Efficiency of Fe-zeolite and Fe-bentonite on Co-stabilization of As, Cd and Pb in Contaminated Soil

Lead (Pb), Cadmium (Cd) and Arsenic (As) have been known as a malicious environment and their toxic effects for plants, animals and humans has been demonstrated. This metals in soil have a different behaviours. The normal concentrations of Cd, As and Pb in agricultural soil were 1.1, 20 and 67 mg.kg respectively. In situ immobilization of Lead and Cadmium (by zeolite and bentonite) and Arsenic (by iron) in soil is well recognized. However, studies on soils that are simultaneously contaminated with lead, cadmium and arsenic are fewer, and assessment of the sorbents effectiveness on costabilization of As, Cd and Pb is also necessary. In this study, local bentonite and zeolite were converted to Fe-zeolite and Febentonite. A Pb-, Cdand As-contaminated soil has been treated with modified bentonite and zeolite separately in 1 and 6 wt% rate. After one month of incubation in at 80% of field capacity moisture, Sunflower (Helianthus annuus. L) plant was transplanted into each pot. The result showed that Fe-zeolite and Fe-bentonite decreased concentration of Pb and Cd extractable with DTPA-TEA; however, Fe-bentonite in the soil reduced water-soluble arsenate, but Fe-zeolite increased it. Finally application of Fe-bentonite can be an effective approach to co-stabilize Pb, Cd and As, in contaminated soils. Keyword: Fe-Bentonite, Fe-Zeolite, Lead, Cadmium, Arsenic


Introduction
Lead (Pb), Cadmium (Cd) and Arsenic (As) have been known as a malicious environment and their toxic effects for plants, animals and humans has been demonstrated. Input sources of these elements to agricultural land include industrial production, pesticides and mining waste. [1]. The normal concentrations of Cd, As and Pb in agricultural soil were 1.1, 20 and 67 mg.kg −1 respectively [1,2].
Chemical behavior of Pb and Cd in the soil was similar while As chemical behavior are very different in comparison of these two elements. So in soils thate contaminated with Pb, Cd and As at the same time, stabilization strategies of heavy metals is different from other contaminated soils [3].
In situ stabilization is a low cost method to clean up and reduce the toxic effects of heavy metals in the contaminated soil [4][5][6]. Generally soil amendments have been used to stabilize of Pb and Cd include phosphate compounds [7], zero-valent iron [8], ferrous sulfate [9], metal oxides [10] and clay minerals [11]. These agents prevent the entry of lead and cadmium into the food chain (plant uptake and leaching to subsurface water) in the long-term [12]. Keller et al. (2005) founded that the addition of 1% zeolite significantly decreased the Cd content of tobacco leaves [13]. In pot experiment it was shown that applying of bentonite to soil, reduced the significant percentage of exchangeable Cd and Pb was converted into unavailable forms [14]. Also in contaminated soils with As have been done many studies for stabilize of As [4,15]. Other studies founded that in situ fixation of As can be achieved by addition of iron amendments, which can also reduce bioaccessibility of As [10,16]. Reaserchers demonstrated that clay minerals with negative surface charges are not efficient for remediation of metalloids such as anionic pollutants. So they by change of surface properties of clays improved absorption of As [17] and anionic pollutants [18][19][20][21][22][23]. Further studies are needed to application of sorbents that co-stabilize metals with different chemical behavior such as Pb, Cd and As in contaminated soil. It seems that if iron is loaded on the zeolite, and bentonite (half a cation exchange capacity), at the same time lead and cadmium absorbed by negative charges of surface clays and arsenic stabilized by iron. Therefore, the objectives of the present experiment were to investigate, i) the Fe loading on the zeolite and bentonite at 50% of their cation exchange capacity, ii) the application of Fe-bentonite and Fezeolite in contaminated soil and co-stabilization of As, Pb and Cd.

Materials
Soil was collected from agricultural site in a gold mine (Takab Town, West Azarbaijan province, NW Iran). Soil properties were determined according to standard methods [24]. The bentonite and zeolite were obtained from Anarak and Firouzkoh mines in central and northern Iran, respectively. Natural bentonite and zeolite were separately put in contact with 0.3 M NaCl solution (1:6.5 solid-liquid ratio) under reflux for 12 h. This process was repeated one more time changing the NaCl solution. The phases were separated by centrifugation and the solid phases were washed until Clelimination was achieved using the AgNO 3 test. Then bentonite and zeolite were dried at 333 K for 24 h. After modification by Na, the clays were treated by (0.024M FeCl 3 for zeolite, 0.019 M for bentonite) under agitation for 24 h to prepare Fe-bentonite and Fe-zeolite (equivalent to 50% of their CEC). The solid phases were separated by centrifugation and washed by distilled wáter. This step was repeated so many times until amount of iron in the suspension supernatant was no ditected. The solid phases dried at 323 K for 17 h. Then each sample was thermally treated at 373 K for 24 h. In final Fe-bentonite and Fe-zeolite were prepared ( Figure 1) [25].  Table 2).

