Valorização de resíduos lignocelulósicos como bioadsorventes de íons cobre e desenvolvimento de sensor colorimétrico a base de levulinato de sódio para sua detecção

Abstract

In recent years, humanity has advanced in many ways, but it has also caused serious damage to the environment. Human activities such as burning fossil fuels, deforestation and waste production have generated pollutants that affect air, water and soil quality. In this scenario, the contamination of water bodies has become a serious environmental problem due to issues related to the improper disposal of effluents contaminated by heavy metals. As a result, this heavy metal pollution has serious consequences for the health of people and ecosystems. In light of these facts, this thesis aimed to evaluate the potential of using bioadsorbents obtained from cashew leaf (CL), carnauba straw (CS) and the extraction of lignin from green coconut fiber (GCF) to remove copper ions (Cu2+) from synthetic effluents. Nevertheless, the feasibility of using an organic molecular sensor obtained from the synthesis of sodium levulinate (NaLev) for the colorimetric detection of Cu2+ was evaluated, which constitutes a simple and efficient alternative for on-site analysis of metal ions in aqueous solution. To this end, adsorption studies were carried out in a batch system and the effects of pH, adsorbent concentration, particle size and initial adsorbate concentration were evaluated. In order to extract the lignin from the GCF, an alkaline pre-treatment with 2% sodium hydroxide was carried out, followed by impregnation with sodium octanoate to improve the adsorption capacity of this bioadsorbent. NaLev was synthesized by hydrating levulinic acid with distilled water and adding 20 g of sodium hydroxide. The concentrations of copper, NaLev and then the copper-NaLev mixture were determined by molecular absorption spectroscopy in the ultraviolet and visible (UV-Vis) regions. The effects of different concentrations of Cu2+ and NaLev on the maximum absorbance and wavelength were evaluated. The experimental CS data were best fitted by the Langmuir monolayer model, while the CL data were fitted by the Freundlich model. For the same experimental conditions, the maximum adsorption capacity for Cu2+ was 9.51 mg.g-1 and 1.73 mg.g-1 for CS and CL powders, respectively. For GCF lignin impregnated with surfactant, the results of the adsorption study revealed that the adsorption capacity improved due to the increase in the number of functional groups on the bioadsorbent surface. The results of the colorimetric sensor evaluation showed that the addition of NaLev significantly increased the absorbance as the wavelength decreased from 810 to 575 nm in the samples without and with NaLev, respectively. In addition, the color intensity of the solution containing Cu2+ increased as the concentration of NaLev increased. The results showed that it is feasible to use lignocellulosic biomass to remove Cu2+ since the removal capacity is similar to other traditional sources of adsorbents. It was also observed that alkaline pre-treatment and impregnation using the surfactant carried out in the GCF was effective in obtaining a bioadsorbent with adequate Cu2+ removal capacity. Finally, the NaLev colorimetric sensor for detecting Cu2+ appears to be a promising class of biobased heavy metal-free colorimetric sensors.