Arsenic immobilization by calcium arsenic precipitates in lime treated soils

Arsenic immobilization by calcium arsenic precipitates in lime treated soils

ARTICLE IN PRESS Science of the Total Environment xx (2004) xxx–xxx 0048-9697/03/$ - see front matter ᮊ 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.03.016 Arsenic immobilization by calcium–arsenic precipitates in lime treated soils Deok Hyun Moon*, Dimitris Dermatas, Nektaria Menounou W.M. Keck Geoenvironmental Laboratory, Center for Environmental Systems, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, USA Accepted 18 March 2004 Abstract Lime-based stabilizationysolidification (SyS) can be an effective remediation alternative for the immobilization of arsenic (As) in contaminated soils and sludges. However, the exact immobilization mechanism has not been well established. Based on previous research, As immobilization could be attributed to sorption andyor inclusion in pozzolanic reaction products andyor the formation of calcium–arsenic (Ca–As) precipitates. In this study, suspensions of lime–As and lime–As–kaolinite were studied in an attempt to elucidate the controlling mechanism of As immobilization in lime treated soils. Aqueous lime–As suspensions (slurries) with varying CayAs molar ratios (1:1, 1.5:1, 2:1, 2.5:1 and 4:1) were prepared and soluble As concentrations were determined. X-Ray diffraction (XRD) analyses were used to establish the resulting mineralogy of crystalline precipitate formation. Depending on the redox state of the As source, different As precipitates were identified. When As (III) was used, the main precipitate formation was Ca–As–O. With As(V) as the source, Ca (OH)(AsO ) •4H O formed at CayAs molar ratios greater 42422 than 1:1. A significant increase in As (III) immobilization was observed at CayAs molar ratios greater than 1:1. Similarly, a substantial increase in As (V) immobilization was noted at CayAs molar ratios greater than or equal to 2.5:1. This observation was also confirmed by XRD. Lime–As–kaolinite slurries were also prepared at different Cay As molar ratios. These slurries were used to specifically investigate the possibility of forming pozzolanic reaction products. Such products would immobilize As by sorption andyor inclusion along with the formations of different As precipitates. Toxicity Characteristic Leaching Procedure (TCLP) tests were used to evaluate As leachability in these slurries. XRD analyses revealed no pozzolanic reaction product formation. Instead, As immobilization was found to be precipitation controlled. The same Ca–As precipitate, Ca–As–O, identified in the lime–As slurries, was also identified when As ( III) was used as the As source, at CayAs molar ratios greater than or equal to 2.5:1. When As (V) was used as the contamination source in the lime–As–kaolinite slurries, the formation of NaCaAsO •7.5H O was 42 observed. The effectiveness of both As (III) and As (V) immobilization in these slurries appeared to increase with increasing CayAs molar ratios. ᮊ 2004 Elsevier B.V. All rights reserved. Keywords: X-Ray diffraction (XRD); Arsenite; Arsenate; Lime; Ca–As–O; Ca (OH)(AsO ) •4H O; NaCaAsO •7.5H O 42422 42 *Corresponding author. Tel.: q1-201-216-8097; fax: q1-201-216-8212. E-mail address: dmoon@stevens-tech.edu (D.H. Moon). ARTICLE IN PRESS 2 D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx 1. Introduction Arsenic (As) is known to be a very toxic element and a carcinogen to humans (Mollah et al., 1998). Even trace amounts of As can be harmful to human health (Karim, 2000). In nature, As is released in the environment through weath- ering and volcanism (Juillot et al., 1999). Arsenic is also released by anthropogenic activities. It was used extensively for agricultural applications such as herbicides and insecticides (Leist et al., 2000) and has thus created problems through leaching and infiltration to subsurface soils and ground water (Murphy and Aucott, 1998). Arsenic is also produced as a waste by-product from the mineral processing and smelting industries. As (III) and As (V) are the most widespread forms in nature (Boyle and Jonasson, 1973; Cherry et al., 1979), with