Remoção de fósforo de efluente utilizando biocarvão de casca de café modificada com magnésio e ferro

Abstract

Phosphorus (P), despite being an essential nutrient for plant growth, when in excess in aquatic environments can lead to eutrophication. Thus, its removal from wastewater is essential to prevent contamination of water bodies. While conventional wastewater treatment processes are usually inefficient in removing phosphorus, biochars have shown to be very promising in removing this pollutant, especially when chemically modified to increase adsorption performance. Thus, this work aimed to produce a biochar composite from coffee husks, modified with magnesium and iron compounds, and its application in the post-treatment of swine slaughterhouse wastewater to remove phosphorus. The performance of the composite was compared with unmodified biochar (BC) and with a mixture of Mg and Fe compounds (M-Mg/Fe). The pyrolysis conditions for preparing the materials were a residence time of 2 hours at 480ºC, with an initial heating rate of 22.8ºC min-1. For the characterization of the materials, analysis of yield, pH, point of zero charge (PCZ), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (DRX), scanning electron microscopy (SEM) and dispersive energy (EDS). Adsorption studies of the materials with synthetic phosphorus solution were carried out, evaluating the effect of pH, ionic strength, as well as adsorption kinetics and equilibrium studies. Phosphate adsorption in slaughterhouse wastewater was evaluated by adsorbent dosage test and adsorption kinetics assays. By morphological analysis, BC had a very irregular surface with exfoliation and pores, while BC-Mg/Fe had a rough structure, with the presence of channels and amorphous surface deposits. M-Mg/Fe showed the formation of cubic phases and overlapping layers. The pH effect test showed that BC has a low phosphate adsorption capacity, with the highest observed efficiency of 15.7% (pH 12.0), while BC-Mg/Fe and M-Mg/Fe obtained 99, 3 and 98.8% adsorption efficiency, respectively, at pH 2.0 and 7.0). The presence of chloride ions, in the ionic strength test, did not influence the phosphate adsorption by BC-Mg/Fe and M-Mg/Fe (removals ≥ 99.0%). The Elovich model best fitted the experimental data for BC-Mg/Fe and M-Mg/Fe, indicating that chemisorption may be an important mechanism in the removal of phosphate by these adsorbents. The adsorption equilibrium for these materials was reached, respectively, after 360 and 600 minutes. The Sips isotherm best fitted the experimental BC-Mg/Fe data, with an estimated maximum adsorption capacity (qmax) of 216.47 mg g-1. For M-Mg/Fe, it was the Redlich-Peterson isotherm, with qmax of 133.52 mg g-1. For the tests with swine slaughterhouse wastewater, the dose of 4 g L-1 of BC-Mg/Fe was enough to remove 97.0% of phosphate present in the effluent, while 6 g L-1 of M-Mg/ Fe to remove 96.7% of the phosphate. The pseudo-second order model presented a good fit to the experimental data of BC-Mg/Fe with the effluent, while for M-Mg/Fe it was the Elovich model, corroborating that the chemical mechanisms may have been important in the phosphate adsorption, adsorption equilibrium being reached, respectively, in 240 and 600 minutes. Thus, the high adsorptive capacity of BC-Mg/Fe and the efficiency of phosphate removal in effluents evidenced the great potential of this material for post-treatment of effluents.