solidification technique on highly contaminated sediments with environment risk assessment

Science of the Total Environment 684 (2019) 186–195

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Long-term application of stabilization/solidification technique on highly contaminated sediments with environment risk assessment Dunja Rađenović ⁎, Đurđa Kerkez, Dragana Tomašević Pilipović, Miloš Dubovina, Nenad Grba, Dejan Krčmar, Božo Dalmacija University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Dositej Obradovic Square 3, 21000 Novi Sad, Serbia

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Evaluation of long term S/S process of contaminated sediment in novel mixtures • Environmental risk assessment of contaminated sediment (Igeo, ERi, DIN, TCLP, BCR) • BCR determination of mobility level and bioavailability of metals in the sediment • Kaolinite, quicklime and Portland cement proved to be effective immobilization agents.

a r t i c l e

i n f o

Article history: Received 28 February 2019 Received in revised form 20 May 2019 Accepted 23 May 2019 Available online 24 May 2019 Editor: F.M. Tack Keywords: Remediation technique Stabilization/solidification testing Leaching Sequential extraction

a b s t r a c t After dredging of contaminated sediment, additional remediation technique is required before its final disposal. For this purpose, this research was based on the long-term stabilization/solidification (S/S) process of highly contaminated sediment (dominantly by heavy metals) from a European environmental hot spot, the Great Bačka Canal. Due to optimisation of remediation techniques, this sediment is treated with selected immobilization agents: kaolinite, quicklime and Portland cement. The use of pseudo-total metal content (selected priority substances: Cr, Ni, Cu, Cd, Zn, Pb and As) in untreated sediment, determined that sediment urgently requires remediation. Short-term (after 7 and 28 days) and long-term (after 7 years) monitoring were done in order to estimate the concentrations of metals and effect on biota from S/S mixtures during this processes. The environmental risk assessment encompassed the application of several appropriate analytical methods: the pseudo-total metal content, the German standard leaching test - DIN 3841-4 S4 and Toxicity Characteristic Leaching Procedure - TCLP test leaching tests and sequential extraction procedure (BCR) on S/S mixtures, testing the aging process and toxicity effects. After simulating real environmental conditions using all tests in all three mixtures, metals do not exceed the prescribed limit values and as such S/S mixtures are classified as nonhazardous waste. Sequential extraction procedure showed that the highest percentage of metals are in the residual phase, bound to silicates and crystalline structure. After 7 years of S/S mixture aging, kaolinite showed the highest binding capacity that was reflected in the content of metals in the residual phase (34.8% of Ni to 77.6% of Cr). DIN and TCLP leaching tests confirmed that the exchangeable phase has a minor effect on the environment.

⁎ Corresponding author. E-mail address: [email protected] (D. Rađenović).

https://doi.org/10.1016/j.scitotenv.2019.05.351 0048-9697/© 2019 Elsevier B.V. All rights reserved.

D. Rađenović et al. / Science of the Total Environment 684 (2019) 186–195

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Accordingly, this remediation technology could be well applied for final disposal of this and similar extremely contaminated sediment dominantly with inorganic pollutants. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Pollution of sediments with metals is a global problem with a tendency to grow (Fernandes et al., 2008; Birch, 2017) and it is thought that because of its persistence, toxicity and ability of bioaccumulation through the food chain, they lead to long-term endangerment of the state of ecosystems and human health (Suthar et al., 2009; Guédron et al., 2014; Valdes et al., 2014; Tashakor et al., 2014; Won et al., 2016). The removal of sediment from the watercourse is most often carried out within the regular maintenance of watercourses in order to reduce the risk of flooding and maintenance of waterways, but also when it is estimated that there is a significant degradation of the aquatic system that causes or represents a potential negative ecotoxic effect on the living world (Saravanan et al., 2018; Libralato et al., 2018). The highest percentage of metal ions in the aquatic system are bound to suspended particles and sediment, while a very small percentage of free metal ions can be found in water. Heavy metals have the ability to immobilize by coagulation, flocculation or adsorption on the surface of the sediment, and also be incorporated into the lattice structure of minerals and form insoluble fractions, such as metal sulfides (Zhang et al., 2014). When revitalizing the aquatic body, certain quantities of sediment must often be removed, and depending on the degree of pollution of the sediment, remediation is sometimes necessary (Dubovina et al., 2018). Metals can be effectively immobilized in the sediment using various remediation techniques such as stabilization/solidification and in this way reduce the negative impact on the environment. The processes and techniques of stabilization/solidification (S/S) have been developed in an important part of environment technology and represent a very efficient remediation technique from the aspect of high efficiency and low cost (Wang et al., 2018a; Wang et al., 2019a). This technology, which involves mixing some binder materials (e.g. Portland cement, lime, kaolin etc.) with contaminated material, reduces environmental risk in aquatic systems from increased mobility of harmful components. Solidification/ stabilization (S/S) treatments involve the conversion of contaminated material into chemically more stable forms and require the addition of binding agents to chemically fix or physically encapsulate contaminants within treated materials. S/S does not remove contaminants, but is used as a technique that modifies the physical and/or chemical properties of materials to reduce component mobility. The goals of solidification/stabilization are to achieve and maintain the desired physical properties of the waste, to maintain chemical stabilization or permanently bind contaminants (Ma et al., 2018). Cement-based stabilization/solidification, except that it is costeffective, it is also very efficient for recycling contaminated waste into useful construction materials (Wang et al., 2018b). The use of Portland cement (OPC) in S/S technology can immobilized potentially toxic elements in the OPC-based matrix due to chemical fixation and physical encapsulation mechanisms (Wang et al., 2019b). The cement can create different interactions between the trace elements and cement moisture products, including adsorption, coprecipitation and chemical incorporation (Wang et al., 2018; Liu et al., 2018). Additionally, cement posed good mechanical and structural characteristics and ecological performance (Malliou et al., 2007; Chen et al., 2009). Clays are often used to treat waste, because they have an ability to bind water and create a solid form. Clays have an important role in the environment because they represent a natural sponge for pollutants, linking their anions or cations through ion exchange or adsorption. Kaolinite is one of the most commonly used clays for adsorption of metals, but also other

