Effect of ultrasound-assisted EDTA and citric acid washing on heavy metal removal, residual heavy metal mobility, and sewage sludge quality Open Access

The management and disposal of sewage sludge is one of the most complicated tasks for wastewater treatment plants. Each stage of the sewage treatment process produces sludge, and current sludge disposal methods and utilization technologies are unable to efficiently consume large quantities of sludge, causing a huge buildup of sewage sludge. Currently, sewage sludge disposal methods include land application, disposal in landfills, incineration, and the creation of building materials (Alvarenga et al. 2015). To examine and dispose of sewage sludge appropriately while the conventional landfill approach is being phased out, a new mindset is urgently required (Kelessidis & Stasinakis 2012). Given that sludge contains a large amount of nutrients, the application of fertilizers in barren soil can be greatly reduced by land application of sludge (Seleiman et al. 2013; Hoang et al. 2022). The nitrogen (N) content of sewage sludge is reported to be about 4% of the dry matter (DM), while the total phosphorus (P) content is about 2% of the DM (EPA 1994). Between 15 and 85% of this, N is available to plants, while approximately 50% of P is usually available to plants (Gilbert et al. 2011). The toxic and detrimental components of sewage sludge, however, restrict its use in agriculture. Among all kinds of pollutants in sewage sludge, heavy metals (HMs) are particularly prominent because of their toxicity, in-degradability, portability, and accumulation in the environment and in living organisms (Rutgersson et al. 2020; Zhang et al. 2022). Hence, the development of a technology that not only quickly and effectively reduces HMs in sludge but also maintains sludge nutrients will greatly contribute to appropriate land application of sludge.

Chemical washing is one of the few permanent treatment options for removing HMs from sewage sludge (Zhang et al. 2018; Klik et al. 2021). Traditional chemical washing technology involves thorough mixing and scrubbing of sewage sludge particles with eluents. Mechanical mixing is a key step in the chemical washing of HMs. Several studies have investigated the use of ultrasound for the removal of HMs in sewage sludge washing procedures. Ultrasound has several benefits in terms of increased effectiveness in removing HMs, operation time, energy use, and washing fluid use (Son et al. 2012; Park & Son 2017; Choi et al. 2021). Due to powerful desorption at macroscopic and microscopic scales, the ultrasound washing procedure has a reasonably high removal rate for pollutants in particle pores as well as those on the surface of sewage sludge particles (Son et al. 2011; Wang et al. 2015).

Washing agents, which are crucial components of chemical washing, have been widely employed to remove HMs from sewage sludge. These include chelation agents, surfactants, organic acids, and inorganic acids (Kuan et al. 2010; Tang et al. 2018; Yang et al. 2021). A representative substance in washing agents, ethylenediaminetetraacetic acid (EDTA), can create solid, water-soluble complexes with alkali metals, rare earth elements, and transition metals. However, EDTA has low biodegradability in the natural environment, and only a few bacterial isolates could degrade EDTA on a laboratory scale (Bloem et al. 2017; Freitas & Nascimento 2017). EDTA pollutes the environment mainly through wastewater generation; the average concentration of EDTA detected in groundwater, lakes, and river water was 2–100 μg/L, and it has become a prevalent anthropogenic compound in water samples (Kari & Giger 1995; Oviedo & Rodríguez 2003; Al Sakkaf et al. 2021). It is thus critical and urgent to develop a washing agent that not only effectively removes HMs from sludge but is also biodegradable and environmentally friendly. Citric acid (CA), both an organic acid and a chelation agent, has been extensively explored in HM washing because it is biodegradable and does not cause secondary pollution (Kong et al. 2013; Bao et al. 2019). Ke et al. (2020) found that the removal efficiency of HMs was 89.1% for cadmium (Cd), 26.8% for lead (Pb), 41.7% for zinc (Zn), and 14.2% for copper (Cu) in smelter soil when 2.5 L CA (0.1 M and pH 5) solution was leached in five batches. Moreover, washing Pb-contaminated soil with a low concentration (0.01 M) of CA modified with potassium chloride resulted in a maximum Pb removal of 77.6 ± 3.4% (Etim 2019).

