Study of catalytic processes for biodiesel production wastewater treatment

Abstract

The study of the possibility of removing organic compounds from wastewater originating from the biodiesel purifi cation stage by two catalytic processes, HSO5 -/transition metal and Fenton method has been presented. The source of the ion HSO5 is potassium monopersulphate (2KHSO5∙KHSO4∙K2SO4) (Oxone) that may be decomposed into radicals (OH., SO4 -., SO5 -.) by means of transition metal as Co (II). Different concentrations were used for both compounds and the combination ([Co2+] =1.00μM/[HSO5 -] =5.00×10-2 M) achieved the highest COD removal (60%) and complete decomposition of the oxidant was verifi ed for contact times of 45 min. This process has some advantages comparing to the conventional Fenton method such as the absence of the costly pH adjustment and the Fe(III) hydroxide sludge which characterize this treatment process. The Fenton process showed that the combination of [H2O2] =2.00M/[Fe 2+] = 0.70 M was the best and archived COD removal of 80%. The treatments studied in this research have achieved high COD removal, but the wastewater from the biodiesel purifi cation stage presents very high parametric values of Chemical Oxygen Demand (667,000 mgO2/L), so the fi nal COD concentration reached is still above the emission limit of discharge in surface water, according the Portuguese Law (Decree-Law 236/98). However, both treatments have proved to be feasible techniques for the pre-oxidation of the wastewater under study and can be considered as a suitable pre-treatment for this type of wastewaters. A rough economic analysis of both processes was, also, made. Archives vol 45 no 2 a4 srodki.indd 66 2019-05-09 14:53:01 Study of catalytic processes for biodiesel production wastewater treatment 67 2012, Chi et al. 2016, Wang and Chu 2012) thereby providing mechanisms for reaction with some advantages comparing to hydroxyl radicals (Hu and Long 2016, Oh et al. 2016). Oxone can also be decomposed into radicals by means of transition metals, such as Co (II), Ru (II) and Mn (II) (Rivas et al. 2012), according Eqs.1, 2 and 3: HSO5 + M n+ → SO4 .+M(n+1) + + OH(Eq. 1) HSO5 + M n+ → SO4 +M(n+1) + + OH. (Eq. 2) HSO5 + M n+ → SO5 +M(n+1) + + H+ (Eq. 3) M = Co (II), Ru (II) or Mn (II) Co (II) has been reported as one of the most effective metal catalysts for the activation of HSO5 promoting a radical sulphate complex mechanism (Rivas et al. 2012, Sun et al. 2009). So, the main objective of this study was to evaluate the performance of two catalytic treatments based in advanced oxidation processes; the traditional Fenton process and the emerging treatment that uses the compound monopersulphate (HSO5 -) on the removal of wastewater COD from the biodiesel purifi cation stage. Several combinations of the amounts of oxidant/catalyst have been investigated and optimum conditions assessed. Materials and methods Materials and chemical reagents Table 1 summarises the physicochemical characteristics of the biodiesel wastewater used, provided by PRIO (Prio Energy Co). The reagents used in this work were: Sulfuric acid, H2SO4, 95–98%, (Panreac); Ultrapure water, Mili-Q. Resistivity 10 MΩ cm; Sodium hydroxide, NaOH, 98%, (Panreac); Potassium iodate, KIO3, (Panreac); Potassium iodide, KI, 99.0–100.5%, (Panreac); Ammonium molybdate, (NH4)2MoO4∙4H2O, 93.1–101.0% (Panreac); Oxone, 2KHSO5∙KHSO4∙K2SO4, 99.9%, (Sigma Aldrich); Cobalt(II) sulphate, CoSO4∙7H2O, (Sigma Aldrich); Sodium thiosulphate, Na2S2O3, 98–102%, (Panreac); Hydrochloric acid, HCl, 36% (Panreac); Barium chloride, BaCl2∙H2O (Panreac); Hydrogen peroxide, H2O2, 30% w/w, (Scharlab); Iron(II) sulphate, FeSO4∙7H2O, (Sigma Aldrich); Potassium dichromate, K2Cr2O7 , (Panreac) and Silver nitrate, AgNO3, (Panreac). All reagents used were analytical grade. Analytical method Different physico-chemical parameters such as pH, Conductivity, Redox potential, Turbidity, Chemical Oxygen Demand (COD), Oil and grease and Chlorides were analysed for the sample characterization. The characterization was performed according to Standardized Water and Wastewater Methods (APHA 2012). The pH and Redox Potential were determined by the potentiometer pH – Metron 654 pH meter and WTW – Inolab potentiometer respectively, while Conductivity was measured by the conductivity meter Methohm, and the Turbidity using a WTW Turb.550 turbidimeter. In the determination of sulphates the furnace, Thermo – Heraeus, model K 114 was used. For both catalytic treatments (HSO5 -/Co2+ and Fenton process) several concentrations of Oxone and hydrogen peroxide were tested and its evolution over time of reaction was monitored using the iodometric method. This method consists in adding to the sample an excess of iodide which is titrated with sodium thiosulphate solution. Thus, 5 mL of sample was taken, 10 mL of H2SO4, 2.0 N, 25 mL of KI (20 g/L) and a few drops of saturated ammonium molybdate solution were added, which acted as a catalyst. The reaction that occurs is: HSO5 + 2I+ 2H+ → I2 + H2O + HSO4 (Eq. 4) H2O2 + 2I + 2H+ → I2 + H2O + OH (Eq. 5) Then it is titrated with sodium thiosulfate, previously measured with KIO3 and the desired concentration is obtained by: C = (VS2O3 2∙ CS2O3 2-)/2 ∙ Vsample (Eq. 6) where VS2O3 2is the volume of thiosulphate spent in the titration, CS2O3 2is the concentration of the thiosulphate solution used and V sample is the volume of the sample. At the end of treatments organic matter degradation was carried out by COD determination (APHA 2012). The sulphates were determined using the gravimetric method (APHA 2012) and the chlorides with the Mohr method (APHA 2012). Methodology of research Both experiments were carried out in a discontinuous way using 300 mL of wastewater with stirring at a constant speed of 250 rpm to avoid concentration gradients. The assays started Table 1. Biodiesel wastewater physicochemical characteristics and the respective Limit Emission Values (LEV) of wastewater discharges in surface water according Decree-Law 236/98 (Portuguese legislation) Parameter Biodiesel wastewater Decree-Law 236/98 (LEV)