pollutants. Quicklime is used as an immobilization agent, as this may reduce the mobility and bioavailability of the trace elements present (Wang et al., 2015), which is a result of enhanced adsorption and/or precipitation. Additionally, hydroxide based binders represent good CO2 capturing materials lowering its content in the atmosphere as one of the greenhouse gases, which represents one more environmental benefit (Yang et al., 2019). The subject of this study includes the study of the influence of aging and ripening (7, 28 days and 7 years) of solidified and stabilized mixtures of sediment contaminated with heavy metals (Cr, Ni, Cu, Cd, Zn, Pb and As) used selected immobilization agents (kaolinite, Portland cement and quicklime). These metals have been identified as priority substances for a long-term trend monitoring in sediments and/or biota (European Union Directive, 2013/39/EU). Pseudo total metal concentrations were analyzed in untreated sediment and sediment mixtures after 7 years of aging by microwave digestion and ICP-MS analysis (Perkin Elmer Sciex Elan 5000). Two indices, the geo-accumulation index (Igeo) and potential ecological risk factor (ERi) developed by Hakanson (1980), were calculated based on pseudo total metal concentrations, in both the initial (non-treated) and treated sediments (mixtures), in order to establish how the S/S treatment reduced the risks posed by the sediment to biota (Zhou et al., 2018). TCLP and DIN 38414-S4 leaching tests have been applied to simulate real environmental conditions, as well as sequential extraction to determine the mobility and binding of metals in the sediment. 2. Materials and methods 2.1. Study area and sample collection This research investigated the sediment of the Great Bačka Canal in the town Vrbas. The Great Bačka Canal is part of the Danube-TisaDanube system intended for water supply during drought periods. As a result of the rapid development of industry in the second half of the 20th century and the release of untreated wastewater, around 400,000 m3 of sediments contaminated with heavy metals has accumulated. Based on literature data (Serbian Environmental Protection Agency, 2011), the 6 km long section of the Great Bačka Canal in Vrbas represents a European environmental hot spot and is one of the most endangered water bodies in Europe. The high concentrations of priority substances in the sediment, particularly heavy metals (Krčmar et al., 2017), require special attention for immobilization, with the extremely polluted sediment requiring remediation. This research is based on inventive optimum remediation techniques that could be applied to an investigating section of the 6 km of the Great Bačka Canal, reducing environmental risks to a minimum. The similar results were obtained from near region locations in North Serbia (Dubovina et al., 2018; Grba et al., 2017). For this purpose, a technique of solidification/stabilization with various immobilization agents and a perceived degree of bonding was applied for a period of 7 years, in order to determine the mobility of metals after the material was dredged and deposited in the environment. Physico-chemical characterization of the sediment included sampling of the surface sediment (0 to 0.50 m) on 10 profiles. Samples were mixed in order to obtain a representative composite sample of the critical section. Samples were collected using Eijkelkamp core sampler from each location, according to the standard method for sediment sampling ISO 5667-12: 1995. Sediment samples were packaged in plastic containers (for heavy metal

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analysis). Samples were stored and transported to a laboratory at a temperature of 4 °C until their analysis. All experiments were performed in triplicate. Mean values were used and the RSDs (n = 3) were below 5%. 2.2. Sample preparation At the beginning of the experiment, the sediment from the Great Bačka Canal was dried at 105 °C. The samples for analysis were prepared by mixing kaolinite (K20), quicklime (L10), Portland cement (C5) and sediment in certain quantities so that the total sample mass is 30 g, as shown in Table 1. Samples were labelled with capital letters: K20-kaolinite, C5-cement and L10-quicklime, with the number representing the percentage of immobilization agent added to the contaminated sediment. The chemical composition of the immobilization agents used in the solidification/stabilization treatment is shown in Supplementary data Table S1. After homogenization, the optimal water content was added to each mixture according to the ASTM D1557-00 procedure. Typically, cohesive soils and sediments at the optimum water content can be squeezed into a lump that barely sticks together when hand pressure is released, but will break cleanly into two sections when “bent”. The mixtures were then compacted (ASTM D1557-00, 2000), applying pressure of 2700 kNm/m3 (56,000 ft. lbf/ft3). Monolithic samples were placed in inert plastic bags, at a temperature of 20 °C. 2.3. Characterization of sediment The pseudo-total metals content of the sediment was determined in a dried sample digested according to the EPA 3051a (USEPA, 2007) microwave digestion method (Milestone, star E), followed by analysis in which ICP-MS technique (Perkin Elmer Sciex Elan 5000) was used, in accordance with USEPA Method 6020B (USEPA, 2014). The results of pseudo total metal content in untreated sediment are compared with the relevant natural, national and various international regulatory values. In S/S mixtures, after 7 years of aging, the pseudo total metal content was also analyzed. 2.4. Leaching tests Leaching tests were applied to the mixtures after 7 days and 28 days of maturation (K20-7D, K20-28D, C5-7D, C5-28D, L10-7D, L10-28D) and after 7 years (K20- 7Y, C5\\7Y, L10-7Y). The S/S treatment was applied after 7 days, because in that period there is often development of strength due to the possible formation of pozzolanic reactions that tend to stabilize the sediment in the long-term (Xia et al., 2017). It has been determined that the drying S/S waste up to 28 days results in an even better performance improvement, which causes the reduction of leaching of heavy metals and an increase of the strength of the samples (Malviya and Chaudhary, 2004). The analysis of the S/S mixtures was conducted after a long period of time, since the long-term stability of heavy metals and arsenic could be uncertain, and their precipitates are often not long-term stable as their crystalline forms (Drahota and Filippi, 2009; Schmukat et al., 2013; Wang et al., 2014). 2.4.1. The German standard leaching test - DIN 3841-4 S4 Based on the German standard test for leaching (DIN 38414-S4, 1984), the monolithic samples are comminuted to a maximum particle

