D1,D2, H1, J1, J2, Y1). At the initial stage of adsorption(0~24 h), DOM and SMX were mainly dominated by competitive adsorption on paddy soil, DOM inhibited the adsorption of SMX on paddy soil. For paddy soils with small adsorption amount of SMX, after adding DOM, DOM and SMX were mainly dominated by accumulated adsorption or coadsorption with the extension of the adsorption time,DOM promoted the adsorption of SMX on paddy soils. However, after adding DOM, for paddy soils with large adsorption amount of SMX, DOM and SMX were mainly dominated by complexation. At this time, DOM inhibited the adsorption of SMX on paddy soils significantly. |
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