As (III) being both more mobile and toxic (Boyle and Jonasson, 1973; Pantsar-Kallio and Manninen, 1997). More specifically, As (III) is 25–60 times more toxic than As (V)(Dutre and ´ Vandecasteele, 1995; Corwin et al., 1999). Stabilizationysolidification (SyS) is one of the most effective methods to reduce the mobility of heavy metals (Yukselen and Alpaslan, 2001). Var- ious combinations of type I portland cement (OPC), lime, type F fly ash, silica fumes, iron (II) or (III), silicates and blast furnace slag have been used in the treatment of soils contaminated with As (Akhter et al., 1997; Leist et al., 2000). Several researchers have shown that As immo- bilization is mainly controlled by the formation of Ca–As precipitates. Dutre and Vandecasteele ´ (1995, 1998), Dutre et al. (1999) and Vandecas- ´ teele et al. (2002) demonstrated that the formation of Ca (AsO ) and CaHAsO precipitates controls 342 3 the immobilization of As in contaminated soils, which have been treated with cement, lime and pozzolanic material. At the high pH levels (12– 13) induced by lime treatment, where a large fraction of As (III) occurs as HAsO , the precip- 2y 3 itation of CaHAsO will take place. Within the 3 same pH range, the formation of Ca (AsO ) 342 occurs in the presence of As(V) ions. These precipitates were found to be responsible for the observed reduction in As leachability. Also, research by Bothe and Brown (1999) has suggest- ed that lime addition reduces As mobility in contaminated slurries due to the formation of low solubility Ca–As precipitates such as Ca (OH)(AsO ) •4H O and johnbaumite, 42422 Ca (AsO )(OH). 543 Moreover, the reaction of alumino-silicious material, lime and water results in the formation of concrete-like products described as pozzolanic (LaGrega et al., 1994). Dermatas and Meng (2003) have demonstrated that in quicklime SyS applications, the formation of pozzolanic reaction products may be associated with heavy metal immobilization by sorption and inclusion in poz- zolanic reaction products. Therefore, there seems to be three possible As immobilization mecha- nisms to be considered. These are Ca–As precip- itation, sorption or inclusion in pozzolanic reaction products. In this study, the prepared lime–As and lime–As–clay slurries were tested by X-ray dif- fraction (XRD) analyses and analyzed for soluble As in order to evaluate these mechanisms. Kaolin- ite was chosen as the clay that will provide the available alumina and silica for the possible for- mation of pozzolanic reaction products. The objectives of this study are: (1) to investi- gate the formation of Ca–As precipitates in lime– As and lime–As–kaolinite slurries prepared at different CayAs molar ratios; (2) to investigate pozzolanic reaction product formation in lime– As–kaolinite slurries as a function of CayAs molar ratios; (3) to then correlate soluble As concentra- tions with the type of crystalline phases (precipi- tation vs. pozzolanic reaction products) as identified by XRD analyses; (4) to examine the possible oxidation of As(III) in contaminated soils as a result of lime treatment; and (5) to investigate the aging effect on Ca–As precipitates to evaluate whether the various phases formed persist or redis- solve with time. 2. Experimental methodology 2.1. Reagents and materials Three different commercially available As com- pounds were used as As contamination sources: arsenic oxide (As O ), sodium arsenite (NaAsO ) 23 2 and sodium arsenate (Na HAsO •7H O). The last 242 ARTICLE IN PRESS 3D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx two are very soluble and provide two different As oxidation states, As (III) and As(V). The As (III) source, As O was chosen because of its low 23 solubility (1.2–3.7 gy100 ml at 20 8C)(Interna- tional Programme on Chemical Safety, 1997) com- pared to NaAsO , which is highly soluble. This 2 was done in order to evaluate the difference in solubility between various As (III) forms present in the soils. These chemicals were obtained from Fisher Scientific Company (Suwanee, GA). Kao- linite was provided by Dry Branch Company (Dry Branch, GA). Chemical grade hydrated lime (Ca(OH))powder was obtained from the Belle- 2 fonte Lime Company (Bellefonte, PA). 