Ultrasound-Assisted Extraction of Cr from Residual Tannery Leather: Feasibility of Ethylenediaminetetraacetic Acid as the Extraction Solution

Abstract

In this work, the use of ultrasound energy for chromium removal from residual tannery leather was investigated. The following parameters were evaluated: complexation temperature (60-90 °C), chromium/complexant molar ratio (1:0 to 1:6), complexation time (30-120 min), washing steps (1-14), washing temperature (25-80 °C), and washing time (1-10 min). For all evaluated conditions, chromium removal was monitored by flame atomic absorption spectrometry. The residual tannery leather after different extraction strategies were characterized using a scanning electron microscopy. For the proposed method, the optimized conditions were: 3 g of residual tannery leather to be treated, 100 mL of extraction solution (chromium/complexant molar ratio of 1:3), at 80 °C and 30 min of sonication. To complete the chromium removal, only five washing cycles (50 mL of water at 50 °C) of 3 min were required. Using these conditions, a chromium removal higher than 98% was achieved. Under the same reaction conditions, the results were compared with mechanical stirring (100 rpm), which allowed observing the significant effects of ultrasound for chromium removal. Comparing to the conventional method, the total time of the process (including extraction and washing steps) was decreased from 150 to 45 min and the water volume for the washing was reduced from 450 to 250 mL. Therefore, the proposed ultrasound-assisted process can be considered as a suitable alternative for chromium removal from residual tannery leather.

Figure 1

Influence of the Cr/EDTA molar ratio on the Cr removal by conventional extraction (n = 3). Conditions: 100 mL of extraction solution and 3 g of residual tannery leather stirred for 120 min at 70 °C, followed by 14 washing steps with 50 mL water at 25 °C and 10 min.

Figure 2

Influence of the number of washing steps on the Cr removal by conventional extraction (n = 3). Conditions: 3 g of residual tannery leather in 100 mL of extraction solution (Cr/EDTA molar ratio of 1:3). The water volume in each washing was 50 mL, the temperature set at 25 °C, and time of 10 min.

Figure 3

Effect of complexation time and temperature in the conventional extraction (n = 3). Conditions: 3 g of residual tannery leather in 100 mL of extraction solution (molar ratio 1:3) with 9 washing cycles using 50 mL of water at 25 °C and 10 min.

Figure 4

SEM micrographs (magnification of 300 times) obtained for (a) nontreated residual tannery leather; (b) residual tannery leather after conventional extraction; and (c) residual tannery leather after UAE.