Removal of pharmaceuticals in urine using activated carbon from rice husks (Oryza Sativa)

Abstract

ABSTRACT : Water is a scarce natural resource, indispensable for human life and for the environmental sustainability, which with the passage of time, has suffered an alarming deterioration as a result of the rapid human and economic development. For years, quantities of pharmaceutical compounds, such as acetaminophen (ACE), ciprofloxacin (CIP), diclofenac (DIC) and sulfamethoxazole (SUL), have been present in municipal, hospital and industrial wastewater, and due to the inefficiency of the conventional water treatment methods, these compounds have been detected, in both natural and drinking waters, producing adverse effects on the living beings of the aquatic and terrestrial ecosystems. Therefore, the study of the behavior of these compounds in the environment, and their possible elimination in wastewaters, is a subject of growing interest for researchers around the world. Alternatives such as membrane filtration, advanced oxidation processes and adsorption by using activated carbon are among the most promising options to deal with pharmaceutics in wastewaters. The use of activated carbonsis of particular interest thanks to their high efficiency and easy application. In addition, activated carbons can be obtained from organic wastes, such as coconut husks, bamboo, fruit seeds and peels, sawdust, coffee pulp and rice husks (RH). Among them, RH are of special interest, because in many developing countries, including Colombia, it is considered an environmental problem. In fact, large volumes of this waste are produced annually, and a significant part of them are not used or inadequately disposed, generating environmental pollution. Therefore, the use of this waste for the production of activated carbon for water treatment allows the simultaneous treatment of two environmental concerns: the elimination of waste generated by the RH and the elimination of contaminants, such as pharmaceuticals, from waters. Considering the aforementioned, this work focused on the preparation of activated carbons from RH to be used in the elimination of various pharmaceuticals in aqueous solution. Activated carbons prepared from RH wastes were activated with NaOH, ZnCl2 and FeCl3 at 800°C. Initially, these materials were applied in the elimination of a mix of ACE, CIP, SUL, and DIC in distilled water. The results showed that NaOH activated RH carbon is the most effective in removing all of four contaminants. Then, several NaOH activated RH carbons with different ash content, RH-NaOH-800°C (activated carbon from RH activated with NaOH at 800°C), RH NaOH-800°C pretreated and RH-NaOH-800°C post-washed, were tested on its ability to remove the organic pollutants. The results confirmed that a decrease in the percentage of ash increases the BET surface area, and consequently the adsorption capacity of activated carbon. The best activated carbon, RH-NaOH-800°C post-washed, was then tested on adsorption of the pharmaceuticals in both distilled water and synthetic urine, to determine the effect of the chemical structure of the contaminant and the complexity of the matrix on the adsorption capacity. CIP, due to its zwitterion structure, was the pharmaceutical that presented the highest affinity with the adsorbent surface, while ACE was the one with the lowest affinity. Both, ACE and CIP, were then selected to better understand the process by investigating isotherms (Langmuir, Freundlich and Redlich-Peterson models), kinetics (pseudo first-order, pseudo second order and intraparticle diffusion models), and thermodynamics involved during the adsorption. The best fit, of both pharmaceuticals, for the Langmuir isotherm, which was confirmed by the Redlich-Peterson model, suggested a monolayer-type adsorption, presenting maximum adsorption capacities of 555.56 and 210.55 mg g-1 for CIP and ACE, respectively, in RH-NaOH-800°C post-washed. Concerning the kinetic models, the best fit, of ACE and CIP, to the pseudo second order model suggests that the adsorption capacity is proportional to the number of active centers of the adsorbent. Furthermore, the intraparticle diffusion model indicated that ACE and CIP adsorption occur both on the surface and within the pores of the activated carbon. Thermodynamic studies such as activation energy (Ea), enthalpy change (ΔH°) and Gibbs free energy change (ΔG°) suggest that the physisorption of both pollutants takes place in a spontaneous process. However, the thermodynamic data showed that ACE physisorption is an exothermic process, while the CIP physisorption process is endothermic, suggesting that the material developed temperature-dependent functional groups. These results presented in this work show the possible application of RH residues as an promissory option for the elimination of pharmaceutical pollutants from water and urine. The best activated carbon prepared from RH and activated with NaOH can be effectively used for adsorption, in a complex matrix like urine, of various types of pharmaceutical contaminants with different chemical structures, such as ACE, CIP, SUL, and DIC, presenting adsorption percentages of 71.97, 98.36, 94.23 and 97.87%, respectively, in 5 minutes of treatment.