Evaluation of a combined chemical, electrochemical, and mechanochemical approach for metal extraction from contaminated dredged sediments: preliminary studies

Vol.:(0123456789) https://doi.org/10.1007/s11368-024-03915-6 SEDNET 2023 Evaluation ofacombined chemical, electrochemical, andmechanochemical approach formetal extraction fromcontaminated dredged sediments: preliminary studies IreneLlorente1· JorgeRuiz‑Fernandez2· RomanNevshupa2· MartaCastellote2 Received: 19 January 2024 / Accepted: 28 September 2024 © The Author(s) 2024 Abstract Purpose This study investigates the efficacy of a combination of chemical, electrical, and mechanical methods for extracting specific metal contaminants from marine dredged sediment. Materials and methods Samples of muddy contaminated sediment from a Spanish harbor were characterized, including the mode of occurrence of heavy metals, using sequential chemical extraction. Desorption tests were conducted using the sediment in its fresh state, in a custom-built cell/reactor filled with an electrolyte—either a solution of 0.25M citric or acetic acid, or deionized water. Electrical current, ultrasonic energy, and circulating flow were applied in various combinations, and the efficacy of such combinations on the metal desorption was evaluated. After the experiments, the solutions were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES). X-ray diffraction (XRD) and thermogravimetric analysis/differential thermal analysis (TG/DTA) were performed on the sediment. Results The sequencial extraction revealed that most metals (excluding Hg, Pb and Zn) were primarily bound to the residual fraction (fraction V), with As and Cu exhibiting the highest concentrations. Cadmium and mercury were preferentially extracted in fraction IV, associated with organic matter. Pb and Zn exhibited their highest percentages in fraction III, bound to Fe–Mn oxides. Regarding desorption, the results indicated that increasing treatment time enhanced metal desorption, with the most significant effect observed during the initial stages. The electrolyte used emerged as the most influential factor. Citric acid proved more effective for As, Cr, and Ni, while acetic acid favored Cu, Pb, and Zn extraction. As, Pb, and Zn exhibited preferential desorption in the presence of ultrasounds, while Cr, Ni, Pb, and Zn desorbed more readily under electrical current. Application of electrolyte circulation had a positive effect on the extraction of all metals. Conclusions Changes in theelectrolyte's chemical composition are the most significant factor influencing metal desorption. In addition to the electrolyte used, the application of some form of energy had a slight positive effect on metal desorption. However, at this stage it seemed that the synergistic effect of both electric field and ultrasounds appears to be only relevant for Cu. Electrolyte circulation had a positive impact on metal desorption for all metals tested. The concentration of recalcitrant and refractory organic matter decreased after all the tests, indicating its degradation into more labile matter. Keywords Contaminated marine dredged sediment· Heavy metals· Desorption· Electrical current· Ultrasounds· Recirculation· Enhancing solutions· Treatment technology 1 Introduction Marine sediments are complex mixtures of organic matter, mineral particles of various sizes, and interstitial water. Their composition can vary widely depending on location. Harbors often contain sediments contaminated with substances that do not readily dissolve in water. Instead, these contaminants tend to bind to or accumulate within the sediment. As a result, sediments can act as contaminant sinks, Responsible editor: Susanne Heise * Marta Castellote mar[email protected] 1 National Centre ofMetallurgical Research (CENIM-CSIC), Avda. Gregorio del Amo, 828040Madrid, Spain 2 Eduardo Torroja Institute ofConstruction Sciences (IETcc-CSIC), Spanish National Research Council, Serrano Galvache 4, 28033Madrid, Spain / Published online: 11 October 2024 Journal of Soils and Sediments (2024) 24:3863–3876 with concentrations sometimes exceeding regulatory limits (Bortone 2007). Decontamination processes aim to remove contaminants from sediments through desorption and dissolution. Extensive research has been conducted over the past 40years to develop effective sediment decontamination techniques. A comprehensive overview of 19 such techniques, including their technical feasibility, maturity, environmental