# Adsorption of Metolachlor and Its Transformation Products, ESA and OXA, on Activated Carbons
## 1 Introduction  
Pesticides have become indispensable for intensive agricultural areas around the world, since they increase the production and improve the quality of agricultural production. However, when they are sprayed on crop plants, a fraction of pesticides is transferred through the soil and can reach aquifers, affecting the groundwater quality, making it unsuitable for human and agricultural uses [1, 2, 3]. Additionally, pesticides' transformation products (TPs) have also emerged as an area of concern due to their high mobility and their less degradation compared to the parent molecule [4]. For this reason, the protection of water resources is an essential issue to be treated at worldwide scale. There are different methods to remove the presence of contaminants in surface water and groundwater, including electrochemical processes, photocatalysis and adsorption [5, 6, 7, 8], as most representatives.  
Metolachlor--belonging to the acetamide class of herbicides--is applied as a mixture enriched in S isomer (S-metolachlor), widely used for selective weed control, and it represents one of the most common organic compounds found in groundwater [1, 9, 10]. Its degradation in water is promoted by microbial decomposition, provoking the formation of metolachlor oxanilic acid (OXA) and metolachlor ethane sulfonic acid (ESA) which are commonly quantified in groundwater at concentrations over that of metolachlor, which can reach up to 5 \(\upmu\)g/L [11, 12, 13, 14, 15]. Concerning the water intended for human consumption, the EU (Drinking Water Directive 98/83/EC [16]) established a maximum legal concentration of 0.1 \(\upmu\)g/L for individual pesticides and their relevant TPs, and 0.5 \(\upmu\)g/L for total pesticides and total relevant TPs. Hence, it seems clear that there is a need for an efficient methodology to remove the excess of pesticides and their TPs from water.  
Several works have reported on water remediation by the adsorption of metolachlor on different porous materials. [PERSON] and co-workers reported the adsorption of S-metolachlor on two mesoporous materials, a phenolic resin and a mesoporous carbon, revealing a high adsorption capacity due to their porous structure [17]. Similarly, [PERSON] et al. employed porous alginate-montmorillonite beads for the adsorption of metolachlor for water purification, showing the adsorption of the herbicide on the montmorillonite component [18]. More recently, activated carbons and biochar have been employed as efficient adsorbents for this herbicide [19; 20]. However, the differences in physicochemical properties between the metabolites ESA and OXA and their parent molecule may cause different adsorption mechanisms, since the parent pesticide is electrically neutral, whereas its two metabolites are anionic in aqueous environment.  
To the best of the authors' knowledge, the OXA and ESA metolachlor metabolites' adsorption, applied to environmental remediation, has not been studied. Accordingly, the purpose of this work is to analyze the TPs adsorption on four different highly porous activated carbons (commercially available and synthetized from biomass). For comparison, the parent compound (metolachlor) was also tested. The adsorbents were exhaustively characterized, and the relationship between the adsorbent's physicochemical properties and the yield of adsorption of the different organic compounds was studied.