(188a) Use of an Adsorbent Produced from Residual Sludge from the Aluminum Industry for the Removal of Chlorpyrifos

The aluminum industry produces a high number of wastes that are concentrated in the sludge of their water treatment plants. The industry is one of the most prosperous in recent years, being aluminum one of the most widely used non-ferrous metals worldwide. Its production has increased in the last decade from 37,606 thousand metric tons in 2009 to 63,697 thousand metric tons in 2019, showing a growth of 59% for this period [1]. The main applications include transportation, packaging, construction, and electrical engineering [2]. This sludge is rich in aluminum, which could be extracted and purified. This work aims to evaluate, by different extraction methods, the recovery of the aluminum contained in a wastewater sludge derived from a metallurgical industry. For this purpose, Bayer, HCl, and isopropanol extraction methods are compared for different reagent ratios. Besides, the extracted product has been evaluated for its application as adsorbent for the removal of an organophosphate pesticide (chlorpyrifos, CPS) from contaminated waters. In this work, three extraction methods have been evaluated to recover the aluminum from the sludge samples provided by a metallurgical industry from Medellín, Colombia. The sludge has been characterized to confirm its high aluminum content. The Bayer extraction method has been carried out, using 1:10, 1:15 and 1:20 sludge/solution mass ratios, and HCl extraction, by using 1:15 and 1:20 ratios. The extraction with isopropanol has also been evaluated. The extraction agent for this method was analytical grade isopropanol. The tests were carried out in accordance with the procedure proposed by Saiz et al. [3]. The goal of this process is to produce aluminum monohydroxide by reacting sludge with isopropanol to produce aluminum isopropoxide. Because of its widespread use as a pesticide in agriculture, CPS has been used as a reference water pollutant in adsorption tests, particularly in developing countries such as Colombia. Adsorption tests were performed with different weights (0-5 g) of the precipitate obtained from the Bayer extraction process in a 1:15 ratio. All samples were made in quadruplicate to ensure reproducibility. The vials were filled with 100 mL of the standard solution, the flasks were shaken, and samples were taken at 5, 30, 60, 90, 120, 150, 180, 210, and 240 minutes. The equilibrium parameters for the Freundlich and Langmuir models, have been optimized by minimizing an objective function, OF, defined as the sum of the squares of the differences between the experimentally measured adsorbate concentration in the liquid phase, and the values calculated by the model. In MATLAB, a calculation algorithm based on the Nelder-Mead algorithm has been implemented, which uses the ode subroutine to solve the mass balance equationsand the fminsearch subroutine to calculate the without restrictions minimum of the objective function. According to XRF results, Al (68.52%) stands out among other species, with concentration levels comparable to those found in other studies. As a result, this sludge contains a high concentration of Al, which has a high potential for extraction and little interference from other metallic species. The high concentration of S (28 wt%) observed could be attributed to neutralization treatments performed in the company at its origin. The amounts of Al, Fe, Cu, and Ni present in the aqueous phase have been measured to determine the recovery of the metals, as well as the selectivity of the extraction method. The highest metal recovery (99.3%) is achieved by HCl extraction for a sludge/solution ratio of 1:20, while the Bayer method (1:15 ratio) has shown the best selectivity. On the other hand, the extraction with isopropanol has not been successful for any of the ratios evaluated. The extracted material has been tested as an adsorbent for the removal of chlorpyrifos (200 mg/l), where the removal percentages have been higher than 95% for all the weight ratios studied. Hence, this Al-enriched material shows good prospects to be used as an adsorbent in the treatment of polluted water.

Among the various Al extraction methods investigated, the Bayer method proved to be the most practical, particularly for 1:15 and 1:20 sludge/NaOH ratios, where an Al recovery of 80% and 100%, respectively, was obtained. However, the selectivity for the 1:20 ratio was significantly lower than that for the 1:15 ratio. Furthermore, with a 1:20 ratio, an average recovery of 60% iron, 40% copper, and 2.5% nickel was obtained. Based on these findings, the Al samples extracted by the Bayer method with a ratio of 1:15 (BES15) were tested for CPS adsorption, and a removal of 99% was achieved for 5 to 240 mins, with no evidence of desorption. According to the Langmuir isotherm, a good fit has been established for the adsorption process, in which a single layer is formed at the start of the process.

References

1. IAI, Primary aluminum production, Int. IAI Inst. (2019). http://www.world-aluminium.org/statistics/primary-aluminium-production/.
2. V. Marques, R.L. Barcelos, G.O.C. Parma, E. Girotto, A.C. Junior, N.C. Pereira, RF. Magnago, Recycled polyethylene terephthalate and aluminum anodizing sludge-based boards with flame resistance, Waste Manag. 92 (2019) 1–14. https://doi.org/10.1016/j.wasman.2019.05.013.
3. S. Saiz, G.C. Vargas, J.C.M. Piraján, Obtención de pseudoboehmita mediante el método de sol-gel empleando dos catalizadores diferentes, Av. Investig. En Ing. 1 (2010) 35–44.