size of 10 mm. Deionized water is added and mixed on a continuous mixer for 24 h. The liquid/solid ratio is 10:1 (l kg−1). This test was performed according to EN 12457 (BS EN 12457-2, 2002), which was given in the decision of the Council of the European Union (2003/33/EC, 2003) as the standard leaching test for the determination of the general characteristics of waste materials and sludge. The Serbian regulation on categories, testing and classification of waste (Official Gazette 56/2010, 2010) also uses this test to determine the hazardous characteristics of waste, as well as to classify the waste intended for disposal. 2.4.2. Toxicity characteristic leaching procedure - TCLP test For this test, a particle fraction of b1 cm was used. In order to perform the TCLP test (USEPA, 2002) it is necessary to determine which extraction fluid should be applied, depending on the alkalinity of the waste material. Highly alkaline materials are treated with a certain volume of glacial acetic acid solution with a pH of 2.88 ± 0.05. Other materials are treated with a glacial acetic acid solution whose pH is adjusted to 4.93 ± 0.05 by the addition of 1 N NaOH solution. The dried sample and the extraction fluid were in a ratio of 1:20 and were shaken at room temperature for 18 h on a continuous stirrer. After extraction, the solution was filtered through a 0.45 μm membrane filter and conserved, after which it was analyzed for metal contents by ICP-MS (Perkin Elmer Sciex Elan 5000). According to the Serbian regulation (Official Gazette 56/2010, 2010), which is in accordance with the provision 40 CFR §261.24 of the Law on Resource Conservation and Recovery Act (US Federal Register) (40 CFR Parts 261, 2005), if the concentration of one or more contaminants is greater than the limit value prescribed, the waste is characterized as dangerous due to its toxic characteristics. 2.5. Sequential extraction procedure To investigate the fractionation of metals in the sediment, a sequential extraction procedure (Rauret et al., 1999) was used in untreated sediment and treated mixtures after 28 days and 7 years of aging of the samples to examine the efficiency of remediation techniques applied. The procedure consists of four steps: Step 1. In the first step, the sequential extraction of 1 g of the dry sample is mixed with 40 ml of acetic acid 0,11 mol/l in a 100 ml vessel and 16 h extraction takes place at 22 ± 5 °C. The samples are then centrifuged. The supernatant is decanted and used for analysis. Step 2. The sediment from step 1 is used in the second step by adding 40 ml of hydroxylamine hydrochloride to 0.5 mol/l and extracting 16 h at 22 ± 5 °C. Then, centrifugation is performed, the supernatant decant and analyzes the metal content. Samples from this phase are used in step 3. Step 3. Add 10 ml of hydrogen peroxide 8.8 mol/l, digest at room temperature for 1 h with shaking. Continue to evaporate 1 h on a water bath at a temperature of 85 ± 2 °C to 3 ml. Add 10 ml of hydrogen peroxide 8.8 mol/l, continue to digest for 1 h in a water bath at a temperature of 85 ± 2 °C, continue to evaporate to a volume of 1 ml. Add 50 ml of ammonium acetate 1 mol/l and extract 16 h at 22 ± 5 °C. Centrifuge the sample, decant the supernatant and use it for analysis. Step 4. The samples from the third stage were digested with aqua regia (1:3 mixture of HCl and HNO3) and then the samples were filtered and analyzed for metal content. 3. Results and discussion

Table 1 Composition of sediment mixtures with kaolinite, Portland cement and quicklime. Samples

K20 C5 L10

Mass (g) Kaolinite

Portland cement

Quicklime

Sediment

6 – –

– 1.5 –

– – 3

24 28.5 27

3.1. Pseudo-total metal content of the untreated sediment and S/S mixtures after long-term treatment Table 2 shows the pseudo-total metal content of the untreated sediment sample, as well as the pseudo-total metal content in sediment treated with kaolinite, cement and quicklime after 7 years of aging (Fig. 1).