In addition, rather than focusing exclusively on total metal content, the mobility and bioavailability of HMs before and after washing procedures are widely used to evaluate remediation efficacy (Guo et al. 2018; Hazrati et al. 2020). However, it is noteworthy that increased mobility and plant utilization of the remaining HMs in EDTA- or CA-washed soils has been demonstrated (Lei et al. 2008; Zhang et al. 2010; Wang et al. 2017; Hazrati et al. 2020). At the same time, the successful removal of HMs resulted in a substantial relative increase in total and exchangeable nutrients (nitrogen – N; phosphorus – P; potassium – K) in sewage sludge. The magnitude of these modifications was determined by the washing agent used and the washing procedure (Wang et al. 2016). As a result, trade-offs between the decrease in overall metal content, rise in metal mobility, and loss of nutrients during ultrasound-assisted EDTA and CA washing of sewage sludge pose novel issues for remediation design.

We thus aimed to comprehensively evaluate the effect of using eluent (EDTA and CA) and ultrasound-assisted washing on HM removal and residual HM mobility in sewage sludge. In addition, the quality of washed sewage sludge (pH, organic matter – OM; available P – AP; available K – AK; alkali N – AN) was evaluated, and the nutrient contents in the eluate measured. We further refined the procedure for ultrasound-assisted washing of HMs from sewage sludge.

Samples of sewage sludge were collected from a municipal sewage treatment plant using the sequencing batch reactor-activated sludge method, which is located in Guiyang, Guizhou, China. Following collection, the sewage sludge was freeze-dried until it reached a constant weight. Prior to use, all samples were evenly mixed and passed through a standard 100-mesh sieve. The physicochemical characteristics of the sewage sludge are shown in Table 1.

Various solution concentrations, ultrasound times, and washing agents (including CA and EDTA) were used for the experiments. A solid–liquid ratio of 1:4 (g:mL), an ultrasound time between 2 and 90 min, and a CA concentration between 0.1 and 0.6 M or an EDTA concentration between 0.05 and 0.3 M were used. We employed an ultrasound system with 600 W of power and a 40-kHz ultrasound transducer module. To maintain a constant operating temperature, the reservoir was filled with 2 L of tap water and the water was replaced, since the temperature in the reservoir was slightly increased (5–7 °C) when the ultrasound was running. All washings were performed in 50 mL centrifuge tubes at room temperature (25 °C). The mixture was then washed by centrifugation at 4,000 × g for 10 min and filtered (0.45 μm).

After determining the optimal washing parameters from the first-round washing experiment, the sewage sludge was subjected to eight continuous washings. We employed washing in a solid–liquid ratio of 1:4, an ultrasound time of 30 min, and a CA concentration of 0.4 M or an EDTA concentration of 0.2 M. All washings were performed in 50-mL centrifuge tubes at room temperature (25 °C). The ultrasound system parameters were consistent with those of the first-round washing experiment. The eluents were made up to the same initial scale after each washing. The mixture was then washed by centrifugation at 4,000 × g for 10 min and filtered (0.45 μm).

The pH values of the sewage sludge were measured at a solid–liquid ratio of 1:5 using a pH meter. Organic matter was determined by the Tyurin method. Alkali N was determined using the alkaline diffusion method. Available P was determined using the anti-spectrophotometric sodium hydrogen carbonate–Mo–Sb solution method. Available K was determined using flame photometry. The three-phase ratio of the sewage sludge was determined using soil three-phase tester.

The total metal concentration was determined using acid digestion (HCl/HF/HNO₃/HClO₄) followed by coupled plasma optical emission spectrometry (ICP-OES, Thermo Scientific iCAP 7000 Series ICP Spectrometer, USA). HM was extracted sequentially using the modified BCR-sequencing extraction method (Zhang et al. 2017). In short, four operationally defined fractions were extracted sequentially: acid exchangeable (F1, 0.11 M HAc), readily reducible (F2, 0.5 M NH₄OH·HCl, pH 2.0), oxidizable (F3, 8.8 M H₂O₂ + 1.0 M NH₄Ac, pH 2.0), and residual fractions (F4, HCl, HNO₃, HClO₄, and HF).

Total K was determined using flame atomic absorption spectrophotometry. Nitrate N was determined using UV spectrophotometry. Ammonia N was determined spectrophotometrically using Nessler's reagent. Total P was determined spectrophotometrically using ammonium molybdate. Total organic carbon (TOC) content was determined using a TOC analyser (Shanghai METSH, TOC-2000).

All experiments were repeated three times. All tubes and glassware were soaked in either dilute HNO₃ (10%) or H₂SO₄ (10%) overnight prior to the start of each experiment and rinsed with deionized water. Average values of the results are presented with error bars. Standard reference material for soil composition analysis (GBW07404 (GSS-4) was used for quality control. Matrix spikes were tested for each batch of samples, and recoveries of HMs were all within 80–120%. BCR SEP recoveries were within 85–105% via comparison of the sum of the four fractions with total metal concentration. Statistical analysis of the data was performed using SPSS 22.0 software (SPSS Inc., Chicago, IL, USA). The experimental results were presented as mean ± standard deviation (SD). Least significant difference tests were used to separate means where ANOVA showed significance at P < 0.05.

