Synthesis of montmorillonite-alginate beads applied to the kinetic study of paraquat adsorption

MARIANA ETCHEVERRY; GRACIELA ZANINI

Wuhan-Modalidad Virtual

Conferencia; The 11th International Conference on Interfaces Against Pollution (IAP2021); 2021

Huazhong Agricultural University (HZAU)

Paraquat (1′-dimethyl-4,4′-bipyridinium dichloride, PQ) is a quaternary ammonium herbicide. Althoughits use is restricted within the European Union, as it is considered a highly hazardous pesticide (HHP), itis still used worldwide, so further research into this pollutant is needed. Water pollutants removal ismainly carried out by adsorption processes. These processes require the use of adsorbents with somefeatures such as low cost, accessibility and non-toxicity. Alginate (Alg) - Montmorillonite (Mt) beads havebeen effective for the adsorption of PQ [1].The aim of this work is synthesise and characterise alginate beads loaded with a 70 % of montmorillonite(Alg-Mt70) at different resident times in CaCl2 in order to analyse the effect of the ionic gelation processon the PQ adsorption process. In addition, the adsorption kinetic of the pollutant paraquat on Alg-Mt70beads was evaluated. Beads were synthesised by dissolving sodium alginate in 0.1 M NaCl to form a 1.0% by weight alginate solution. A set amount of montmorillonite clay was added to this solution and themixture was homogenised on a mechanical stirrer overnight. This homogenised solution was addeddropwise to a 0.1 M CaCl2 solution with a peristaltic pump to produce calcium alginate-montmorillonite(Alg-Mt) beads. The residence times studied of the beads formed in this CaCl2 solution were 30; 60; 120minutes; 1; 4 and 7 days, respectively. The beads were characterised by elemental composition,thermogravimetric analysis (TG-DSC) and scanning electron microscopy (SEM-EDX). In order toperform the morphological analysis, the beads were freeze-dried (pressure of 0.060 mmHg for 72 h),therefore the qualitative and quantitative composition of the beads is not altered during drying. Thebeads keep their shape and collapse of the structure is prevented, so that the micrographs can showtheir internal structure. The results clearly show the internal cavities formed. Semi-quantitative analysisshows that the bead surface has a higher percentage of carbon from alginate. The interior has a higherpercentage of aluminium and silicon from montmorillonite. These analyses show that Mt is mainlylocated inside the bead. After beads characterisation, the adsorption of PQ as a function of time wasstudied for 3 initial concentrations of the herbicide (C0 = 19, 38 and 50 ppm) on the beads with gelationtimes of 30 minutes and 7 days. The experiments were carried out by contacting the PQ with thematerials with constant stirring, then 4 mL of solution were withdrawn at set times, the PQ in that solutionwas quantified and then the volume was returned to keep the adsorbate/adsorbate ratio constant. Allexperiments were carried out at room temperature (25 ± 1°C) and 1´10-2 M NaCl ionic strength. Kineticsadsorption studies were also carried out using alginate beads without montmorillonite (Alg-Mt0). The PQwas spectrophotometrically quantify by UV/vis at 257 nm. Adsorption isotherms were obtained byperforming batch adsorption experiments. For that, Alg-Mt70 composites beads were placed inpolypropylene centrifuge tubes. After that proper volumes of 1´10-2 M NaCl and appropriate aliquots ofstock solution of PQ were added to cover concentration ranges from 5.0´10-5 to 3.0´10-3 M. The pH was5.5 ± 0.2 in all experiments. The tubes were shaken enough time for it to reach equilibrium. Allexperiments were carried out at room temperature (25 ± 1 ºC). The experimental results of herbicideadsorption as a function of time were fitted with the Shrinking Core Model (SCM) [2]. The concentrationprofile as a function of time was obtained. The results obtained showed a very good agreement betweenthe model prediction and the experimental data. The effective diffusion coefficients of the adsorbentmaterial were obtained as fitting parameters. No major differences were observed in the adsorptionbehaviour with the gelation time of the beads, so it can be concluded that Alg-Mt70 beads with only 30minutes in CaCl2 is a stable and efficient material for PQ adsorption, not requiring long gelationprocesses.References[1] Etcheverry M, Cappa V,Trelles J, Zanini GP. Montmorillonite-alginate beads: Natural mineraland biopolymers based sorbent of paraquat herbicides Journal of Environmental ChemicalEngineering 5 (2017) 5868.[2] Dominguez MA, Etcheverry M, Zanini GP. Evaluation of the adsorption kinetics of brilliant greendye onto a montmorillonite/alginate composite beads by the shrinking core model. Adsorption 25(2019) 1387.The authors thank CONICET, ANPCYT, UNS