International Journal of Environmental Research University of Tehran ISSN: 1735-6865 EISSN: 2008-2304 Vol. 4, Num. 3, 2010, pp. 455-462

International Journal of Environmental Research, Vol. 4, No. 3, July-September, 2010, pp. 455-462

Article

Insights in leaching characteristic assessment of solidified wastes using different leach tests

Saeedi, M . ^1* , Rezaei Bazkiaei, A . ² and Torkaman, Z . ³

¹ Environmental Research Laboratory, Department of Hydraulics and Environment, School of Civil Engineering, Iran University of Science and Technology, 16846, Narmak, Tehran, Iran
² Environmental Engineering, State University of New York at Buffalo (UB), 218 Furnas Hall, USA
³ Department of Chemistry, Shahid Beheshti University, Tehran, Iran

Correspondence Address: * Environmental Research Laboratory, Department of Hydraulics and Environment, School of Civil Engineering, Iran University of Science and Technology, 16846, Narmak, Tehran, Iran
msaeedi@iust.ac.ir

Date of Submission: 25-Nov-2009
Date of Decision: 05-Apr-2010
Date of Acceptance: 15-Apr-2010

Code Number: er10050

Abstract

Keywords: Cement, Solidification, Fuel oil combustion, Residue, TCLP, SCE, Alkalinity, Solubility, pH, Leach tests

Introduction

Cement based solidification/stabilization process has long been used for different hazardous materials and have proven to be one of the most cost effective methods for handling of several waste types because of comparatively lower price of required material (ce-ment, and water in the case of dry materials) than many other remediation technologies. Scope of utilization of this technology, as best practical, comprise sludge (Diet et al., 1998; Asavapisita et al., 2004; Coz et al., 2004; Zain et al., 2004; Tanapon et al., 2005; Athanasios & Evangelos, 2006), mean radioactive wastes (Spence & Shi 2004, Zhoua et al., 2006), low volume generating wastes (Jang & Kim, 2000), polluted soils (Jing et al., 2004 & 2006). Differences are also in several binders and mixtures used in the cement- waste matrix (Shin et al., 1995; Duchesne & Laforest, 2004; Shawabkeh, 2005; Sarvinder & Pant, 2006) but the governing assump-tions of encapsulation and fixation of hazardous con-stituents in microscopic and macroscopic scale due to chemical and physical processes happening inside the mixtures remains relatively the same. Several different leaching characteristics of stabilized waste bodies has been reported elsewhere; some because of obviously different nature of waste being treated by cement ma-trix, and some because of unpredictable characteristics of final mixture of cement and waste. In almost all cases leach tests designed to evaluate hazard identification are just considered as predefined sets of regularly used leaching tests with assumptions not in accordance with the real management scenario.

Truth is that in recent years many evaluation pro-cesses on leaching characteristics of waste materials and their stabilization products have been conducted with less notice on real management scenario desig-nated to that remedied wastes. Recently, international concerns on achieving efficient remedial actions and real evaluation of leaching condition of stabilization products is emerging and need for development of com-prehensive characterization approaches is more and more revealed. Kosson (2002), in his paper on necessi-ties of development of a case specific leaching frame-work for every waste management case, highlights that: "In evaluating the leaching potential of wastes based on a single, plausible worst-case mismanagement sce-nario via TCLP, the USEPA seeks to provide environ-mental protection for unregulated wastes. However, wastes are managed in many different settings, and under a range of conditions that affect waste leaching. The reliance of the USEPA on a single, plausible worst-case, management scenario for leach testing may be generally protective, but often at the cost of over regu-lation. It has also proven to be inadequately protec-tive in some cases". That is exactly the reason why TCLP test has come to criticism by underlying assump-tion of municipal solid waste (MSW) co-disposal which does not happen in many cases. Moreover, Kosson emphasis that neither the TCLP nor any other test per-formed under a single set of conditions can provide an accurate assessment of waste hazards for all wastes. Importance in different interpretation of results reveals when researchers care about different assumptions on management scenarios. First, whether a single scenario is available for destination receptor environment or dif-ferences in scenarios have considerable effects on leach-ing condition of dumped materials. More importantly, whether regional parameters affecting leaching, or sim-plifying assumptions in developed leach tests have con-siderable effect on final assessment of solidification products. Assumptions such as: the fraction of the con-taminant extracted during a batch extraction is equal to the fraction that will leach (USEPA 1986), or leachable fractions are divided in predefined sets of bounds in dumped material (Tessier et al., 1979). The presumed leaching assessment approach must be consistent enough to appropriately assess leaching potentials (not necessarily exact measured amounts of release based on what test procedures define), and cover different conditions in possible management scenarios.