2.2. Slurry preparation and analysis Lime–As slurries were prepared at five different CayAs molar ratios (1:1, 1.5:1, 2:1, 2.5:1 and 4:1). This was done by using a liquid to solid (L:S) ratio of 10:1, by weight. Three separate sets of lime–As slurries were prepared by using three different As compounds as previously discussed. Likewise, Bothe and Brown (1999) evaluated the formation of Ca–As precipitates at CayAs molar ratios that ranged from 1.5:1 to 2.5:1. The prepared slurry samples were then aged and periodically shaken at 20 8C. After 4 days of continuous mixing using an Orbital incubator (Gallenkamp), a sub- sample was taken with a 5 ml pipette and filtered through a 47 mm polycarbonate filter (pore size: 0.4 mm). The residue retained on the filter was air-dried and characterized by XRD analysis. The filtrate was analyzed for soluble As concentration. All the experiments focused on 4-day test results, but the effect of aging was also considered. Time allowed for the aging experiments was 4 months for the lime–As O (lime–As(III)) slur- 23 ries. Initial results indicated no significant change in As concentrations between 4-month and 4-day samples. Thus, equilibrium was attained within 4 days. As a result, the time allowed for aging was shortened to 2 months for the lime–NaAsO 2 (lime–As(III)) and 40 days for the lime– Na HAsO •7H O (lime–As(V)) slurries. At the 242 end of the aging experiments, the same procedure followed in the initial 4-day experiments was used with regards to XRD and soluble As concentration analyses. Lime–As–kaolinite slurries were prepared at CayAs molar ratios (1:1, 2:1, 2.5:1 and 4:1) similar to those used in lime–As slurries. The amount of kaolinite used for the preparation of the slurry was 30 g. The As source was NaAsO (10 2 wt.%) and Na HAsO •7H O (10 wt.%). The sam- 242 ples were adequately mixed with water to enhance the hydration process. All the prepared samples were cured in plastic bottles at 20 8C for 1 month in order to enable comparison of data with other studies. A small portion, approximately 10 g, was removed and air-dried. A fraction of this portion was used for XRD analysis. The effectiveness of lime at immobilizing As in lime–As–kaolinite slurries was evaluated using TCLP tests. A fraction of the portion removed and air-dried from lime–As–kaolinite slurries was used for this purpose (L:S ratio was 20:1). More spe- cifically, 60 ml of the extraction fluid (pH 3) was added to3goftheair-dried sample. After 18 h of tumbling, the leachate was filtered through a 0.4 mm pore-size membrane filter to separate the solids from the leachate solution. The leachate solution was then analyzed and soluble As concen- trations were measured. The concentrations of soluble As and TCLP As were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES)(Varian Vista-MPX, Palo Alto, CA). A number of blanks and check standards were prepared with each batch of samples for quality control purposes. 2.3. X-Ray diffraction analyses A Rigaku DXR 3000 computer-automated dif- fractometer was used. Step-scanned XRD data were collected using Bragg–Brentano geometry. Diffractometry was conducted at 40 kV and 30 mA using Cu radiation with a diffracted beam graphite-monochromator. The data were collected between 5 and 658 in 2 u with a step size of 0.058 and a count time of 5 s per step. All the samples were pulverized and sieved through a 200-mesh sieve (0.075 mm diameter opening) prior to XRD analyses in order to obtain a uniform particle size distribution. ARTICLE IN PRESS 4 D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx It is important to note that when a phase is not detected by XRD analyses, this does not mean that it is not there, but rather that it may be there only in quantities below the detection limit of XRD. As a general rule, the detection limit of the XRD analyses of crystalline phases is considered to be 5% of the total weight of the mixture. However, several factors do have a significant effect on the actual detection limit for any partic- ular crystalline phase. These factors include: degree of crystallinity, hydration, surface texture of the sample, sample weight, particle orientation, mass absorption coefficients of different minerals, etc. (Mitchell, 1993). Any of these factors may have an influence on the resulting peak height and broadness, and thus the detection limit. Carter et al. (1987) established a 0.01% detection limit for quartz and a 0.03% limit for cristobalite. Converse- ly, pozzolanic mineral phases may very well be either poorly crystallized or amorphous, especially at the early stages of formation, which makes their detection by XRD more difficult. 