Ultrasound-enhanced combined removal of typical heavy metals from tannery sludge by organic acids
The rapid development of the tanning industry has led to a dramatic increase in the production of tannery sludge,improper disposal will lead to a large number of heavy metals and nitrogen and phosphorus elements in the tannery sludge leachate into the surrounding environment and jeopardize the ecological environment and human health.Therefore,the problem of rational disposal of tannery sludge has become a serious obstacle to the development of the industry.Among the solid waste disposal methods,chemical washing is a commonly used method for removing heavy metals from solid waste,but it is often ineffective in removing the compound pollution of multiple heavy metals in tannery sludge.In this study,two typical organic acids such as polycarboxylic acid(citric acid)and dicarboxylic acid(oxalic acid and tartaric acid)were used as drenching agents combined with ultrasonic enhancement for the targeted drenching of various characteristic heavy metals(Cr,Zn and Pb)and major plant nutrients(ammonium nitrogen,rapidly available potassium,organic matter)in tannery sludge.To investigate the main factors influencing the effectiveness of the combined chemical-physical drenching process on the removal of heavy metals from tannery sludge(concentration of washing agent,pH,solid-liquid ratio,ultrasonic enhancement)and loss of plant nutrients from tannery sludge.The experimental results showed that there were significant differences in the removal of different types of heavy metals in tannery sludge by different types of organic acids under ultrasound-enhanced conditions,among which the optimal removal of Cr,Zn and Pb from tannery sludge was 49.11％,63.74％and 31.88％for citric acid(CA),76.25％,65.24％and 9.98％for oxalic acid(OA)and tartaric acid(TA)was 32.79％,67.53％and 18.34％.At the same time,the use of ultrasound-enhanced removal of tannery sludge resulted in a significant increase in heavy metal removal efficiency(increase 8.75％～22.67％)and a highly significant reduction in reaction equilibrium time(from 720 min to 90 min)compared to the conventional heavy metal washing method.The chemical forms of heavy metals(mainly in residual and oxidized states)remaining in the tannery sludge were still in a relatively stable form after washing with three organic acids.The results of Fourier transform infrared(FTIR)spectroscopy showed that the carboxyl and carbonyl groups contained in the organic acids removed heavy metals from the tannery sludge by mechanisms such as ion exchange,chelation and electrostatic adsorption.Retention of most of the plant nutrients in tannery sludge after washing with organic acids.Among them,the pH,alkaline dissolved nitrogen(AN),and quick-acting potassium(AK)contents of the drenched sludge were decreased,whereas the organic matter(OM)and quick-acting phosphorus(AP)contents or ratios were increased in relative terms.The experiments confirmed that the optimized chemical-physical combined washing can achieve high heavy metal removal rate,low nutrient loss rate,and not easy to produce secondary pollution,and the related research results can provide a reference for the efficient removal of heavy metals in tannery sludge and the utilization of the washed sludge as a plant substrate.
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