The most commonly fuel oil type used in Iran is Fuel oil. Characterization and stabilization of a thermal power plant air heater washing waste in Iran was car-ried out previously (Saeedi & Amini, 2007a; Saeedi & Amini, 2007b; Saeedi & Amini, 2008). Fuel oil combus-tion residues in Iran's thermal power have also been first characterized before (Saeedi & Rezaei Bazkiaei, 2008). Noticeable results were achieved in the case of heavy metal contents (Fe, Ni, Na ₂ O, Cr ₂ O ₃ ; 7.01, 6.02, 4.15, 2.14 % respectively), high loss on ignition or car-bon content (36.76%), high Sulphur content (21.59 weight percent), high acidity (mean pH: 2.71), low mois-ture content (2.02 weight percent), and more interest-ingly metal complexation formation in crude analyzed waste (controlling complex phase: Calcium Vanadium Oxide; CaO.17V ₂ O ₅ ). It seems that compared to other studied residual waste forms in coal-fired thermal power plant wastes; bottom and fly ashes and MSW incin-eration residues (Ilham et al., 2001; Asokan et al., 2005; Dincer et al., 2006; Jing et al., 2006), our investigation of fuel oil combustion residue owns apparently differ-ent characteristics, thus far. USEPA TCLP test results on Oil Combustion Wastes (OCWs) in the category of bottom ash alleges characteristic of toxicity only infre-quently in analyzed samples and that exceedences were spread across a relatively large number of sites (USEPA, 1999). USEPA results for elemental Cd (mean: 0.130 mg/L), Cr (mean: 0.387 mg/L), Pb (mean 1.23 mg/ L), and Ni (mean: 30.7 mg/L) are noticeably less than leached amounts in our previous study (Saeedi and Rezaei Bazkiaei, 2008); Cd (mean: 5.55 mg/L), Cr (mean38.54 mg/L), Pb (mean 19.3 mg/L), and Ni (mean: 472.5 mg/L). Although a comprehensive study on other thermal power plants in the characterization effort has not been conducted, but highly effective difference in concentrations was reasonable evidence on need for utilization of other test methods and case specific leach-ing assessment approaches, at least in particular waste management practices. Reportedly, the most common management scenario for OCW bottom ash wastes is mono-disposal in landfills or surface impoundments (USEPA, 1999), which is far from assumption of co-disposal with municipal wastes in development of worst case leaching scenario in TCLP test. This is what Kosson believes to serve as misrepresentation reason in many cases.

In the present investigation, a special combustion residue type waste material; fuel oil combustion bot-tom ash of a thermal power plant in Iran, was first un-dergone cement solidification process with two ma-trixes; Ordinary Portland Cement (OPC) and mixed OPC-Pozzolan matrixes, in different cement to waste ratios. Then a leaching assessment approach was designed to evaluate crude waste material and its cement solidi-fication/stabilization products to assess efficiency of stabilization processes.

Materials and Methods

In January 2006, 20 waste samples were taken from bottom ash of combustion furnace of Shahid Rajaee power plant in Qazvin Province, Iran. The collected samples were stored in a cool place in sealed bags until analysis. Four composite samples were prepared by homogenizing and combining every fifth sample. The samples were air dried at room temperature (< 40^° C) to constant mass before being divided and screened. The water content of samples used for leaching was determined on a parallel sample by drying at 110°C overnight. Moisture content then was determined us-ing ASTM 4254 (ASTM 2000). The natural pH of samples was determined using Cyber Scan PC510 pH meter. The chemical composition of dried samples was determined by X-ray fluorescence (XRF; Philips PW 2404) and X-ray Diffraction measurement was used to identify possible mineral phases in the waste sample. Two sets of solidification matrixes with OPC and poz-zolan added OPC, with cement to waste ratio (C/W); 0.5, 0.75, 1, 3 and 5 were completely mixed with water based on ASTM-C109-90 standard mix design. Poz-zolanic samples contained 5% weight pozzolan com-pared to cement amount in mixture. Prepared matrices are then pored in 5Χ5Χ5 cm standard molds to form final stabilized samples, enough curing time in wet en-vironment of sealed box is provided and samples were broken after 28 days. Required amounts of samples for analysis were gained from core of broken samples. XRF analysis was performed for determination of chemical composition of crude waste samples and resulting ce-mentation products. XED analysis was executed on matrixes to identify possible cement hydration prod-ucts, their variation in different matrixes and possible metal complexation with hydration products.