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Table 2 An overview of the concentrations of heavy metals (pseudo total metal content (mg/kg)) with relevant natural, national and different international regulation values of untreated sediment (S0). Untreated sediments from this study (comparison data) Sampling (S0) and relevant native values (Sback)

Ni Zn Cd Cr Cu Pb As

Legislation values

Natural values

Sediment quality guidelines (SQGs)

S0

Sback

Remediational value (RS 50/2012)

UCC

Northern Serbia

ERL

ERM

TEL

PEL

417 ± 18.2 1290 ± 35.6 29.2 ± 0.564 772 ± 18.7 416 ± 11.3 1970 ± 55.2 58.0 ± 0.986

29.6 ± 0.976 131 ± 4.13 0.94 ± 0.013 39.5 ± 0.852 37.2 ± 1.12 18.4 ± 0.519 12.5 ± 0.471

210 720 12.0 380 190 530 55.0

44.0 71.0 0.10 85.0 25.0 17.0 5.10

36.8 76.3 2.85 41.0 28.9 17.0 /

30.0 120 5.00 80.0 70.0 35.0 33.0

50.0 270 9.00 145 390 110 85.0

18.0 123 0.60 37.3 35.7 35.0 5.90

36.0 315 3.53 90.0 197 91.3 17.0

Note: Sback was sampled on 25 to 50 cm depth, and is located north-west of the Great Backa Canal at the confluence of the Tisa and Danube rivers (Krčmar et al., 2017).

The pseudo-total concentration of heavy metals and arsenic in the initial sediment samples (Table 2) decreased in the following order: Pb N Zn N Cr N Ni N Cu N As N Cd, although this does not necessarily reflect their relative mobility. Based on the Regulation on Limit Values of Pollutants in Surface and Ground Water and Sediment and Deadlines for their Completion (“Official Gazette of RS” No. 50/, 2012) in the untreated sediment, concentrations of metals (Cr, Cu, Cd, As, Pb, Zn, Ni) are above the remediation value. For this reason, sediment should be subjected to remediation treatment due to high concentrations of heavy metals and As. The mobility, toxicity or biodegradability of metals varies depending on the sediment phase for which they are bound (Darmawan and Wada, 2002; Kim et al., 2011). When the values obtained in untreated sediment with S/S mixtures (Fig. 1) are compared, there has been a significant reduction in metal concentrations after 7 years of S/S mixtures aging. The concentration of all metals decreases linearly with time. S/S mixtures are characterized by the value of the pseudo total metal content according to the Criteria for assessing the quality of sediment and the permitted methods of treatment of sediment (“Official Gazette of RS” No. 50/, 2012). In all three S/S mixtures the concentrations Cd, Cr, Cu, Pb and Zn are above the intervention values. Also, the concentration of Ni is above the intervention value in the mixtures C5 and L10, while in the mixture K20 the value of Ni is between the verification level and the remediation

value. Concentration of As in all three mixtures after 7 years of aging is between the reference value and the limit value. The use of pseudototal concentrations of metals in the sediment as a measure of their toxicity and the ability to bioaccumulate do not provide information about their possible origin, nor about the method of their binding in the sediment. For this reason, sequential extraction (Fig. 2) was used to determine the binding of metals in the sediment, as well as the level of mobility and bioavailability of metals in the sediment. 3.1.1. Comparison of pre-treated and treated sediments after 7 years with relevant natural and legislation values The metal content of the pre-treated sediment (S0) and sediment 7 years after the S/S process was analyzed by ICP-MS. In order to evaluate their toxicity, a linear comparison with relevant natural, national and international regulations values was made (Table 3, Supplementary data, Table S2). In the untreated sediment (So), As was the only parameter analyzed with a value (58 mg/kg) less than the ERM, while all heavy metals were present at higher levels and likely to have a great impact on biota, according to the sediment quality guidelines (SQGs), relevant natural and remediation values (RS 50/2012, 2012). As present is primarily of crustal origin, and is slightly higher than the remediation value (RS 50/2012, 2012), indicating partly geogenic origins.

Fig. 1. Pseudo-total metal content in sediment mixtures with immobilization agents after 7 years of S/S treatment, results from the ICP-MS analysis.

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Fig. 2. Results of sequential extraction procedure for untreated sediment and S/S mixture after 28 days and 7 years of aging.

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Table 3 Comparison of geological and ecological synthetic factors from the untreated and treated sediments, 7 years after the stabilization/solidification treatment (S/S), based on ICP-MS analysis.

Ni Zn Cd Cr Cu Pb As

Untreated sediment (S0)

Quality of mixture sediments with locked contaminants (metals) after stabilization/solidification treatment (S/S)

S0

L10 7Y

C5 7Y

K20 7Y

Igeo

ERi

Igeo

ERi

Igeo

ERi

Igeo

ERi

3.23 ± 0.14 2.72 ± 0.11 4.37 ± 0.17 3.70 ± 0.16 2.90 ± 0.14 6.16 ± 0.22 1.63 ± 0.07

177 ± 7.61 15.0 ± 0.60 855 ± 33.4 67.6 ± 2.84 68.0 ± 3.20 613 ± 21.5 51.1 ± 2.35

2.62 ± 0.10 2.33 ± 0.11 3.41 ± 0.12 3.12 ± 0.13 2.38 ± 0.07 4.68 ± 0.22 1.54 ± 0.05

46.3 ± 1.71 7.60 ± 0.37 478 ± 17.2 26.1 ± 1.07 39.2 ± 10.7 193 ± 9.26 43.7 ± 1.44

2.53 ± 0.08 2.34 ± 0.07 3.45 ± 0.12 3.11 ± 0.11 2.39 ± 0.07 4.72 ± 0.13 1.57 ± 0.03

43.2 ± 1.43 7.60 ± 0.22 492 ± 16.7 25.9 ± 0.91 39.4 ± 1.14 198 ± 5.54 44.7 ± 0.80

1.99 ± 0.04 2.00 ± 0.07 3.19 ± 0.12 2.82 ± 0.06 2.13 ± 0.07 4.53 ± 0.18 1.47 ± 0.06