3. Results and discussion 3.1. Formation of different phases in the lime–As slurries Lime–As slurries produced Ca–As precipitates at all CayAs molar ratios tested. The different phases identified by XRD and the corresponding soluble As results at 4-days of mixing and after aging are summarized in Tables 1 and 2, respec- tively. XRD patterns are presented in Figs. 1–6 for all lime–As slurries. In the lime–As O (lime–As(III)) slurries, fol- 23 lowing 4 days of continuous shaking, three major phases were observed: portlandite (Ca(OH)), cal- 2 cium arsenite (Ca–As–O), and calcite (CaCO ) 3 (Table 1 and Fig. 1). Arsenolite (As O ) was only 23 identified at the lowest CayAs molar ratio (1:1), due to its limited solubility, as shown in Fig. 1 and Table 1. However, at CayAs molar ratios greater than 1:1, the peaks of arsenolite disap- peared (Fig. 1). Also, following 4 months of aging, no arsenolite peaks could be identified for samples having a CayAs molar ratio of 1:1 (Table 1 and Fig. 2). Aside from the disappearance of arsenolite peaks, there were no significant differences in the observed XRD patterns between the sub-samples tested following 4 days and 4 months. For the lime–NaAsO (lime–As(III)) slurries, 2 three major phases were identified by XRD: Ca(OH) , Ca–As–O, and CaCO (Table 1 and 23 Fig. 3). No NaAsO was identified due to its high 2 solubility (Table 1 and Fig. 3). No obvious differ- ences were observed in the XRD patterns between the sub-samples tested at 4 days and 2 months (Fig. 4). Overall, regardless of the arsenite source used, whether readily soluble NaAsO or less 2 soluble As O , the same Ca–As precipitate, Ca– 23 As–O, was identified (Table 1). In lime–Na HAsO •7H O (lime–As(V)) slur- 242 ries, five phases were identified (Table 1 and Fig. 5). CaCO and Ca(OH) were identified at all Cay 32 As molar ratios. Due to its high solubility, no Na HAsO •7H O was identified (Table 1 and 242 Fig. 5). NaCaAsO •7.5H O was identified at Cay 42 As molar ratios up to 1.5:1. Johnbaumite, Ca (AsO ) OH, was observed only at a CayAs 543 molar ratio of 1:1. Calcium arsenate hydroxide hydrate, Ca (OH)(AsO ) •4H O, was observed at 42422 CayAs molar ratios greater than 1:1 (Table 1 and Fig. 5). Following 40 days of aging, NaCaAsO •7.5H O was no longer detected by 42 XRD at CayAs molar ratios greater than 1:1. Ca (OH)(AsO ) •4H O and johnbaumite were 42422 still detected following 40 days of aging (Table 1 and Fig. 6). Bothe and Brown (1999) also found Ca (OH)(AsO ) •4H O at CayAs molar ratios 42422 between 2:1 and 2.5:1 as well as minor amounts of johnbaumite. According to their research, the formation of johnbaumite was clearly observed in lime–As(V) slurries at CayAs molar ratios between 1.7:1 and 1.9:1. 3.2. Formation of different phases in lime–As– kaolinite slurries In lime–NaAsO –kaolinite (lime–As(III)–kao- 2 linite) slurries, the formation of Ca–As–O was observed at CayAs molar ratios greater than or equal to 2.5:1 (Table 3 and Fig. 7). Kaolinite is composed of alumina and silica, which become quite soluble at the high pH levels ARTICLE IN PRESS 5D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Table 1 Mineral formations in lime–As O , lime–NaAsO and lime–Na HAsO •7H O slurries following 4 days of mixing and after aging 23 2 2 4 2 CayAs Lime–As O wAs(III)x 23 Lime–NaAsO wAs(III)x 2 Lime–Na HAsO •7H OwAs(V)x 242 molar Phases Phases Phases ratio Phases following 4 days of mixing 1 As O , Ca–As–O, CaCO , Ca(OH) 23 3 2 Ca–As–O, CaCO , Ca(OH) 32 NaCaAsO •7.5H O, Ca (AsO ) OH, CaCO , Ca(OH) 42 543 3 2 1.5 Ca–As–O, CaCO , Ca(OH) 32 4 NaCaAsO •7.5H O, Ca (OH)(AsO ) •4H O, CaCO , Ca(OH) 42 42422 3 2 2 44 Ca (OH)(AsO ) •4H O, CaCO , Ca(OH) 42422 3 2 2.5 444 4 444 Phases after aging 1 Ca–As–O, CaCO , Ca(OH) 32 Ca–As–O, CaCO , Ca(OH) 32 NaCaAsO •7.5H O, Ca (AsO ) OH, CaCO , Ca(OH) 42 543 3 2 1.5 44 Ca (OH)(AsO ) •4H O, CaCO , Ca(OH) 42422 3 2 2 444 2.5 444 4 444 ARTICLE IN PRESS 6 D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Table 2 Soluble As concentrations in lime–As O , lime–NaAsO and lime–Na HAsO •7H O slurries following 4 days of mixing and after 23 2 2 4 2 aging CayAs Lime–As O wAs(III)x 23 Lime–NaAsO wAs(III)x 2 Lime–Na HAsO •7H O wAs(V)x 242 molar As conc. (mgyl) As conc. (mgyl) As conc. (mgyl) ratio Soluble As concentrations following 4 days of mixing 1 589 2783 5165 1.5 2.5 40.8 3699 2 2.1 21.2 1739 2.5 0.9 16.6 6.8 4 0.5 3.6 1.5 Soluble As concentrations after aging 1 489 2488 2278 1.5 ND 32.1 1979 2 ND 20.3 1077 2.5 ND 12.7 4.7 4 ND 3.4 1.8 Note-NDsnot detectable. Fig. 1. XRD patterns for lime–As O slurries with different CayAs molar ratios following 4 days of continuous mixing. 23 induced by lime addition (Keller, 1964). Upon lime treatment a wide variety of calcium aluminate silicate hydrate pozzolanic phases will form depending on reaction conditions (Transportation Research Board, 1976). Several types of calcium aluminate hydrate and calcium silicate hydrate pozzolanic phases have been identified in previous research that focused on lime treatment of kaolinite ARTICLE IN PRESS 7D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Fig. 2. XRD patterns for lime–As O slurries with different CayAs molar ratios following 4 months of aging. 23 soils and arsenic immobilization (Mitchell and Dermatas, 1992; Dermatas and Meng, 1996). In the present study, however, no pozzolanic reaction products were identified. It is important to note that such an observation does not exclude the possibility pozzolanic phases exist within the mix- ture. If pozzolanic phases do exist within the mixture, they may be in quantities that are below the detection limit of XRD analyses. As previously discussed in the XRD analyses section, many factors may affect the detection limit of XRD analyses for any given crystalline phase. The degree of crystallinity is probably the most appli- cable factor in this case, since Ca–As precipitates are more likely to have a higher degree of crystal- linity than pozzolanic phases. Overall, the lack of detectable quantities of pozzolanic phases may suggest that a significant portion of the available Ca ions were consumed during Ca–As–O forma- tion and thus could not participate in pozzolanic reactions. For the lime–Na HAsO •7H O–kaolinite 242 (lime–As(V)–kaolinite) slurries, the formation of NaCaAsO •7.5H O, Ca(OH) and CaCO was 42 2 3 confirmed for all samples tested (Table 3 and Fig. 8). Similar to the lime–As(III)–kaolinite slurries, as discussed in the previous paragraph, no pozzo- lanic reaction products were detected. The formation of johnbaumite was only detected ataCayAs molar ratio of 1:1. No Ca (OH) 42 (AsO ) •4H O was detected, even though its for- 42 2 mation was confirmed by XRD in lime–As(V) slurries. Ca–As precipitates in the lime–As(V)– kaolinite slurries were somewhat different from those found in the lime–As(V) slurries at the same CayAs molar ratios. For the first time in a SyS study of arsenic contaminated soils, a Ca–As–O precipitate was identified. This formation seems to be closely associated with decreasing As concentrations. Some of the Ca–As precipitates observed in this study confirmed previous research findings. Akhter et al. (1997) also identified NaCaAsO •7.5H O 42 formation when cement and cement–fly ash bind- ers were contaminated with sodium arsenate (10 wt.%). NaCaAsO •7.5H O was detected even 42 when As (III) was used as the As source in cement–fly ash mixes, but not in cement-only ARTICLE IN PRESS 8 D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Fig. 3. XRD patterns for lime–NaAsO slurries with different CayAs molar ratios following 4 days of continuous mixing. 