impact, and economic viability, is presented by Hakstege (2007). These techniques encompass a wide range of approaches: – in-situ methods: chemical treatments (Pensaert etal. 2003), biological remediation (Myers etal. 2000; Hakstege 2006), and capping (Palermo 1998; Cornelissen etal. 2011; Zhang etal. 2016); – ex-situ physical methods: separation (Olin-Estes and Palermo2001; Averett and Estes 2011), dewatering (natural: Vermeulen etal. (2003), and mechanical: Detzner etal. (1998)); – ex-situ thermal treatment: desorption (Rienks 1998; Covelli etal. 2021), immobilisation to produce bricks (Hamer and Karius2002), light-weight aggregates (Hamer etal. 2003), artificial basalt (Balkaya 2019) or cement (Mensinger 2008); – ex-situ chemical or biological treatment: washing extraction / biogenesis (Gatchett and Banerjee 1995; Stern etal. 1998; Jones etal. 2001), stabilization / chemical immobilization (c2004; Palansooriya etal. 2020; Kumar etal. 2021), bioremediation (Ferdinandy-van Vlerken1998; Hakstege and Van Geldermalsen 1998), landfarming and phytoremediation (Baker etal. 1994; Vervaeke etal. 2001; Harmsen etal. 2007), confined upland disposal (Detzner and Knies 2004) and sub-aquatic confined disposal (Environmental Commission Working Group 52002; Palumbo 2007). The selection of an appropriate treatment technology is complex due to the interplay of various factors, including sediment characteristics, contaminant types, site-specific conditions, economic considerations, and regulatory requirements. For instance, washing is effective for sandy sediments with primarily non-residual metal contamination. However, the presence of higher clay, silt, or organic matter content can significantly hinder the effectiveness of washing and many other decontamination techniques. While most methods are efficient for organic pollutants, remediating metal contamination presents a greater challenge. The diverse nature of pollutants and the potential for synergistic interactions between contaminants and treatment agents further complicate the decontamination process. For example, the presence of microplastics can influence the removal of polycyclic aromatic hydrocarbons (PAHs) and phthalate esters through adsorption or co-precipitation (Ren etal. 2021). This highlights the need for a tailored approach that considers the specific characteristics of each contaminated site and the potential interactions between contaminants and sediment matrix. To address these complexities, integrated treatment strategies combining multiple techniques often prove more effective than single-method approaches (Bhandari etal. 1994). Electrokinetic remediation emerges as a promising option for sediments with predominantly fine-grained particles (clay and silt). Over the past decades, substantial research efforts have been dedicated to developing and refining electrokinetic remediation technologies for contaminated sediments. These studies have yielded significant advancements in understanding the underlying mechanisms, optimizing treatment parameters, and expanding the applicability of this technology to a wider range of contaminants and sediment types (Saichek and Reddy 2005; Nystroem etal. 2006; Pazos etal. 2008; Reddy and Cameselle 2009; Benamar and Baraud 2010; Kirkelund etal. 2010; Ribeiro etal. 2011; Cameselle and Reddy 2012; Rozas and Castellote 2015; Garcia-Blas etal. 2020a, b; Benamar etal. 2020; Kanbar etal. 2023). This technique leverages the application of an electric field to induce the movement of contaminants within the sediment through electromigration, electro-osmosis, and electrolysis and concentrate them in an electrolyte (Reddy and Cameselle 2009). Metals are primarily removed through electromigration, while organic pollutants are extracted via electro-osmosis or oxidized at the anode. A primary challenge is converting immobile metals into a mobile form for extraction (Virkutyte etal. 2002; Wen etal. 2021). To address this, enhancing agents like acids, chelates, redox agents, or their combinations are often added to the electrolyte for metal solubilization (Pazos etal. 2008; Cang etal. 2013; Garcia-Blas etal. 2020b). For organic contaminants such as PAHs, PCBs, pesticides, and herbicides, surfactants, biosurfactants, cosolvents, or cyclodextrins can be employed (Alcántara etal. 2008; Rozas and Castellote 2015; Cameselle and Gouveia 2018). Additionally, efforts have been made to simplify conventional remediation methods by reducing the number of