Different leaching tests utilized for a comprehen-sive coverage of waste and stabilization products leaching characteristics. These are tests in which in-terpretation of leached amounts based on close exami-nation of different conditions can provide insights on several possible management scenarios and site spe-cific conditions. Detailed specifications of tests are as follows:

a) Toxicity Characteristic Leaching Procedure (TCLP) (USEPA, 1992) a commonly used leaching procedure to identify waste characteristic under a specific sup-posed worst case field condition; Acetic acid as leachant in pH 2.88 ± 0.05 over Liquid to Solid (L/S) ratio 20/1 and tumbling in an end-over-end fashion for 18 hours.

b) Sequential Chemical Extraction; According to (Tessier et al., 1979); elements can be classified into the following five operationally defined fractions: 1) exchangeable: The sample was extracted for 5 h with 0.5 M MgCl ₂ at pH 7.0 and a L/S ratio of 8; 2) bound to carbonates: The solid residual from step (1) was ex-tracted with 1M NaAc at pH 5 and a L/S ratio of 8 for 5 h; 3) bound to iron and manganese oxides: The re-sidual from step (2) was extracted with 0.04 M NH ₂ OH.HCl in 25% (v/v) HAc for 6 h at 96°C. The L/S ratio was 20; 4) bound to organic matter and sulfides: The residual from step (3) was extracted with 30% H2O2 for 6 h at 85°C, then extracted with 3.2 M NH ₄ Ac in 20% (v/v) HNO ₃ with continuous agitation for 30 min; and (5) residual: this fraction was calculated from di-gestion of remnant of step (4)

c) Alkalinity Solubility and Release as a function of pH (Kosson et al., 2002) ; The equivalents of acid or base are added to a combination of deionized (DI) wa-ter and the particle size reduced material (< 2mm), with Nitric acid solution 2N in situation where natural pH of samples is to be reduced or Potassium Hydroxide so-lution 2N where natural pH is to be raised, to cover desirable range of pH values which may occur under different field conditions, with final L/S ratio 10 in an end-over-end fashion for 24 hours to identify leaching characteristics of samples over a broad range of pos-sible pH value (2 to 12) and in fact more realistic condi-tions over different plausible management scenarios. Performing the tests, elemental concentrations of Ni, Fe, Cr, Pb and Cd in eluates from different tests were determined using Atomic Absorption Spectrometry (Buck Scientific 210 VP model).

Results and Discussion

Crude waste and solidification products with Ce-ment to Waste ratio (C/W) 0.5, 3 and 5 are subjected to XRF analysis. Results illustrated in [Table - 1]. indicate high concentrations of elemental Fe, Ni, Cr respectively. Constituents with concentrations less than 0.01 % has been under detection limits of XRF apparatus and has not been reported though, but as it is stated in TCLP test results, elemental Cd and Pb concentrations has leached to eluate solution indicating presence of these elements in lower amounts than 0.01 % fraction in crude waste. Leaching results for solidification products are predictably lower than concentrations in crude waste; depletion of concentrations is nearly in ratio of dilu-tion with cement and water. One of important features of characterized waste is its comparatively high car-bon content. Reported carbon content in same origin wastes (combustion residue type wastes) have shown notably lower weight percent carbon content than 23.5 in this case (Asokan et al., 2005; Dincer et al., 2006; Jing et al., 2006).