30 ± 0.66 6.30 ± 0.22 409 ± 15.1 21.2 ± 0.47 32.8 ± 1.12 173.4 ± 6.94 39.8 ± 1.67

3.1.2. Geological and ecological indices of untreated and treated sediments after 7 years of S/S processes All immobilization agents showed a high potential of the heavy metals immobilization process. The sediment treated with kaolinite had the highest metal binding capacity, confirmed by the lower metal concentrations after 7 years of S/S treatment (reductions ranging from 11% for As to 68% for Pb, Table 3). After 7 years of S/S treatment, the geological and ecological synthetic factors (Table 3 and Supplementary data, Table S2) significantly decreased in comparison to non-treated sediment, again suggesting a significant degree of binding the heavy metals in the sediment and the stabilization agents after the S/S treatment. All parameters showed dominantly decreasing trend of contaminant compared to untreated sediment (Supplementary data, Table S2). For the most effective kaolinite mixture sediment the following trend was noted: Igeo has a downward trend compared to the untreated sediment from 10% (As) to 38% (Ni) and round 35% for other metals and from 22% (As) to 83% (Ni) and round 60% from other metals for ERi after the 7 years of S/S process. As it is showed in Table 3, kaolinite has similar, but lowest Igeo factor compared to quicklime and Portland cement. The use of this immobilization agent as most appropriate is confirmed. S/S sediments significantly reduced the risk that sediment posed to biota. Based on Igeo class categorization, Ni decreases 2 classes from strongly to moderately polluted and other metals decrease one class. Arsenic has the same moderately polluted status indicating dominant crustal origin. ERi for Ni decreases 3 classes from high to low ecological risk, while Cr, Cu and As qualify as low risk one class lower. ERi values of Pb changed its position, reaching the high-risk category, while Cd had a very high-risk category. Note that although the Igeo for Zn classifies the untreated sediment as moderately to strongly polluted (Igeo = 2.72), the ecological risk evaluation suggests that the risk for biota from Zn is low (ERi = 15), similar to all S/S treated sediments. According to these indices, the Pb pollution is of great concern. The use of kaolinite

agent from the aspect of lowest input based on ecological influences on biota is presented in Table 3 where steep fall of the curve is dominant for Ni, Cd and Pb confirming its best performance. The overall conclusion of potential toxicity is also related to sequential extraction data, including Pb concern. 3.2. Metal leaching Based on the LAGA criteria (LAGA, 1996) prescribed by the German State Waste Working Group (LAGA, 1996), it can be concluded that seven years after the applied S/S treatment with kaolinite (K20), the concentration of Cr, Cu, Zn, As, Cd and Pb in sediment doesn't exceed prescribed values. However, the concentration of Ni in the sediment slightly exceeds the allowed concentration. Regarding the criteria prescribed by the European Union (2003/33/EC, 2003), as well as the Serbian legislation, the Regulation on categories, testing and classification of waste (Official Gazette 56/2010, 2010), the kaolinite treated waste is classified as inert for Cr, Cu, Zn, As, Cd and Pb (Table 4, A* values), and non-hazardous waste after the long-term S/S treatment for nickel (Table 4, B* values). Based on this test, effective immobilization of metals was established using kaolinite as an immobilizing agent. The excess negative charge of kaolinite is compensated by bonding cations located on the outer surface of the crystal lattice through an ion exchange or adsorption or both processes. In this way, safe metal ions such as Na+ and K+ are liberated and bound to the crystalline structure of kaolinite heavy metals (Rouquerol, 2014). Sorption of heavy metals using clay involves a series of adsorption mechanisms; direct bonding of metal cations with the surface of clay minerals, surface complexation and ion exchange. After 7 years of aging of the sediment/cement mixture (C5), the metals which had higher concentrations of LAGA criteria (LAGA, 1996)

Table 4 Leaching of metals from the S/S sediment mixtures with immobilization agents according to the DIN 3841-4 S4 test. Samples

K20 7D K20 28D K20 7Y C5 7D C5 28D C5 7Y L10 7D L10 28D L10 7Y LAGA Z2* values A* B*

leaching concentration metals in mg/kg Cr

Ni

Cu

Zn

As

Cd

Pb

0.52 ± 0.018 0.08 ± 0.001 0.07 ± 0.002 1.04 ± 0.033 1.17 ± 0.042 0.12 ± 0.004 1.52 ± 0.022 0.73 ± 0.029 0.10 ± 0.003 1.00 0.50 10.0–70.0

2.37 ± 0.107 1.88 ± 0.074 1.26 ± 0.054 6.65 ± 0.228 4.12 ± 0.104 0.91 ± 0.038 12.4 ± 0.523 5.54 ± 0.211 0.59 ± 0.024 1.00 0.40 10.0–40.0

3.51 ± 0.154 2.24 ± 0.107 1.49 ± 0.047 16.6 ± 0.753 11.2 ± 0.486 6.15 ± 0.264 23.4 ± 0.985 3.43 ± 0.046 2.13 ± 0.102 2.00 2.00 50.0–100

2.36 ± 0.102 2.83 ± 0.111 1.54 ± 0.052 4.70 ± 0.156 0.80 ± 0.033 0.27 ± 0.010 2.35 ± 0.107 0.61 ± 0.025 0.15 ± 0.002 4.00 4.00 50.0–200

0.36 ± 0.014 0.41 ± 0.018 0.03 ± 0.001 1.95 ± 0.069 1.24 ± 0.053 0.45 ± 0.019 0.92 ± 0.031 0.74 ± 0.025 0.17 ± 0.006 0.05 0.04 1.00–5.00

0.35 ± 0.014 0.07 ± 0.002 0.08 ± 0.001 0.01 ± 0,001 0.01 ± 0.001 0.004 ± 0.001 0.01 ± 0.001 0.002 ± 0.001 0.004 ± 0.001 1.00 0.50 10.0–50.0

1.15 ± 0.049 0.09 ± 0.003 0.06 ± 0.002 0.06 ± 0.003 0.06 ± 0.002 0.02 ± 0.001 0.11 ± 0.002 0.05 ± 0.001 0.02 ± 0.001 0.50 0.50 2.00–25.0

A* - Maximum allowed concentration of accepting waste as inert L/S = 10 (l kg−1); B* - Maximum allowed concentration of accepting waste as non-hazardous L/S = 10 (l kg−1); Z2* upper recommended value of usage.