consecutive steps, eliminating the use of expensive environmentally unfriendly reagents and solvents and enhancing their efficiency (Falciglia etal. 2017). However, for certain sediments, either due to their composition or the specific combination of contaminants present, electrokinetic treatments alone may not yield satisfactory results. This has prompted the exploration of combining electrokinetics with other unconventional energy sources, such as mechanochemical treatments (MCTs), to trigger synergistic physical and chemical processes (Ares etal. 2019). MCTs have been employed as processing technologies in various industrial sectors, especially in powder metallurgy, 3864 Journal of Soils and Sediments (2024) 24:3863–3876 extraction of minerals and pharmaceutics (Boldyrev 2006; Balaz 2008; Zhang etal. 2023b). MCTs typically involve the application of mechanical energy through milling, crushing, ultrasounds, sonication, or other forms of mechanical action to activate chemical reactions or facilitate chemical processes under milder conditions. While the application of MCTs to dredged sediments is relatively uncommon, several studies have explored its utilization for activating inert high-plasticity clay sediments in the production of geopolymer mortar (Hosseini etal. 2021; Zhang etal. 2023a). MCTs have also been employed in a two-stage treatment for the detoxification of marine sediments heavily polluted with polychlorinated biphenyls (Cagnetta etal. 2015) and for solvent-free destruction of perand polyfluoroalkyl substances (PFAS) in sediments via piezoelectric ball milling (Yang etal. 2023). Therefore, it is reasonable to hypothesize that MCT could also be effective in desorbing metals from sediments, despite their potentially diverse composition and properties compared to soils. To the best of our knowledge, no published studies have investigated the combined application of MCTs and electrokinetics for sediment remediation. This study presents preliminary findings on the possible synergistic effects of combining various energy sources – ultrasound, electrokinetics, and fluid circulation – with three different electrolytes (water, citric acid, and acetic acid) to enhance metal desorption from contaminated sediments. The research aims to understand the underlying mechanisms responsible for the enhanced contaminant removal and optimize the remediation process by identifying the most effective combination of these parameters. 2 Materials andmethods 2.1 Sampling andcharacterisation ofthesediment Samples of the sediment were collected from a Spanish harbor. The exact location of the sediment sample is protected by a None Disclosure Agreement (NDA) with the supplier.Sediment samples were stored in hermetic plastic bags and frozen at -20ºC. Detailed information regarding the sampling and storage conditions, as well as the characterization of the sediment, can be found in GarciaBlas etal. (2020a). The comprehensive characterisation revealed key characteristics such as particle size distribution (67% of the sediment was ≤ 63μm and around 30% was ≤ 2μm), total organic carbon content (TOC) of 2.1% and carbonate content of 10.2g kg−1. The concentrations of organic substances (in dry mass) were: tributyltin (TBT) 57μg kg−1; total petroleum hydrocarbons (TPH) concentrations for fractions C10-C40 70mg kg−1; polycyclic aromatic hydrocarbons (PAHs) EPA 16 below 0.48mg kg−1; and polychlorinated biphenyls (sum PCBs 28, 52, 101, 118, 138, 153, 180) below 0.007mg kg−1. EPA selected 16 PAHs, which are frequently found in environmental monitoring samples, namely acenaphthene, acenaphthylene, anthracene, fluoranthene, fluorene, naphthalene, phenanthrene, pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[ghi]perylene, benzo[a]pyrene, chrysene, dibenz[a,h]anthracene, and indeno[1,2,3cd]pyrene. These 16 EPA PAHs are used as indicator and serve to summarize the exposure to total PAH by a small selection. The metals content in the dry sediment was determined to be (in mg kg−1 d.w.): As 224.9, Cd 1.35, Cr 49, Cu 2228, Pb 317.5, Ni 47.1, Zn 2897.8, and Hg 1.3. To further characterize the mode of occurrence of the metals in the sediment and to assess their bioavailability, mobilization, and transport, Tessier’s sequential chemical extraction procedure was employed (Tessier etal. 1979). The method involves extracting the metals into five different fraccions: exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter, and residual. The metal