Results of TCLP leach test are presented in [Table - 2]. The very first conclusion from results is identifica-tion of both waste material and solidification products as hazardous in almost all samples and in the case of all elements except for Cd in all samples. Compared to hazard limit for Pb in TCLP test (5 mg/Lit), only cement samples with C/W ratios 1, 3 and 5 has met regulatory leaching limits. In the case of Cr, with TCLP leaching limit 5 mg/L, the only sample meeting the threshold is pozzolanic sample with C/W ratio 5. Nickel concentra-tions in all samples are above limits posed by EPA based on 100 times safe potable water concentration; equal to 10 mg/L (EPA, 2003). Till this step of charac-terization procedure, regardless of other characteristics of samples, all these samples must be rejected for co-disposal with municipal solid wastes in landfills, but examination of percent removal of elements com-pared with leached amounts in natural waste can pro-vide clues for future practices. With close consider-ation of removal percents in all samples, strange be-havior of matrixes from C/W ratios 0.5 to 1 is revealed. In both cement and pozzolanic samples, leached amounts increase with increase in cement content of mixture till the ratio of 1 and thereafter in ratio 3 and 5 leached amounts has decreased. It may be contributed to high percentage of carbon in natural waste material where its hindering effect on hydration processes may have caused difficulties on generation of Calcium Sili-cate Hydrate (C-S-H) gels. Considering the fact that with increase in cement content of mixture, required water content also increases, it can be concluded that cement addition till ratio 1, not only has not helped the mixture to improve its leaching characteristics, but also has caused more release because of presence of nec-essarily higher water content in the mixture. Till C/W 1, solidification products were a mixture of not com-pletely formed or enough generated hydration prod-ucts and waste material with no satisfactory bound to cement hydration products. But after C/W 1, it seems that hindering effect of carbon has been weakening by higher cement hydration products and leaching char-acteristics have improved in mixtures with C/W 3 and 5. Although measures with higher C/W ratios was not carried out in this practice because of time limits, but it seems that with increase in C/W ratios to about 10 many samples may meet regulatory leaching criteria of TCLP test. Here stands one of trivial but important features of a successful waste management practice with cement mixtures; choose reasonably adequate range of matrixes to cover unpredictable range of ob-servations in practice.

Changes in hydration processes and formed crys-tals in XRD analyses of cement samples with C/W 0.5, 3 and 5 ascertain our assumption of hydration prod-ucts. As it is depicted from XRD analysis, the most observable crystal phases formed in 0.5 sample are just some isolated phases of cement hydration; Cal-cium Sulfate Hydrate, Gypsum, and Bassanite. XRD analysis reveals samples with C/W 3 and 5, crystal phases formation with elemental Aluminum and Man-ganese are reported; Calcium Magnesium Aluminum Oxide Silicate. This can be evidence on better hydra-tion processes in cement ratios 3 and 5. Although the XRD results does not contain a weight percent evalu-ation of different mineral phases in the mixture but this extent qualitative interpretation of results can also be of help to better conclude on real occurring phenom-enon in mixtures.

The principle of the Sequential Chemical Extrac-tion test is that a series of increasingly more aggres-sive extraction solutions are applied successively. Ide-ally, only the metal from a particular matrix should be released and other forms of metal should remain in the solids. However, these assumptions may not be true in practical systems (Li et al., 2001). Although SCE test was originally developed for sediments and its appli-cation for S/S-treated wastes needs further study (Li et al., 2001), but in a wise assessment framework for waste management practices, results of this test can help to achieve a more efficient final strategy. This method may be of help in determination of likelihood of presence of an element in easily leachable fraction or in hard bounds. It can help on decision making when practical technologies have noticeable operational cost differences. Results can also provide researcher with supplementary insights on interpretation of results of other leach tests. Results of SCE test for all samples are depicted in [Table - 3]. Close examination of results for solidification matrixes and natural waste reveals some clues on how solidification processes have ex-changed leachable amounts of elements between dif-ferent fractions.

In the case of Ni fixation, more than 65% weigh fraction of total leachable Ni in all samples is present in last fraction (Residual fraction) where based on Tessier assumption is the least likely leachable frac-tion of element. Highest percent in this fraction be-longs to samples with C/W ratio 1 and 3 in both ce-ment and pozzolanic matrix. Compared to this amount in crude waste, 57% weight percent, solidification pro-cesses have shifted about 20% of leachable Ni from first 4 fractions to residual fraction. Although leaching amounts of Ni in TCLP test have not shown compli-ance with regulatory limits, but it is likely that with addition of cement to higher ratios, this trend may help to more definite fixation of Ni in matrix.

Results for Cr are more interesting, solidification process has made shifts on leachable fractions that are not generally in favor of a successful fixation in matrix. Results for crude waste show that about 57.5% of total leachable amount of Cr is present in residual fraction of test and remaining are relatively evenly dis-tributed between bounded to carbonates (12.57%), bounded to Iron and Manganese oxides (12.2 %) and bound to organic matter and Sulfides (11.98%). Re-sults for cement matrix show that leachable amount has shifted more from residual fraction to fraction 2 and 3 where between 19.9 % to 30.78% of Cr has shown to be in bound to carbonates (fraction2) and amount of Cr in residual fraction has reduced to even 39.24 % in sample with C/W 3. Generally, about 10% of leach-able amount of Cr has shifted from residual form to other 4 more easily leachable fractions, and this trend is observed in almost all samples. Results are interest-ing when compared with TCLP. Although some samples (Pozzolanic C/W 5, 4.2 mg/L) has met regulatory leach-ing limit (5 mg/L), and some (Cement C/W 5, 6.65) has leached near the limit, but considering likelihood of presence of Cr in more leachable fractions with addi-tion of cement can not definitely meet the regulatory compliance under only the assumption of landfilling of solidified samples. It is likely that with change in management scenario, real condition of ambient envi-ronment is likely to leach more easily leachable frac-tion of bound to carbonate, where it seems to be one of important features of TCLP test that brings it to criticism.

