Abstract
The residual effluents from the fur stage of the bovine leather tannery industry are characterized by having a high concentration of sulfides. The objective of this study was to evaluate the effects of aeration time and pH in the residual effluents of the leather stage of the tannery, with the catalysts MnO2 and MnSO4 separately; as well as, determine adjustment models through the response surface methodology and the optimal intervals of the best conditions that lead to a higher percentage of sulfide removal. For this reason, the sulphide removal percentage was evaluated from samples extracted from the pellet stage, by means of catalytic oxidation treatments; varying the catalyst, pH and aeration time. The catalysts used were manganese dioxide (MnO2) and manganese sulfate (MnSO4) and for each catalyst the pH was varied in the values of 8.5; 9.5; 10.2 and 13.4; likewise, the aeration time was varied in the values of 30, 60, 90, 120, 150, 180, 210 and 240 minutes. 64 treatments were carried out, with 3 repetitions each, reporting the average values of the sulfide removal percentage. The response surface methodology was used to adjust the correlation of the variables to a quadratic model; Likewise, through contour graphs the regions with the highest percentage of sulfide removal were easily identified and by superimposing contour graphs the optimal ranges of the variables pH and aeration time were determined for removal percentages greater than 98%. Based on this evaluation, it is proposed for treatments with manganese dioxide, aeration times between 160 to 240 min and pH between 8.5 to 9 and for treatments with manganese sulfate, aeration times between 110 to 240 min and pH between 8.5 to 9.8. The coefficients of multiple determination R2 for the models with catalyst MnO2 and MnSO4 were 97.51% and 95.12% respectively. With the MnSO4 catalyst, higher removal percentages were achieved at a shorter aeration time, compared to the treatments carried out with the MnO2 catalyst.
Highlights
The tanning industry is considered one of the most polluting industrial activities in the world, which generates effluents that contain persistent pollutants and has been recognized as a serious environmental threat due to high levels of salinity, organic and inorganic matter, suspended solids, dissolved ammonia, total nitrogen, sulfur, sodium chloride, chromium, among others. (Kothiyal et al, 2016; Salas, 2005)
It should be noted that tannery wastewater can vary significantly, depending on the nature of the tanning process, the processing capacity, the amount of water used and the storage process of the hides. (Caliari et al, 2019)
It is observed that as the pH increases, the removal percentages decrease both for the treatment with MnO2 and with MnSO4, since in several studies it has been shown that a decrease in the pH of the reaction solution increases the oxidation capacity. the catalyst; in turn, both the speed and the final oxidation product depend on this parameter. (Herszage and Dos Santos, 2003; Ahmad, 2009; Qiu, 2011; Wilk et al, 2020)
Summary
The tanning industry is considered one of the most polluting industrial activities in the world, which generates effluents that contain persistent pollutants and has been recognized as a serious environmental threat due to high levels of salinity, organic and inorganic matter, suspended solids, dissolved ammonia, total nitrogen, sulfur, sodium chloride, chromium, among others. (Kothiyal et al, 2016; Salas, 2005). In its wastewater from this process, sulfur (S2-) becomes one of the most important pollutants in different aqueous solutions and aquatic media, as well as in the air, because it is released in the form of hydrogen sulfide (H2S) at pH
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