concentrations in the extracted solutions obtained from each sequential step were determined using unductively coupled plasma optical spectrometry (ICP-OES). The procedure for analyzing 1g of dry sediment sample is the following: – Fraction I (exchangeable): Dissolve the sediment in 8ml of magnesium chloride solution (1M MgCl2, pH 7.0) with continuous agitation for 1h at room temperature. – Fraction II (bound to carbonates): Filter the solution obtained in Fraction I to isolate the residue. Leach the residue with 8ml of 1M NaOAc adjusted to pH 5.0 with HOAc with continuous agitation for 1h at room temperature. – Fraction III (bound to Fe–Mn oxides): Filter the solution obtained in Fraction II to isolate the residue. Leach the residue with 20ml of 0.04M NH2OH•HCl in 25% (v/v) HOAc at 96 ± 3 ºC with agitation for 2h. – Fraction IV (bound to organic matter): Filter the solution obtained in Fraction III to isolate the residue. Add to the residue 3ml of 0.02M HNO3 and 5ml of 30% H2O2 adjusted to pH 2 with HNO3. Heat the mixture to 85 ± 2 ºC for 2h with occasional agitation. Add 3ml of 30% H2O2 (pH 2 with HNO3) and heat the mixture to 85 ± 2 ºC for 2h with occasional agitation. After cooling, add 5ml of 3.2M NaOAc in 20% (v/v) HNO3 and dilute the solution to 20ml. Agitate the solution continuously for 30min. – Fraction V (residual). The residue remaining after the extraction of Fraction IV is considered as the residual fraction. It has been calculated by substracting the sum of the metal concentrations obtained in fractions I-IV 3865Journal of Soils and Sediments (2024) 24:3863–3876 from the total metal content obtained after digestion of the sample. 2.2 Desorption experiments Desorption tests were conducted under various conditions (Table1) using the setup illustrated in Fig.1. A 180 cm3 cylindrical methacrylate cell/reactor with an internal diameter of 50mm and a length of 260mm, enabling continuous recirculation of the solution was used. To prevent sediment leakage during recirculation, nylon membranes were placed on both ends of the cell. Two openings on the front of the cell provided access for introducing the electrodes. In all the experiments, 50g of contaminated sediment in its fresh state, without drying or grinding, were added to the cell and filled with an electrolyte solution, either 0.25M citric or acetic acid, or deionized water. Ultrasonic energy was introduced into the setup using an ultrasonic bath that employed high-frequency modulated sound waves. The electrical current was applied in galvanostatic mode using activated titanium electrodes. The positive electrode (anode) was placed within the sediment bulk, while the negative electrode (cathode) was situated in the upper portion of the cell in the supernatant electrolyte. Two levels of electrical current were applied: 0.15 A and 0.3 A. After the experiments ran for the specific duration, 2 or 4h, (Table1), a sample of the electrolyte was collected, and all energy sources switched off. The results obtained at this point have been labelled as Step 1. The leachant and sediment were allowed to be in contact for 20 additional hours. After this time, the supernatant was removed (Step 2). The liquid samples were filtered and analyzed by ICP-OES, and the sediment was dried at 100 ºC for 24h before being characterized using X Ray Table 1 Labeling and experimental conditions of the trials performed 123456789 Electrical Current (A)0.15 0.15 0.15 0.15 0.15 0.15 0.30 Ultrasounds XXXX XXX Electrolite (0.25 M) Citric Acid Citric Acid Acec Acid Citric Acid Citric Acid Citric Acid Citric Acid WaterCitric Acid Recirculaon flow rate (4 L/hr)XXXXXX XX Time (h)244444444 EXPERIMENT Fig. 1 Schematic setup for the desorption tests 3866 Journal of Soils and Sediments (2024) 24:3863–3876 Diffraction –XRDand Thermogravimetrical/Differential thermal analysis -TG/DTA. 3 Results anddiscussion 3.1 Sequential extraction oftheoriginal sediments The percentage of metal extracted in each fraction is presented in Fig.2. It can be observed that for most metals (excluding Hg, Pb and Zn), the highest proportion is found in the residual fraction. This fraction represents the portion that remains in the sample after all other extraction steps have been completed. For As and Cu, the residual fraction contains the highest metal concentrations, indicating that their removal is challenging and the contamination is recalcitrant in this muddy sediment. For As, the remaining metal is primarily associated