The role of CA (both chelating agent and weak acid) in the removal of HMs may be the result of two mechanisms: acid dissolution and metal complexation. On the one hand, only HMs that form more stable complexes with the chelator than with functional groups in the sewage sludge can be washed out (Wang et al. 2015); on the other hand, H⁺ ions can compete with HM ions for adsorption sites and affect the adsorption of HMs, thus promoting the desorption of HM ions from the sewage sludge (Zhai et al. 2018). At lower concentrations, CA forms complexes with HMs to chelate them from the sewage sludge. With increasing CA concentrations, HMs were gradually removed due to acid dissolution and HM chelation. Therefore, HM removal via organic acids is not only due to their acidity but also due to their chelating properties. EDTA is a strong chelator, and its mechanism in HM washing is similar to that of conventional chelators.

CA and EDTA were both efficient in Pb removal, as shown by Equations (1) and (2). CA washing, however, did not completely remove Cd and Cu from the sewage sludge. This indicates that these HMs were present as very strong adsorption cations and formed more stable complexes with biosolids as compared to CA. The same was true for Cu and Cr washing with EDTA. However, when compared to CA, EDTA was more efficient in removing Pb at low concentrations (e.g., <0.1 M, Pb removal >31%). The removal of Cd, Cr, Cu, and Zn did not increase significantly with increasing EDTA concentration (P > 0.05), similar to the patterns observed for CA. This could be due to the co-solubilization effect of target HMs and co-existing metals (Fe, Ca, Mg, and Mn) (Wu et al. 2015).

It is worth noting that the percentage of removed HMs increased with increasing ultrasound time and tended to stabilize after a while. A previous study reported entrapment of dissolved gas molecules in pores during ultrasound-assisted washing, which enhanced the micro-scale contact of the washing liquid in the interior of sewage sludge particles (Choi et al. 2021). The acoustic-physical effects of ultrasound (including micro-jets and shock waves) may also break particles into smaller ones with a larger surface area (Son et al. 2012). However, it has been reported that pollutants that are strongly bonded to soil particles via trenching in pores could hardly be desorbed by mechanical mixing alone (Son et al. 2011). This may also pertain to sewage sludge. When compared to traditional mechanical mixing, however, ultrasound-assisted washing was 7–10 times faster and more energy efficient (Zhang et al. 2017; Kou et al. 2020). This could save time, reduce energy consumption, and improve HM removal efficiency from sewage sludge in a short period of time.

The first round of ultrasound-assisted washing for 30 min significantly reduced the total amount of HMs (Figure 2) but could not reduce environmental risks because the sewage sludge contained more HMs in the mobile fraction (Beiyuan et al. 2018). Continuous washing could be used to reduce HM bioavailability.

Washing affected the quality of sewage sludge (pH, OM, AP, AK, and AN), but the magnitude and direction of quality changes depended on the washing agents used (Table 2). Because the washing agents were acidic, the pH in the sewage sludge was significantly lower (P < 0.05) after six rounds of washing (the pH decreased by 1.52, on average, after washing with EDTA; and by 4.2 after washing with CA) as compared to the original sludge. The OM content of sewage sludge increased significantly (P < 0.05) after six rounds of washing; unwashed sewage sludge had an OM content of 18.47%, which increased by 4.64% after washing with EDTA and by 8.1% after washing with CA. In a previous study, Wang et al. (2015) also observed an increase in OM after CA washing, which could be attributed to changes in sludge structure and residual CA as a possible source of OM. However, Ren et al. (2015) found that EDTA washing resulted in a decrease in the OM content, which is contrary to the results of the present study.

The nutrient content required for plant growth needs to be considered in addition to pH and OM when applying sewage sludge to land. We found that the contents of AN and AK were significantly lower (P < 0.05) after six rounds of EDTA and CA washing as compared to the untreated sludge; yet, the effect of EDTA washing on the AP content was smaller. It has to be noted that many factors (e.g., pH, redox potential, washing) can affect the removal of N, P, and K from sludge, from which the pH (CA treatment pH between 1.5 and 2.5 and EDTA treatment pH between 5.0 and 6.0) was likely the most important factor that affected the dissolution and conversion of sludge AN, AP, and AK in the present study (Ren et al. 2015).