2 mixes. This may indicate that As (III) was oxi- dized during SyS applications in the presence of fly ash. However, during the present study, the oxidation of As could not be confirmed in any of the precipitates identified. Mollah et al. (1998) identified a Ca (AsO ) formation when arsenate 342 was treated with Portland cement type-V(OPC-V). Vandecasteele et al. (2002) and Dutre et al. (1999) ´ also predicted the formation of Ca (AsO ) and 342 CaHAsO precipitates by using the speciation pro- 3 gram MINTEQA2. However, the formation of CaHAsO has not been established by XRD and 3 no XRD data file describing it currently exists (JCPDS database). None of these precipitates were identified during this study. Bothe and Brown (1999) did not observe the formation of Ca 3 (AsO ) either. They observed Ca (OH) 42 4 2 (AsO ) •4H O and Ca (AsO )(OH) instead, 42 2 5 43 which were also observed herein (Tables 1–3). 3.3. Soluble As concentration in lime–As slurries and the aging effect Soluble As concentration results are summarized in Table 2 and presented in Fig. 9. In lime–As O (lime–As(III)) slurries, the con- 23 centration of As in solution was 589 mgylata CayAs molar ratio of 1:1 (Table 1). However, when the ratio was increased to 4:1, the As concentration decreased to 0.52 mgyl. In lime–NaAsO (lime–As(III)) slurries, the 2 As concentration was high (2783 mgyl) at a Cay As molar ratio of 1:1. The difference in soluble As concentrations, between lime–NaAsO (lime– 2 As(III)) and lime–As O (lime–As(III)) slurries, 23 can be readily explained by the solubility differ- ences between the two As contamination sources. When the CayAs molar ratio was increased to 4:1, the As concentration in the lime–NaAsO slurries 2 ARTICLE IN PRESS 9D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Fig. 4. XRD patterns for lime–NaAsO slurries with different CayAs molar ratios following 2 months of aging. 2 decreased to 3.6 mgyl. Overall, As (III) immobi- lization in lime–As O (lime–As(III)) and lime– 23 NaAsO (lime–As(III)) slurries was more 2 pronounced at CayAs molar ratios greater than or equal to 1.5:1 (Fig. 9). In lime–Na HAsO •7H O (lime–As(V)) slur- 242 ries, the soluble As concentration was very high (5165 mgyl) at a CayAs molar ratio of 1:1, but decreased significantly to 1.5 mgylataCayAs molar ratio of 4:1 (Table 2). Arsenic (V) immo- bilization was more pronounced at CayAs molar ratios greater than or equal to 2.5:1. Soluble As concentrations following the aging step are also summarized in Table 2 and plotted in Fig. 9. After 4 months of aging, soluble As concentrations decreased with increasing CayAs molar ratios for all slurries tested (Table 2) and were very low at a CayAs molar ratio of 4:1. The variable reaction time (aging) appeared to have an important effect on soluble As concentra- tions only in lime–As(V) slurries. In lime–As(III) slurries, no significant differences in soluble As concentrations were observed between the 4-day and 4-month results. This indicates that equilibri- um was probably reached within the first 4 days (Fig. 9). Conversely, As concentration in lime– As(V) slurries after aging, at CayAs molar ratios up to 2.5:1, was significantly reduced when com- pared to 4-day results (Fig. 9). However, no significant As concentration reduction was observed for a CayAs molar ratio of 4:1. These results indicate that equilibrium was probably not reached in lime–As(V) slurries and that reactions were ongoing following 40 days of testing for all CayAs molar ratios, except for a CayAs molar ratio of 4:1. This observation requires further investigation. Even though the formation of Ca–As–O was observed in all lime–NaAsO (lime–As(III)) and 2 lime–As O (lime–As(III)) slurries, significant 23 As immobilization in the presence of this precipi- tate occurs only when CayAs molar ratios are ARTICLE IN PRESS 10 D.H. Moon et al. / Science of the Total Environment xx (2004) xxx–xxx Fig. 5. XRD patterns for lime–Na HAsO •7H O slurries with different CayAs molar ratios following 4 days of continuous mixing. 242 Fig. 6. XRD patterns for lime–Na HAsO •7H O slurries with different CayAs molar ratios following 40 days of aging. 242 [...]