with fraction III (Fe–Mn oxides fraction), with smaller amounts found in fraction I (exchangeable) and fraction IV (organic matter). This observation aligns with previous studies (Bettoschi etal. 2018) on As bound in sediments. For Cd and Hg, the highest percentage of metal extraction is found in fraction IV (organic matter), suggesting that their availability is linked to the organic content of the sediment (Chakraborty etal. 2012; Pinedo-Hernández etal. 2015). For Pb and Zn, the highest percentage of metal extraction corresponds to fraction III (bound to Fe–Mn oxides). In the case of Pb, the total extracted percentage exceeds 100% (118.3%). This Fig. 2 Metal content distribution in the different fractions of the sequential extraction Fig. 3 Evolution of pH and conductivity 0 5 10 15 20 25 0 1 2 3 4 5 6 7 8 123456789 Conducvity(mS/cm) pH step 1 pHstep 2 pHconducvity step 1conducvity step 2 3867Journal of Soils and Sediments (2024) 24:3863–3876 discrepancy has been attributed to the inhomogeneity of the initial sediment. 3.2 Desorption tests 3.2.1 General conditions oftheexperiments Figure3 presents the pH and conductivity of the samples collected after Step 1 (immediately after the test) and Step 2 (after 20h of contact between the sediment and the electrolyte in the reactor). It can be observed that the pH values of the electrolytes after the tests are slightly higher than those of the initial solutions. The pH of the 0.25M citric and acetic acids solutions is around 2 and 3, respectively. The corresponding conductivity values are 9.8 and 7.8 mS cm−1, respectively. Sample 8, tested with deionized water, exhibits a neutral pH after the test. This indicates that the electrodic reactions are primarily driven by the hydrolysis of water, resulting in the generation of H+ at the anode and OH− at the cathode. These ions neutralize each other in the electrolytic solution. When electric current, recirculation, citric acid, and ultrasounds are applied in experiment 2, the conductivity is higher than in the other tests, where some of these treatments are not used. Among these factors, circulation of electrolytes has the most pronounced effect on increasing the removal of ions into solution. Figure4 depicts the electrical potential (V) required to achieve the target current density for each experiment involving electrical current application. As expected, the electrical resistance decreases as the experiment progresses, necessitating lower voltage to maintain the electrical current in instances where fluid circulation is employed. The voltage values for experiments 1 and 2 are identical, as the experimental parameters are the same, with the exception of the duration. In experiment 3, with acetic acid, the initial potential needed to get the same current is higher, decreasing largely as the experiment proceeds, indicating smaller resistance probably due to solubilisation of part of de matrix. Not applying ultrasounds and not recirculating lead to higher resistance. Fig. 4 Electrical potential (V) neededto maintain the current. (0.15 A for tests 1-3, 6-8 and 0.30 A for test 9) Table 2 Percentage of metals desorbed from the sediment during the first step under various experimental conditions. Metals with the highest and lowest desorption rates for each experiment are highlighted in green and gray, respectively. The metal with the secondhighest desorption rate is underlined. The experiment yielding the highest overall desorption for each metal is bolded % desorbed to the electrolyte Experiment As Cr Cu Ni Pb Zn 17.90 8.27 2.44 15.21 8.25 23.61 28.35 10.17 1.29 18.46 8.68 27.83 30.99 6.38 28.38 13.66 26.13 58.53 4 15.41 8.15 0.05 15.99 9.24 28.15 57.14 5.21 0.23 9.65 4.16 16.05 67.17 13.20 0.64 25.46 10.14 41.99 74.22 5.88 1.20 11.98 6.25 19.96 83.92 0.38 0.13 0 0 1.1 98.12 5.86 0.60 13.20 6.79 19.14 3868 Journal of Soils and Sediments (2024) 24:3863–3876 Higher voltage must be applyed, as expected, when using water as the electrolyte. 