Leather is usually made from the raw hides of various animals processed through physical and chemical manufacturing processes and is one of the most important materials used in the manufacture of clothing and footwear (Al-Jabari et al. 2021). The tanning process generates approximately one million tons of fresh tanning sludge with a moisture content greater than 70% per year, and annual sludge production is still increasing by approximately 10% (Zhai et al. 2020). Leather can only absorb 55–75% of the tanning agent, basic chromium sulfate (BCS), which results in a large amount of chromium (Cr) flowing into the effluent and producing a Cr-rich sludge (Kumar et al. 2023). Therefore, the Cr content in tannery sludge is extremely high, usually up to 10,000 mg/kg. Cr entering the environment is mainly present in two valence states, Cr(III) and Cr(VI) (Bartlett 1991). Among these forms, Cr(III) is comparatively immobile, while Cr(VI) is extremely mobile in the environment and once it enters the food chain, it can be mutagenic, persistent, carcinogenic and teratogenic to biological organisms (Monga et al. 2022). In addition, the remaining heavy metals (e.g., Zn and Pb) in tannery sludge also exert a threat to the ecosystem (Xia et al. 2020). However, tannery sludge also contains a large amount of plant-available nutrients, including organic matter, nitrogen and phosphorus (Rigueiro-Rodríguez et al. 2012; Urra et al. 2019). These nutrients can act as soil conditioners and are particularly essential for ameliorating the physicochemical properties of the soil and increasing its productivity indicators (Halecki and Klatka 2021). Currently, land use of sludge is the most ecological and economical way to reduce sludge as well as to provide nutrients to plants (Zhu et al. 2013). Hence, especially in the case of sludge land use, a remediation approach that reduces both toxic metals and nutrient loss is needed.
Many methods of sludge treatment and disposal have been investigated, such as chemical washing, solidification/stabilization, soil replacement, electrokinetic treatment, and phytoremediation (Yoo et al. 2018; O’Connor et al. 2018; Cameselle and Gouveia 2019; Azhar et al. 2022). Chemical washing has attracted attention for its advantages including high removal efficiency, simple operation, economic efficiency and thorough treatment (Khalid et al. 2017; Guo et al. 2022). The suitable washing reagent is particularly essential in the washing process. Presently, the washing reagents for removing heavy metals from soil are chelating reagents, surfactants, inorganic acids and organic acids. (Stylianou et al. 2007; Zhang et al. 2010; Wang and Mulligan 2013; Mao et al. 2015). Although inorganic acids can elute heavy metals contained in the soil in a short time by desorption or dissolution into the liquid phase, it deteriorates the physical and chemical properties of the soil, resulting in a significant loss of minerals and nutrients from the soil (Reed et al. 1996). Surfactants have the advantages of lower biostimulation, good biodegradability and less likely to cause secondary pollution, but are more expensive and have an average removal rate of heavy metals from compound pollution (Paramkusam et al. 2015). However, ethylenediaminetetraacetic acid (EDTA) is difficult to degrade once it enters the environment and stays there for a long time, which has an impact on the properties of soil aggregates (e.g., stability and water retention), leading to the inhibition of microbial and plant growth and bringing the possibility of heavy metal remobilization in the environment (Beiyuan et al. 2018; Guo et al. 2018).
However, organic acids are not only excellent for the removal of heavy metal complex contamination from soil, but also do not lead to secondary pollution of the environment due to self-degradation (Wei et al. 2016; Zhang et al. 2017). Research shows that CA has good biodegradability, with 20% degradation in 1–4 days and 70% degradation in 20 days, depending on the degree of soil contamination (Wen et al. 2009). Although the type of washing reagent is the main influencing factor, the effect of variables such as reagent concentration, S/L ratio, pH and reaction time on heavy metal removal efficiency should also be of interest (Wang et al. 2020).
Studies have shown that the removal/desorption of heavy metals from soils can be enhanced by the sonophysical effects of ultrasound, such as shock waves, microcurrents and microjets, which contribute to the efficiency of removing heavy metals from soils (Arain et al. 2008; Choi et al. 2021). Another study on ultrasound-assisted heavy metal removal by citric acid showed that the time to reach equilibrium for ultrasound-assisted heavy metal removal was reduced from 24 hours to 20 minutes, significantly reducing the time for heavy metal removal by citric acid (Wang et al. 2015). Although many studies have reported the release of heavy metals from sludge by ultrasound-assisted washing, changes in organic matter, nitrogen, phosphorus and potassium in sludge after ultrasound treatment have rarely been studied.
The objectives of this study were (1) to evaluate the ability of citric acid, oxalic acid and tartaric acid to remove heavy metals at different concentrations, pH values, S/L ratios, and reaction times; (2) to explore the changes in the chemical and morphological of heavy metals in sludge before and after washing and the associated ecological risks; and (3) to investigate the nutrient changes in the washed sludge and assess its resource utilization potential.