... Tallman DE, Nicholson RV Arsenic species as an indicator of redox conditions in groundwater J Hydrol 1979;43:373 –392 Corwin DL, David A, Goldberg S Mobility of arsenic in soil from the rocky mountain arsenol area J Contaminant Hydrology 1999;39:35 –38 Dermatas D, Meng X Stabilizationysolidification (SyS) of heavy metal contaminated soils by means of a quicklimebased treatment approach In: Gilliam TM, Wiles... associated with a decrease in soluble As concentration Consequently, As immobilization, in lime As slurries, seems to be controlled by the precipitation of Ca– As phases, whether As exists as As(III) or As(V) 3.4 TCLP As release results for lime As–kaolinite slurries TCLP results for lime As–kaolinite slurries are shown in Table 3 and presented in Fig 10 Similar ARTICLE IN PRESS 14 D.H Moon et al /... concentration However, this phase was not observed in lime As–kaolinite slurries Instead, the formation of NaCaAsO4•7.5 H2O was observed and coincided with decreasing TCLP As concentrations The formation of Ca– ARTICLE IN PRESS D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx As–O found in lime As and lime As–kaolinite slurries coincided with low soluble As and low TCLP As concentrations,... remained high (Fig 9) Therefore, it seems necessary to increase the amount of lime present in order to decrease As solubility This finding could not be confirmed by XRD, since peak intensities of Ca–As–O (Figs 1–4) remained relatively unchanged at the various CayAs molar ratios tested This was probably due to peak overlapping between Ca–As–O, Ca(OH)2 and CaCO3, which obscured any relative changes in. .. Consequently, since no pozzolanic product formation was identified, As immobilization was found to be controlled by precipitation 4 Conclusions Lime As and lime As–kaolinite slurries prepared at different CayAs molar ratios produced crystalline precipitate formations that were identified by XRD analyses In all the slurries tested, the redox state of the As source used appeared to affect the resulting crystalline... peak intensities of Ca–As–O Fig 7 XRD patterns for lime NaAsO2 –kaolinite slurries at different CayAs molar ratios after 1 month curing ARTICLE IN PRESS 12 D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx Fig 8 XRD patterns for lime Na2HAsO4•7H2O–kaolinite slurries at the different CayAs molar ratios after 1 month curing Fig 9 Soluble As concentrations in lime As slurries and aging...ARTICLE IN PRESS D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx 11 Table 3 Mineral formations and TCLP As concentrations in lime As–kaolinite slurries following 1 month curing CayAs molar ratio Lime NaAsO2 –kaolinite Phases Lime Na2HAsO4•7H2O–kaolinite Phases 1 CaCO3, Ca(OH)2 2 2.5 4 CaCO3, Ca(OH)2 Ca–As–O, CaCO3, Ca(OH)2... Remobilization of arsenic from buried wastes at an industrial site: mineralogical and geochemical control Appl Geochem 1999;14:1031 –1048 Karim M Arsenic in groundwater and health problems in Bangladesh Water Res 2000;34:304 –310 Keller WD Processes of origin and alteration of clay minerals In: Rich CI, Kunze GW, editors Soil clay mineralogy Chapel Hill: University of North Carolina Press, 1964 p 3... confirmed in any of the precipitates identified When As (V) was used, however, Ca4(OH)2(AsO4)2•4H2O formed XRD analyses on lime As–kaolinite slurries revealed no formations of pozzolanic reaction products Instead, As immobilization was confirmed to be precipitation controlled Again, Ca–As–O was identified in the presence of As (III), whereas NaCaAsO4•7.5H2O formed in the presence of As (V) Aging had... precipitation of Ca–As–O is closely associated with TCLP As concentration Consequently, in the absence of any detectable pozzolanic product formation, it can be concluded that As immobilization in lime As(III)–kaolinite slurries is precipitation controlled In lime As(V)–kaolinite slurries, TCLP As concentrations decreased with increasing CayAs molar ratios for all slurries tested (Fig 10) TCLP As concentrations . reserved. doi:10.1016/j.scitotenv.2004.03.016 Arsenic immobilization by calcium arsenic precipitates in lime treated soils Deok Hyun Moon*, Dimitris Dermatas,. In this study, suspensions of lime As and lime As–kaolinite were studied in an attempt to elucidate the controlling mechanism of As immobilization in lime