Pre-concentration of nalidixic acid through adsorption-desorption cycles: Adsorbent selection and modeling

A new approach is proposed for pre-concentrating emerging pollutants, taking nalidixic acid as representative. The approach consists of performing successive adsorption-desorption cycles in order to remove the pollutant from the main stream, generating a new stream with higher pollutant concentration during the adsorbent regeneration. For accomplishing these scopes (efficient removal and capability for pre-concentrate), adsorbents presenting both high adsorption capacity and regeneration easiness are needed. In this work, the performance of three different adsorbents is studied: activated carbon (GF-40), multi-walled carbon nanotubes (MWCNTs) and high surface area graphite (HSAG-500). Adsorption experiments were performed at 25 °C, whereas two different desorption temperatures (35 and 40 °C) were chosen in order to evaluate the ability of the materials for concentrating the NAL. The adsorption breakthrough curves were modeled considering different models: Bed Depth Service Time (BDST), Thomas and Yoon-Nelson. Although activated carbon largely presents the highest adsorption capacity, it is difficult to regenerate needing long regeneration times, leading to very dilute regeneration solutions. By contrast, MWCNTs present lower adsorption capacity, but they are more easily regenerated leading to higher NAL concentrations.