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have been arsenic and copper (Table 4). The EU and Serbian regulations (2003/33/EC, 2003; Official Gazette 56/2010, 2010) classify the C5 mixture after 7 years of the applied S/S treatment as inert waste to the leach concentrations of chromium, zinc, cadmium and lead, and as nonhazardous waste based on nickel, copper and arsenic concentrations. Leaching metal concentrations are effectively immobilized due to the formation of pozzolanic reactions and hydration products, probably through coprecipitation and surface interactions (e.g. sorption and ion substitution) (Wang et al., 2018b). The sediment/quicklime mixture (L10) satisfied the LAGA criteria (LAGA, 1996) for chromium, nickel, zinc, cadmium and lead while concentrations of copper and arsenic exceeded the allowed concentrations after 7 years of aging. The European Union and Serbian legislation (2003/33/EC, 2003; Official Gazette 56/2010, 2010) classify the mixture L10 as inert waste for Cr, Zn, Cd and Pb, and as non-hazardous waste in the case of Ni, Cu, and As. Reducing the mobility of metals in the sediment and quicklime mixture can be explained by unwinding the pozzolanic reaction between soil minerals, calcium from lime and water, producing cement compounds with properties such as calcium silicate hydrate and calcium aluminate hydrate (Maubec et al., 2017). This pozzolanic reaction forms a solid, water-soluble gel that has the ability to immobilize trace elements. Table 5 shows the concentrations of heavy metals and arsenic in the S/S mixtures after using the TCLP test. After the S/S treatment, the values are well below the limits set by Serbian regulations (Official Gazette 56/ 2010, 2010) and those set by the EPA (40 CFR Parts 261, 2005), for all mixtures after 7, 28 days and 7 years. It can, therefore, be concluded that these materials do not have toxic characteristics and may be considered non-hazardous and safe for disposal as well as for possible application for construction purposes (Kim and Lee, 2017). Based on DIN and TCLP leaching tests, leaching concentrations of all metals decreased after 7 years. 3.3. Sequential extraction In order to investigate the fractionation of metals in the sediment, a sequential extraction procedure is applied, based on the understanding that metals can form with a solid phase of sediment of a different bond strength, which can be broken using the reagent of increasing strength (Jamali et al., 2009). In untreated sediments, the following reduction of metal mobility can be noticed: Cd N As N Zn N Ni N Cu N Pb N Cr. The percentage of extracted, more mobile metals in this extraction phase ranges from 52% for Cd to 13% for Cr from the total metal concentration. In the untreated sediment Ni, Zn, Cd, and As are in the largest percentage in the exchangeable phase, that is, they are bonded to carbonates, which means they are the easiest available. In an untreated sediment sample, according to the risk assessment code (RAC), cadmium represents a very high risk for aquatic systems;

arsenic, zinc, copper and nickel have a high risk, while chromium and lead represent a moderate risk in the carbonate fraction. Copper shows the highest binding affinity for the third, oxidable phase, respectively in the largest percentage it is bound to organic matter and sulfides (54.67%), which can be explained by the high stability constant of the complex that the copper builds with organic compounds, and in accordance with the literature data (Jain, 2004; Morillo et al., 2004; Consania et al., 2019). Chromium and lead are highly bound in the reducible fraction (43.48% and 57.23% respectively), which means that Fe and Mn oxides play an important role in chromium and lead processes, and that Fe and Mn oxides cause this fraction to be relatively stable. Based on the results of sequential extraction after 28 days and after 7 years (Fig. 2) of applied S/S treatment in sediment/kaolinite mixture (K20), the highest percentage of Cr, Cu, Pb, and As was in residual fraction after 7 years. This means that a large part of the metals was bonded to silicates and therefore became less mobile than the 28-days period. Of all the metals analyzed, Cd is in the highest percentage in the exchangeable phase (46%) compared to other metals, which shows the greatest mobility after 28 days of aging. After 7 years of aging, the percentage of Cd decreased in the exchangeable phase (4.33%) and increased in the oxidable phase (56%), respectively, this metal formed organic complexes and sulfides, and as such it was incorporated in the monolithic mixture after 7 years. The highest content of nickel and zinc is in the reducible fraction, bound to the Fe and Mn oxides and hydroxides after a long-term period. Oxides of manganese and iron are considered important for the binding of nickel in the sediment (Rinklebe et al., 2017), and it is known that Ni can easily be coprecipitated or adsorbed into oxides, whereby Fe2+ and Mn2+ can be replaced by Ni (Frierdich and Catalano, 2012). Based on the risk assessment code (RAC) that provides a better interpretation of the relationship between the biodegradable fraction and the mobility of metals (Liu et al., 2016), all metals exhibit low risk after 7 years of aging. After 28 days in the sediment/cement mixture (C5) (Fig. 2), the highest percentage of most metals (Cr, Ni, Cu, Pb, As) is in the oxidable fraction, while a very small percentage in the residual phase except Pb (34.50%) and Ni (29%). The percentage of zinc after 28 days of aging is the highest in the exchangeable phase (65%), and after 7 years the percentage has decreased to 3%. After seven years of aging, the largest percentage of Zn is in the residual fraction (45.28%) and as such it is unlikely to get into the pore water. Zn and Cr, Cu, Zn, Pb and As, they are in the highest percentage in the residual phase, while Ni (45.41%) is in the oxidable phase after 7 years. Considering the fact that Ni is present in the largest percentage in the organic fraction, Ni are incorporated in the stable humic substances of high molecular weight, which are capable of releasing a small amount of metal over a longer period. The percentage of Ni has decreased from 62.35% after 28 days at 45.41% after 7 years in the oxidable fraction, and after 7 years its percentage increased in the residual and reducible fraction. After many years of aging, cadmium is mostly found in the reducible phase (49.12%)