3.2.2 Metal analysis oftheelectrolytes The analysis of the elecrolytes using ICP-OES has enabled the quantification of the amount of metals desorbed from the sediment. The results (% of the metal concentration measured after sediment acid digestion) at Step 1 are presented in Table2 for As, Cr, Cu, Ni, Pb, and Zn, respectively. The values for Cd and Hg were consistently below the detection limit of the equipment used, which was 0.037mg L−1 and 0.069mg L−1, respectively. It is noteworthy that the overall desorption rates from the sediment are relatively low, as the sediment remains contaminated after the treatments, with a maximum of 58.5% for Zn using acetic acid (experiment 3). This experiment exhibits a distinct pattern, as does that conducted with deionized water (experiment 8). For clarity, Table2 highlights the most and least extracted metals for each experiment in green and gray, respectively. The second most extracted metal is underlined. The experiment in which desorption was greatests for each metal is emphasized in bold. The results indicate that for all metals, except the experiment using deionized water, Zn is the most extracted metal, followed by Ni, both of which have a significant fraction associated with Fe–Mn oxides. Cu is the least extracted metal. The removal of As is higher in experiment 4, with the application of ultrasounds. For Cr and Ni, the highest value is achieved in experiment 6, with the application of electrical current. Cu, Pb and Zn are most effectively desorbed with acetic acid (experiment 3). To illustrate the impact of different parameters on the desorption of each metal, the amount extracted in the electrolyte at Step 1 has been normalysed using experiment 2 (with recirculation, 4h, with ultrasounds, with citric acid and 0.15 A) as a reference, with the value of 100% assigned. If the normalised percentage obtained is less than 100% in a test, it indicates that the test conditions worsen the results of experiment 2. Conversely, if the percentage is higher than 100%, the conditions favour the extraction of that metal. The results are presented in Fig.5, where the effects of decresing the treatment duration, varying the type of electrolyte, adjusting the electrical current intensity, and applying or removing current, ultrasounds, and recirculation are depicted. The findings in Fig.5 reveal that for all metals except Cu, extending the treatment time generally promotes metal desorption. However, even doubling the time, the impact is modest, suggesting the need to optimize process Fig. 5 Evaluation on the desorption results of the effects of a) decresing the time of treatment, b) the electrolyte used, c) the intensitiy of electrical current applied and d) applying ultrasounds, recirculation and taking out both celectrical current and ultrasounds 3869Journal of Soils and Sediments (2024) 24:3863–3876 effectiveness. Electrolyte selection is clearly the parameter that exerts the most significant influence on the results. Employing deionized water proves considerably less effective than using citric or acetic acids, which, in addition to their acidic nature, can form complexes with different metals. Citric acid proves more efficient for As, Cr, and Ni, while Cu, Pb, and Zn are preferentially extracted with acetic acid, in accordance with previous studies (Garcia-Blas etal. 2020a). The effect of electrical current under continuous ultrasound and circulating flow is unexpected. As, Pb, and Zn exhibit preferential desorption without electrical supply, while Cr, Cu, and Ni exhibit their maximum desorption at 0.15 A. Applying 0.3 A is not beneficial in any case. A similar pattern emerges when ultrasound energy is removed, with Cr, Ni, Pb, and Zn exhibiting enhanced desorption. Circulating the electrolyte consistently enhances metal desorption for all metals. Removing both electrical current and ultrasounds leads to reduced desorption in all cases. Figure6 illustrates the increase in metal removal in Step 2 compared to Step 1 (%). Between Steps 1 and 2, the reactor was left undisturbed with the electrolyte in contact with the sediment, without applying any additional energy, to assess the sediment’s tendency to readsorb metals in different media. For As, in general, desorption continues when using citric acid, with experiments 1 and 2 exhibiting a solution that is nearly 50% more concentrated than at the end of the test. When acetic acid or water is used as the electrolyte, all the As is readsorbed, and no detectable As remains in solution. Without applying electrical current, there is minimal desorption, and even some readsorption. This suggests that anode oxidation alters the speciation of As, making it more mobile. Cr, Ni, Pb and Zn readsorbs in all cases except for experiments 6, 7 and 9, which involve removing ultrasounds, eliminating electrolyte recirculation, and applying 0.3 A, respectively. Cu readsorbs in all the cases except for experiment 7. Sorption of metals onto and their