Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Research article Application of thermal lens microscopy (TLM) for measurement of Cr(VI) traces in wastewater L.A. Hernández-Carabalí a,1 , E. Cedeño a,1 , A. Mantilla a,* , S. Alvarado a , H. Cabrera b,c , A.M. Mansanares d , A. Calderón a , E. Marín a a Instituto Politécnico Nacional, CICATA Legaria, Legaria 694, Colonia Irrigación, CP 11500, Ciudad de México, Mexico b National Institute for Nuclear Physics (INFN), Sezione di Trieste, Via A. Valerio 2, 34127, Trieste, Italy c The Abdus Salam International Centre for Theoretical Physics, Trieste 34151, Italy d Gleb Wataghin Physics Institute, U. of Campinas-UNICAMP, 13083-859, Campinas, SP, Brazil ARTICLE INFO Keywords: Thermal lens microscopy Photothermal Photocatalysis Photoreduction CdS photocatalyst Cr(VI) ABSTRACT In this work, we demonstrate for the first time that Thermal Lens Microscopy technique (TLM) can be applied to monitor the dynamics of a photocatalytic process in-situ. The photocatalytic reduction of hexavalent chromium -Cr(VI)- in aqueous solution using CdS and irradiated with visible light is monitored by TLM. Since the values of Cr(VI) concentration obtained after the photocatalytic process were close to those imposed by the international regulations for drinking water, the use of TLM allowed its measurement with a better reliability than with UV spectroscopy, usually used in this kind of analysis. 1. Introduction Chromium is present in natural and anthropogenically modified waters in two oxidation states: Cr(VI) and Cr (III). Due to its toxic, mutagenic, carcinogenic and teratogenic character, Cr(VI) is considered one of the most toxic contaminants that threaten water supply world- wide, being released in most of the cases in the effluent of industrial processes such as electroplating, wood treatment, leather tanning and pigment manufacturing. Chromium (VI) content has been regulated in most of the countries around the world (The Agency for Toxic Subs, 2000), being the maximum concentration in groundwater allowed by the United States Environmental Protection Agency limited to 100 ppb (0.1 mg/L), and for drinking water as low as 50 ppb (0.05 mg/L) (World health organization (WHO), 2004; United States Environmental pro- tection Agency (EPA), 2003; Federal Register and Act (47). Several methods have been reported for the removal of Cr(VI) from wastewater, being the physicochemical one of the most used (Chen and Hao, 2007; Ayuso et al., 2007). However, the final concentration of Cr (VI) in the effluents after this treatment is superior to the maximum limits allowed by most of the environmental legislations. In recent years, several environmentally friendly methods like membrane separation (reverse osmosis) (Kaya et al., 2013), ion ex- change (Zhu et al., 2016; Dai et al., 2015; Pourfadakari et al., 2017), electrochemical, and photocatalytic processes have been proposed to treat contaminated water. Among these, heterogeneous photocatalysis has proven to be an efficient, economical and simple method for the purification of industrial wastewater contaminated with different pol- lutants, such as organic compounds and heavy metals. Photocatalytic processes are based on the reactive properties of the electron-hole pairs generated in a semiconductor material under its illumination by a light source with greater energy than the semiconductor band gap value. It has been reported values of Cr(VI) of few ppm after the photo- reduction of an aqueous solution in presence of different photocatalysts, irradiated with UV and/or visible light (Idris et al., 2012; Idris et al., 2010), and in some cases a total photoreduction, when high efficiency photocatalysts such as CdS are employed, all of them measured by UV–Vis spectroscopy technique (Shaban, 2013; Nagarjuna et al., 2017; Zhang et al., 2013; Mekatel et al., 2012; Challagulla et al., 2016). However, the sensitivity of this technique is in the range of a few ppm, making that these results could be not so reliable, in the case of con- centrations of Cr(VI) below the above mentioned limits established by the international standards for drinking water. Therefore, the search of novel methods for monitoring the de- gradation rate induced by photocatalysis in the sub ppb range has be- come a challenge to meet, where thermal lens microscopy (TLM), has a great potential (Alvarado et al., 2014). https://doi.org/10.1016/j.jenvman.2018.11.044 Received 11 November 2017; Received in revised form 11 November 2018; Accepted 13 November 2018 * Corresponding author. E-mail addresses: angelesmantilla@yahoo.com.mx, angelesmantilla@hotmail.com (A. Mantilla). 1 Contributed equally to this work. Journal of Environmental Management 232 (2019) 305–309 0301-4797/ © 2018 Elsevier Ltd. All rights reserved. T