Table 5 Leaching of metals from the S/S sediment mixtures with immobilization agents according to the TCLP test. Samples

K20 7D K20 28D K20 7Y C5 7D C5 28D C5 7Y L10 7D L10 28D L10 7Y Limit valuesa a

leaching concentration metals in mg/l Cr

Ni

Cu

Zn

As

Cd

Pb

0.34 ± 0.014 0.10 ± 0.004 0.01 ± 0.001 0.16 ± 0.006 0.06 ± 0.002 0.001 ± 0.000 0.06 ± 0.002 0.04 ± 0.001 0.03 ± 0.001 5.00

7.91 ± 0.259 4.32 ± 0.205 1.45 ± 0.066 3.88 ± 0.179 1.50 ± 0.061 0.17 ± 0.005 3.21 ± 0.156 0.54 ± 0.024 0.17 ± 0.007 20.0

0.91 ± 0.037 0.78 ± 0.029 0.14 ± 0.005 0.64 ± 0.031 0.47 ± 0.022 0.33 ± 0.012 0.85 ± 0.036 0.59 ± 0.019 0.39 ± 0.017 25.0

37.0 ± 1.236 7.40 ± 0.316 4.36 ± 0.198 11.9 ± 0.487 1.36 ± 0.058 0.66 ± 0.024 8.12 ± 0.321 0.61 ± 0.028 0.44 ± 0.019 250

0.13 ± 0.004 0.13 ± 0.005 0.08 ± 0.002 0.19 ± 0.007 0.13 ± 0.004 0.06 ± 0.001 0.04 ± 0.001 0.05 ± 0.002 0.009 ± 0.001 5.00

0.58 ± 0.021 0.13 ± 0.006 0.03 ± 0.002 0.33 ± 0.011 0.06 ± 0.002 0.001 ± 0.000 0.24 ± 0.011 0.04 ± 0.002 0.001 ± 0.000 1.00

4.09 ± 0.186 0.82 ± 0.012 0.11 ± 0.003 0.50 ± 0.019 0.23 ± 0.006 0.04 ± 0.001 0.39 ± 0.012 0.26 ± 0.008 0.02 ± 0.001 5.00

Limit values for metals according to TCLP procedure.

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bound to oxides and Fe and Mn hydroxides. The presence of cadmium in the reducing phase after 7 years of aging confirms the formation of a low solubility compound with hydroxides, whereby cadmium is chemically fixed by alkaline precipitation. These results confirm that the percentage of metals increased in the residual phase after 7 years and this mixture exhibits a low risk for all metals based on the risk assessment code. After 7 years in the monolithic sediment/quicklime mixture (L10) (Fig. 2), the percentage of all metals in the residual phase increased relative to the results obtained after 28 days of the applied S/S treatment. Cr, Ni, and Pb are in the highest percentage in the residual phase, respectively, these metals are retained within the crystal lattice of minerals and inside crystallized oxides. The results show that organic matter has a stronger geochemical affinity for Cu, Cd and As, which leads to a higher percentage of these metals in the oxidable fraction. Cu, Cd, and As tend to form organic complexes, indicating that only small concentrations of these metals in the ionic form (b6% in the exchangeable phase) can be found in the solution, which have toxic effects. When comparing the results after 28 days and after 7 years for Cu, Cd and As, the percentage in the residual fraction increased significantly after years of aging. This means that these metals, except as related to the oxidable fraction, are incorporated in a large percentage of silicates which confirms their increased immobilization after 7 years of aging. After years of aging, arsenic is in the highest percentage in the oxidable phase (41%), which confirms that the arsenic tendency is adsorbed or coprecipitated on metal sulphides and generally has a high affinity for other sulphur compounds. Zinc is in 69.27% bound to Fe and Mn oxides and hydroxides (reducible fraction) and based on this, it can be concluded that it represents moderately mobile metal. Zinc forms low soluble compounds with hydroxide, the percentage of which increased significantly in comparison with the period of 28 days of ripening S/S mixture. According to the risk assessment code, all metals exhibit low risk for aquatic systems after 7 years of aging L10 mixture. In all three mixtures, the highest percentage of lead is in the residual fraction after 7 years of aging, while in the mixtures C5 and L10 this metal is present in a higher percentage and in the oxidable phase (about 38%). Lead forms insoluble sulphide compounds that are the most stable solid form within the sediment matrix and are formed in reduction conditions when there are increased sulphide concentrations (Wuana and Okieimen, 2011). Since the lead is in the highest percentage in the residual and oxidable phase, it is well immobilized in monolithic matrices after years of aging.