The three-phase ratio is usually the ratio of the volumes of solid, gas, and liquid phases in the soil, which constitute the whole soil body, and is an important indicator of the looseness of the soil structure. In this paper, the effect of washing process on the physical properties of municipal sludge is judged by the three-phase ratio, mainly considering that the change of physical and chemical properties of sludge after washing process will affect the effect and direction of subsequent land use. If the physical properties of the sludge after washing process are similar to those of the soil, the lower the degree of impact on the physical properties of the soil after its land use, the better the land-use effect.

The generalized soil structure index (GSSI) is a structural change indicator that measures the ratio of solid, gas and liquid phases in the soil. The larger the value, the more desirable the three-phase structure is in that state. Soil three-phase structure distance index (STPSD) is also a measure of the structural change of soil three-phase structure. In contrast to GSSI, the smaller the change in STPSD value, the more desirable the three-phase structure is in that state. These two values can indirectly respond to the trend of washing process on the change of sludge three-phase structure and provide the theoretical basis for the subsequent sludge resource utilization.

In summary, both ultrasound-assisted chemical washing positively influenced the change of sludge three-phase ratio. In terms of washing agent, the three-phase ratio was closer to the ideal state after CA washing of sewage sludge compared to EDTA. In terms of the number of washing, the number of times CA-washed sewage sludge did not change the three-phase ratio significantly (P < 0.05), while the number of times EDTA washed sewage sludge changed the three-phase ratio more significantly (P < 0.05).

Despite the fact that the total HM content of sewage sludge varied greatly, multiple washings dramatically decreased the HMs in the F1 fraction. Multiple washing appears to be the best method for removing all target HMs and reducing their mobility at the same time.

It is worth noting that the P content of the CA eluent was lower than that of the EDTA eluent. After eight rounds of continuous washing, the content of TP in the CA eluate was 114 mg/L, while it was 438 mg/L in the EDTA eluate. This is most likely due to the fact that phosphoric acid is more acidic than CA, preventing the latter from dissolving phosphide, thus reducing the loss of TP.

We discovered that the eluate had a substantial quantity of nutrients produced from sewage sludge, which can have a negative influence on the groundwater ecosystem if not managed appropriately.

Ultrasound-assisted chemical washing is a useful and efficient solution to remove HMs from sewage sludge, instead of stabilizing mobile fractions without reducing the overall amount of HMs. The higher concentration (≥0.4 M) of biodegradable CA eluent could effectively reduce HMs in the sewage sludge compared to the traditional non-biodegradable EDTA eluent. However, 0.4 M CA had a lower single-washing efficiency than 0.2 M EDTA at a solid-liquid ratio of 1:4 and an ultrasound time of 30 min. Ultrasound-assisted washing can reduce the treatment time by 7–10 times as compared to traditional mechanical mixing. Furthermore, during the first round of washing, the mobile fractions (F1) of HMs increased, posing a potential threat to the environment. After continuous washing with CA and EDTA, the total amount of HMs and the F1 fraction was significantly reduced. CA can be a better alternative to EDTA for the removal of HMs from sewage sludge, based on a combination of washing efficiency and removal of mobile fractions. When concentrating on HM removal, it is also important to monitor the quality and three-phase ratio of sewage sludge used for land application (pH, OM, AP, AK, AN, solid: gas: liquid). The type of washing agent utilized had an effect on the quality of the sewage sludge after washing. Furthermore, because the eluate had a significant concentration of HMs (Cd, Cr, Cu, Pb, Zn, etc.) and nutrients (N, P, K, TOC, etc.), secondary contamination and potential recovery should be considered.

All authors agreed to publish this research (including any individual details, images or videos) in Water Science and Technology.

This research has been supported by the Technology R&D project of Guohui Group-GZU (20180726), Guizhou Central Leading Local Science and Technology Development Funds Project ([2022] 4022): Guizhou Karst Environmental Ecosystem Ministry of Education Field Scientific Observation and Research Station Capacity Enhancement Construction, and the high-level innovation talents in Guizhou ([2020] 6002).

All authors contributed to the study conception and design. The first draft of the manuscript and the drawing of the diagram were completed by H. L. and all authors commented on previous versions of the manuscript. Manuscript writing guidance and the first draft revision was completed by the corresponding author Y. W., Y. L., and X. P. Experimental operation, data collation and sample collection were performed by Z. L., Z. W., and Y. Q. All authors read and approved the final manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.