In the case of Cd leaching, the only obvious find-ing from SCE test is that solidification processes have made a slight shift between fraction in bound to Iron and Manganese oxide and fraction in bound to car-bonates. Weight percent of Cd in bound to Iron and Manganese oxides has shifted from 43.57 % to about 26% in Pozzolanic sample with C/W 5 and remaining has shifted to fraction 2 and 4. In other samples also fraction shifting has shown main changes between frac-tions in bound to carbonate, bound to Iron and Man-ganese oxides and slight shift to fraction in bound to organic matte and Sulfides but this change is not as considerable as in fraction 2 and 3. Compared to TCLP results where all samples have met regulatory leaching limits for Cd, it can be concluded that Cd fixation has been done properly and significant change in leaching probability is not made by different mixture formulas. The Alkalinity, Solubility and Release as a function of pH test with its wide range of environmental pH can help to better simulate actual environmental condition of final dump site. In this investigation the test is ex-ecuted just for 4 samples; crude waste, and cement samples with C/W ratio of 0.5, 1 and 3. Results are shown in [Table - 4]. Released amounts in all samples have shown decrease with increase in alkalinity of samples but change rates differ from sample to sample. In the case of crude waste, release has reduced from 22.5 mg/ L for Cr and 8.65 mg/L for Pb in pH 2, to 1.25 and 1.24 in pH 12 respectively. Leached amounts are interestingly lower than 100 times potable water limits (10 mg/L) - a criteria to evaluate harm posed by non-regulatory haz-ardous classified material that are supposed to pose human harm (EPA, 2003) - for Cr in all eluates with pH more than 3. This situation for Pb was just met in pH 12 where leached amount (1.24 mg/L) is lower than 100 times potable water limits (1.5 mg/L). Considerable changes in leached amounts are observed in pH 3 and after that decrease in leached amounts has more rapid rate. In other samples, the leaching decrease rates are relatively the same as in crude waste with exception of considerably lower leached concentration in acidic pHs. This can indicate effective impact of acid attack in leaching of crude waste where with addition of ce-ment hydration products with high buffer capacity this acid attack has shown comparatively lower effect. Uti-lizing result of test in different managerial scenarios can definitely cover overdesign parameters gained from conservative results of TCLP test.

Conclusion

Differences in final results of leaching tests and their underlying assumptions necessitate development of case specific leaching test schemes in criteria of waste management activities. In this way, TCLP test has been commonly used as main leaching test to as-sess effectiveness of almost all solidification/stabili-zation processes internationally. Approach in this in-vestigation was to develop and use a leach testing framework consisting of different leach tests for spe-cial thermal power plant waste, fuel oil combustion resi-due. TCLP has been used as core leaching test and other two sets of tests were used to evaluate its effec-tiveness. Results express deficiencies of TCLP test for final evaluation of these type waste management practices where actual final scenario does not neces-sarily match assumption of co-disposal with MSW in TCLP test. Results of two other designed tests, SCE and "Alkalinity, solubility and release as a function of pH", has shown inadequacy of TCLP test for interpre-tation of actual condition of leaching of elemental Ni, Cr, Pb, and Cd. Comprehensive understanding of real conditions of dump site are the factors that just con-sidering simultaneously can provide decision makers with more cost-effective and not just over-designed waste management scenarios. Conditions like actual pH of surrounding soil or other synthetic material, drain-age condition leading to evaluation of actual Liquid to Solid ratios, actual leachant with respect to type of soil, practical particle size of final products, and so on are of the factors should be considered in choosing the best leach test in each case. It depends on how important this matter will be in future with looming higher waste generation problem world wide.

Acknowledgement

The support of current study by Iran National Sci-ence Foundation (INSF) under contract number 86044/16-5243 is appreciated.^[31]

References

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