Competitive And Complementary Adsorption Of Sulfamethoxazole And Carbamazepine On Carbonaceous Nanomaterials

The environment behavior and risk of antibiotics have attracted special research attention due to their wide occurrence in the environment as well as their toxicity to aquatic organisms and human beings. Adsorption controls the dispersion, migration and degradation of antibiotics in soil/sediment. However, due to the complex composition of soil/sediment, it is difficult to quantify the contribution of each component to the overall adsorption. Many researchers select model sorbent with simple structure to investigate the sorption behaviors of antibiotics. Although simultaneous presence of multiple pollutants is common, the behaviors of the pollutants as affected by other chemicals are still unclear. Organic contaminants with different properties will compete for their common sorption sites, and will complementarily occupy their specific sites. Therefore, this study selected graphene oxide (GO), single-walled carbon nanotubes (SC), graphite (GP) and activated carbon (AC) as model adsorbents, and selected sulfamethoxazole (SMX) and carbamazepine (CBZ) as model adsorbates to study their adsorption mechanisms. We use the new data processing methods to evaluate the complementary adsorption of SMX and CBZ. The energy distribution, desorption hysteresis, thermodynamic analysis and kinetic analysis were incorporated in studying the coadsorption, and attempt to evaluate the accurate sorption affinity between organic contaminants and solid particles.We found that adsorption of CBZ was higher than that of SMX on the same adsorbents, and the order of the adsorption strength of four model adsorbents to SMX and CBZ were:AC> SC> GO> GP. The high hydrophobicity and high π electron accepting ability of CBZ may be the main reason for its higher adsorption. According to the nonlinearity of the sorption isotherms and the characteristics of the adsorbent, it can be concluded that both surface adsorption and pore-filling interaction existed in SMX/CBZ and AC/SC/GO adsorption systems. For GP, SMX and CBZ were mainly adsorbed on the surface sites and edge sites.Competitive sorption was observed in the four sorption systems, with obvious decreases in the sorption of the primary adsorbates after the addition of co-adsorbates. The first observation was the concentration-dependence of the competition. The lower the concentration of the primary adsorbates, the higher degree of inhibition. This phenomenon may be explained by the sorption potential. The second observation is that CBZ is a stronger competitor to SMX than SMX to CBZ. This observation is most likely associated with higher sorption (both higher Q0 and Kd) of CBZ than SMX. A new data analysis approach was used to compare the absolute sorption inhibition of the primary sorbate (△QpΠ) and the amount (Qsec) of competitor sorbed. Complementary sorption was evidenced as the generally observed △QpΠ< Qsec. This is due to the SMX and CBZ have specific adsorption sites, the negatively charged oxygen-containing functional groups could be the specific sorption sites of SMX through charge-assisted H-bonding (CAHB), while the hydrophobic surface of could be the unique sorption sites for CBZ adsorption though π-π EDA interaction. At low concentrations, complementary sorption(△Qp< Qsec) was more obvious, which was due to the preferentially adsorption on specific sites for SMX and CBZ. When CBZ as a competitor, the site energy distribution of SMX was quite different, and the site energy distribution of SMX was not significant change when SMX was the competitor. The competitive sorption was observed at a wide pH range (1-9) and in the presence of DOM (10～60 mg/L), suggesting this phenomenon was ubiquitous in the environment.Since the thermodynamics irreversible coefficient (TⅡFreundlich\(?)TⅡ) does not apply to the coadsorption system that n values of desorption curve greater than the adsorption curve, we use release ratio (RR) to evaluate the change in solid phase concentration during desorption of SMX and CBZ. RR values increased with increasing concentration of the solid phase, indicating that the solute molecules first sorption on high energy adsorption sites, and then distributed to the lower energy sites. For the four adsorbents, SMX release rate significant increase with the CBZ competition, while the release rate is not obvious increase when added SMX to CBZ-adsorbent system, interaction between CBZ and adsorbent was stronger than SMX. Comparison the release ratio between the different adsorbents, the higher the adsorption doesn’t mean the greater change in the release ratio, which is consistent with competition:high absorption does not necessarily lead to high competition. Thus, a large Kd value did not reflect a strong affinity. The release ratio of the adsorbed contaminants provides useful information for understanding sorption strength and associated risks of contaminants.Adsorption of SMX and CBZ on four sorbents under 278 K,298 K and 308 K were comapred. For different sorbate, the degree of influence of temperature is different. We believe the impact of temperature on the different mechanisms are different, the less the type of adsorption sites, and the control mechanism of the adsorption system, the easier to affected by temperature. △G is negative, and △S is positive, indicating that the adsorption SMX and CBZ on four carbonaceous material is Spontaneous and the process of chaotic degree increase. In the GO/GP system, △H is negative, indicated SMX and CBZ adsorbed on the GO/GPwas an exothermic process.The pseudo-second order kinetic (PSOM) showed better fitting result than the pseudo-first order kinetic (PFOM) on kinetic data of SMX and CBZ. Due to the fast adsorption of SMX/CBZ on GP, the kinetic process cannot be fitted with a model. SMX and CBZ adsorption rate constant on the other three sorbents in order of: GO> AC> SC. Adsorption site is characterized by its availability and its accessibility to the equilibrium behavior and the kinetic behavior of adsorption. For SMX and CBZ, although AC adsorption was higher than GO, the adsorption rate is lower than GO. AC adsorption site accessibility was low, because SMX and CBZ into the internal pore of AC required through water film. And GO has the highest accessibility as the flat structure of GO is conducive to SMX and CBZ adsorption. The primary solute adsorption rate on four sorbates significantly decreased as added the competitor. Competition lead to the primary sorbate adsorption rate on four sorbates significantly decreased, extending the residence time of primary sorbate in the liquid phase, increasing the migration risk of the primary sorbate.