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Table 6 Cost estimation for the solution including remediation of dredged contaminated sediment by stabilization and solidification with kaolinite, Portland cement and quicklime. Remediation of dredged sediment

Costs, € Investment Operating

Purchase of land for the landfill Arrangement of disposal area Preparation of access to the landfill Safeguarding the landfill Dredging and disposal Landfill area for subsequent (future) dredging Areas for disposal of S/S sediments Arranging passage between the landfill containers Building to accommodate equipment for sediment S/S Excavator for landfill sediment excavation Sediment transportation trucks Sediment stabilization and solidification plant quicklime Sediment stabilization and solidification plant - Portland cement Sediment stabilization and solidification plant - kaolinite Landfill monitoring

105,350 103,452 5000 17,000 4,906,800 65,331 130,662 19,400 180,000 21,425 85,698 1,367,591

– – – 29,285 118,947 1015 2029 – – 41,204 104,138 774,979

1,538,540

871,851

1,581,277 13,000

896,069 111,824

portion of the total operating costs of sediment remediation solutions. Total investment costs are 7,234,395, 7,191,658 and 7,020,709 € for treatment for kaolinite, Portland cement and quicklime respectively and total operating costs are 1,304,511, 1,280,293 and 1,183,421 in the same order. It is estimated that the amount of accumulated sediment is 400,000 m3 which concludes that the cost of treatment is 18.1, 17.9 and 17.6 €/m3 (20.3, 20.2 and 19.7 $/m3) of dredged sediment regarding investment cost and 3.3, 3.2 and 2.6 €/m3/per year (3.7, 3.6 and 3.3 $/m3/per year) of operating cost for kaolinite, Portland cement and quicklime respectively. Benefits of sediment remediation are contained in (i) improving or facilitating all functions of Canal endangered by the accumulated contaminated sediment and the benefits of a healthier environment, and benefits to the community arising from this; and in (ii) neutralizing or minimizing the hazardous effects of dredged contaminated sediment on the ecosystem, its treatment by stabilization and solidification process and conversion into S/S material that is inert and harmless to the environment, which includes all direct or indirect benefits arising from healthier environment in the given space. The identified benefits of this remediation solution can be evaluated as potentially significant (Rončević et al., 2013). 4. Conclusion

3.4. Cost estimation and comparison of benefits for the solution including dredging of sediment and remediation of dredged contaminated sediment by solidification and stabilization with kaolinite, Portland cement and quicklime Table 6 shows estimation of investment and operating costs for the solution including remediation of dredged contaminated sediment by stabilization and solidification with kaolinite, Portland cement and quicklime. In the cope of investment costs of this remediation solution, disposal of S/S material still accounts for the largest share of costs, almost ~70%. Consideration of possible optimization of investment costs by reducing the disposal cost is finding purpose for the use of S/S materials may be the best solution, because in this case, S/S materials would be taken from the landfill releasing the landfill space for disposal of the newly produced S/S material from the dredged sediment. S/S material as a commercial product would have a price and sales of this material would compensate to some extent for the cost of solution including remediation of sediment. However, if the costs of the sediment treatment plant are not accounting for the largest share of investment costs of this remediation solution, the operating costs of sediment treatment at the plant (including the costs of excavator and trucks) make up a large

The efficiency of stabilization and binding process of several priority substances (Cr, Ni, Cu, Cd, Zn, Pb and As) using selected immobilization agents (kaolinite, Portland cement and quicklime) is showing high potential for a long-time and final disposal of highly contaminated sediment. The high performance of these procedures was confirmed by testing the most toxic and contaminated sediment in Europe, Great Bačka Canal, with great impact on biota, according to the sediment quality guidelines (SQGs). The sediment treated with kaolinite had the highest metal binding capacity, confirmed by the lower metal concentrations (reductions ranging from 11% for As to 68% for Pb). Additionally, S/S sediments classified by the geo-accumulation index (Igeo) and ecological risk factor (ERi) showing decrease trend of contamination from 10% (As) to 38% (Ni) for Igeo and from 22% (As) to 83% (Ni) for ERi after the 7 years of S/S process for most effective kaolinite mixture sediments. The increasingly stable state of S/S mixtures after 7 years of maturation was also confirmed by results from sequential extraction which show that the percentage of metals increased in the residual phase after 7 years, exhibit a low risk for all metals based on the risk assessment code. Additionally, DIN and TCLP leaching tests showed that leaching concentrations were below prescribed values for all agents

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and it could be selected as non-hazardous waste. Comparing the concentration of selected metals with each agent and after all observations, including pre and post-treatment data and indices, it can be concluded that kaolinite is the stabilization agent which displays the best performance. The outcomes of this research have great potential in the selection and definition of a specific approach to sediment management, for this and similar regions and the selection of Best Available Techniques (BAT) for remediation and disposal.

Acknowledgments The authors acknowledge the financial support of the Provincial Secretariat for Higher Education and Scientific Research of the Autonomous Province of Vojvodina (grant no. 142-451-2451/2018-01/02). The authors would like to thank the BioSense Institute from Novi Sad, Serbia, for the use of measurement equipment and data analysis, as well as Goran Kitić, PhD, for his dedicated support and assistance. Also, authors would like to thank Prof. Srđan Rakić, Department of physics, Faculty of Sciences, Novi Sad, for his assistance in performing X-ray diffraction measurements. We are also grateful to Nada Popsavin for preparing the Figures. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.05.351.

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