desorption from a sediment in an electrolyte depends on several factors, some related to the matrix and others to the characteristics to the metal itself (Shaheen etal. 2013). In this study, all experiments were conducted using the same sediment but with different electrolytes, which altered the bulk chemistry and proved to be the most influential factor among those tested. This suggests that pH and the ability of the electrolyte to form complexes with metals play a significant role. Citric acid is able of forming mononuclear, binuclear, or polynuclear and multidentate complexes, depending on the type of metal ion. For instance, Fe and Ni form bidentate, mononuclear complexes with two carboxyl acid groups of the citric acid molecule. Cu, Cd and Pb form tridentate mononuclear complexes with citric acid involving two carboxyl acid groups and the hydroxyl group (Francis etal. 1992). Acetic acid can also form complexes metal ions present in the solution, including Fe, but its formation constants are lower than those of citric acid. Based on this, Pb and Cu should be preferentially extracted with citric acid over acetic acid, which is not the case. This discrepancy could be attributed to various factors. In complex systems like sediments, the anions (citrate and acetate) participate in parallel reactions depending on pH, such as protonation reactions or hydroxocomplex formation. In practice, thermodynamic constants are not entirely accurate, and conditional constants, which are dependent on experimental conditions, would be more appropriate (Arribas Jimeno etal. 2002). In this system with numerous processes involved, including an electrical field in the reactor, ultrasounds and recirculation, conditional constants are not avalilable, making it impossible to explain these results from a theroretical standpoint at this time. 3.2.3 Analysis ofthesediment aftertheexperiments X-Ray diffraction –XRDanalysis of the sediment did not reveal any peaks corresponding to heavy metals. The only crystalline phases detected were quartz and silicates: muscovite (KAl2(AlSi3O10)(OH)2), kaolinite (Al2Si2O5(OH)4), albite (NaAlSi3O8) and halite. No differences were observed in the mineralogical composition of the untreated and treated sediments, except for the disappearance of halite and formation of calcite after experiment 8. This indicates that the sediment structure has not been substantially altered (see Fig.7). Neither citrates nor acetates were detected after the tests. The thermogravimetry results for the untreated sediment and after the desorption experiments are presented in Fig.8. Figures8a, b and c depict the weight loss, its derivative, and the differential thermal analysis respectively. Additionally, Fig.8d shows the fitting of the differential thermal analysis -DTA curvefor the untreated sediment using the Peakfit software, revealing the exotermic reactions and their temperature ranges. The weight loss can be attributed to dehydration of mineral phases and to decomposition of different groups of organic and inorganic substances (Leinweber etal. 1992; Capel etal. 2006). These reactions occur within the following temperature ranges: labile organic matter at 200–400 º C, recalcitrant organic matter at 400–550 ºC, and refractory organic matter at 550–650 ºC. As the sediment is a mixture of minerals and organic matter, it is not possible to determine the exact quantity of each mineral phase using TG. However, the DTA patterns exhibit endothermic peaks corresponding to dehydration, dehydroxylation, and mineral conversion (Plante etal. 2009), as well as exothermic peaks resulting from the oxidation of organic matter. Table3 3870 Journal of Soils and Sediments (2024) 24:3863–3876 provides the percentage weight loss in the different temperature ranges for the samples after the treatments. From the results in Fig.8a and Table3, it can be seen that after experiment 2, 5 and 9, the total weight loss is significantly higher than for the rest. After experiment 3, the weight loss is lower than for the untreated sediment. The thermogravimetric results reveal a distinct trend following treatment with water: the sediment absorbs more interstitial water, labile organic matter diminishes, and calcite a) b) d)c) e) f) Fig. 6 Increase in the concentration of metals in the solution of step 2 with respect to step 1 (%). (Positive values indicate desorption and negative values readsoption in the sediment) 3871Journal of Soils and Sediments (2024) 24:3863–3876