Effect of ultrasonic-assisted organic acid washing on heavy metals and nutrients in tannery sludge

Abstract

Abstract The rapid growth of the tannery industry has led to an increasingly serious problem of how to treat tannery sludge. Washing, which is an effective method for remediating polluted sludge, requires suitable washing reagents. In this study, the removal of heavy metals by washing tannery sludge with three organic acid reagents, namely citric acid (CA), oxalic acid (OA) and tartaric acid (TA), was investigated. The influences of washing reagent concentration, pH, solid‒liquid ratio, and ultrasound assistance on the removal of heavy metals were also analysed. When the optimal single-factor test was satisfied, the removal rates of Cr, Zn and Pb reached 49.11%, 63.74% and 31.88% for CA, 66.25%, 56.72% and 10.44% for OA and 32.79%, 67.53% and 18.34% for TA, respectively. The use of ultrasound-assisted organic acid treatment for heavy metal removal significantly improved the washing efficiency and shortened the reaction time. Moreover, tannery sludge washed by CA, OA and TA, in which the residual heavy metals are less likely to mobile and less harmful to the environment. In addition, the pH and ammonium nitrogen (AN) and rapidly available potassium (AK) contents of the washed tannery sludge decreased, while the organic matter (OM) and available phosphorus (AP) contents increased. The results showed that CA, OA and TA are effective washing reagents and that the washed sludge has the potential for resource utilization.