Removal of micropollutants from wastewater by rice husk biochar: synthesis, characterisation, and adsorption capacity

Abstract

Water is a scarce commodity, continuously exposed to various organic micropollutants, mainly resulting from daily anthropogenic activities. Although these compounds are normally detected in water bodies at very low concentrations, they can also pose a serious threat to the environment and human health. In this regard, pesticides could be considered the micropollutants group with the greatest concern to society. The adsorption process is recognised as a well-known technique for treating pesticide-contaminated waters. However, conventional adsorbents, such as activated carbon from mineral sources, can still considerably harm the environment. In contrast, biochar is a porous, carbonaceous material resulting from the pyrolysis of biomass in oxygen-depleted conditions. The considerable interest in biochar, mainly due to its remarkable physicochemical properties and its low environmental impact, has led to consider it as a promising alternative adsorbent with great potential in wastewater treatment fields. The research derived from this work focuses on the synthesis of biochar from rice husk as feedstock, its characterisation, and its evaluation as an adsorbent material capable of removing different pesticides from wastewater. Biochar was synthesized from pre-treated rice husk feedstock at 500 ºC for 4 h under an N2-H2 (95:5) atmosphere and, subsequently physically activated by CO2 at 800ºC for 1 h. The resulting biochars, both non-activated and activated, were characterised by several techniques to determine their main physicochemical properties. Both presented high alkaline pH values, high ash content and low carbon, hydrogen, and oxygen percentages. The surface area and morphology confirmed the noteworthy change in biochar surface, from 1.22 to 379.95 m2 g -1, upon activation. Moreover, the FTIR spectra confirmed the presence of Si-containing functional groups on activated biochar. Furthermore, it was also evaluated the adsorption performance of activated biochar in the removal of three pesticides with different n-octanol-water partition coefficients (clothianidin, thiacloprid and atrazine). Using a Milli-Q water matrix and assessing the pesticides adsorption individually, the biochar adsorption trend was firstly clothianidin, then thiacloprid and finally, atrazine. On the other hand, when they were evaluated simultaneously from the same aqueous solution, biochar exhibited a greater affinity for thiacloprid, followed by clothianidin, and lastly atrazine. The experimental equilibrium data for thiacloprid and clothianidin was adequately described by the Langmuir model, and the maximum adsorption capacity was quite similar for both, 4068 and 4078 μmol g-1, respectively. In contrast, the atrazine adsorption on biochar was reasonably well adjusted by the Freundlich model, indicating also linear adsorption. Furthermore, in all three cases, the effect of pH on pesticide-biochar adsorption was studied and led to the conclusion that electrostatic interactions were not the main mechanism of adsorption in any case. An experimental study with a real wastewater matrix from a municipal plant was also performed to observe the influence of ions and organic matter on the active sites of biochar. The results evidenced a similar trend of biochar toward clothianidin and thiacloprid, since both achieved around 42-43 % of adsorption at 73 h, whereas for the same time, the adsorption atrazine only was about 23 %. In contrast, for the same conditions but with milli-Q water, the three pesticide adsorption was almost complete within approximately 30 h. In conclusion, the outcomes indicated that CO2-physically activated biochar from rice husk biochar can be used as a readily available adsorbent for the removal of clothianidin, thiacloprid and atrazine from contaminated water, both individually and simultaneously.