Pot Experiments
The modified bentonite and zeolite separately were added to the soil (1kg) in 1 and 6 wt% rate (in three replicate). After one month of incubation at 80% of field capacity moisture, two healthy Sunflower (Helianthus annuus. L) plants were transplanted into each pot. Plants were allowed to grow for 10 weeks in the greenhouse (temperature range 16-30 • C; average photosynthetically active radiation was 825 µmol m −2 s −1 ) and watered daily or as needed. At the end of experiment, shoot, root and soil sample from each pot were collected. Soil pH was determined with a pH meter in a 1:2 (w:w). Water-soluble arsenic [26] as well as Cd and Pb extractable with Diethylenetriaminepentaacetic acid-Triethanolamine (DTPA-TEA) 0.05 M [27] of soil samples were measured by Graphite Furnace Atomic Absorption Spectrometry (GF-AAS). As, Cd and Pb in plant samples (root and shoot) were determined by GF-AAS following HNO 3 -HClO 4 digestion (0.25g dried and ground plant material was digested with 6 mL conc. HNO 3 plus 2 mL conc. HClO 4 ) [28].

Data Processing
The pot experiment was set up in a randomized complete block design with three replications. The ANOVA (SAS program., version 9.2) was employed for statistical analysis of data.

Characteristics of the Soil, Bentonite and Zeolite
Physical and chemical characteristics of soil were presented in Table 1. The pH value of the soil (7.8) was close to neutral. Soil texture was loamy. Comparing of concentration of As, Pb and Cd in soil from Zarshuran area and tolerable levels of metals (20 mg/kg As, 100 mg/kg Pb, and 3 mg/kg Cd) [2] indicated that total As, Pb, and Cd concentration in Zarshuran soil were extremely high. According to chemical properties of natural and modified zeolite and bentonite ( Table 2) the cation exchange capacity (CEC) of zeolite and bentonite were 94 and 76 meq/100gr respectively. The natural zeolite and bentonite had alkaline properties (pH about 8.3 and 8 respectively). After modification of clays by iron, their properties became acidic. Existence alkali and alkaline earth metals on the bentonite bedrock, Due to of high electrical conductivity in bentonite. By removing lime and minerals in raw bentonite, electrical conductivity decreased. According to the XRD pattern ( Figure. 2), the bentonite was composed of 85.5% montmorillonite, 12% quartz and 2.5% illite, the zeolite of 80.6% clinoptilolite, 9.6%. quartz, 5.8% illite and 4% feldspar. Existence of a peak at 13.1 Å in natural bentonite was an indication of high amount of montmorillonite. Several peaks at 2.96, 3.94 and 9 Å in natural zeolite were indication of high amount of clinoptilolite.  Table 3 presented that adding of Fe-zeolite and Febentonite to soil caused a slight decrease of pH. Application of Fe-zeolite and Fe-bentonite in soil has resulted in lowering amount of Pb and Cd extractable with DTPA-TEA. Probably Pb and Cd were adsorbed on exchangeable site and complex with surface functional groups of zeolite and bentonite by following mechanisms (This mechanism occurs in the part of the CEC that is not saturated with iron):

Mobility of Metals in the Soil
To exchange with hydronium ion: ≡ XH+Pb 2+ ≡ XPb + +H + (≡ X is the exchangeable site) To complex with surface functional groups: The adsorption mechanism of Cd on the clays similar to Pb. Usman et al (2006) demonstrated that by the addition of zeolite at 1% (w/w) to a contaminated-soil, the labile Pb and Cd fractions were significantly reduced [29]. Iron bonded to the zeolite and bentonite was affected to the lower mobility of Cd. Mench et al. (1998) reported that the soluble and exchangeable fractions of Cd in two contaminated soils were decreased by the application of hydrous Fe oxide [30]. In this study, Fe-zeolite had a strong effect on reducing of DTPA-TEA extractable Cd compared to the Fe-bentonite.
With application of Fe-bentonite in the soil, reduced watersoluble arsenate, but Fe-zeolite increased water-soluble arsenate. Probably Fe-zeolite even with slight decreasing of soil pH, increased solubility of magnesium and calcium arsenate and arsenate is increased in the solution phase by the following reactions [31]: In bidentite and monodentite inner sphere complex, arsenic species such as HAsO 4 2and H 2 AsO 4 -, are connected with iron (by electrostatic bonding). Sherman and Randall (2003) explained the exact mechanism of the adsorption of As (V) onto iron componds such as goethite, lepidocrocite, hematite and ferrihydrite to be the formation of inner-sphere surface complexes resulting from bidentate corner-sharing between AsO 4 3and FeO 6 polyhedra [32]. In ligand exchange prosses, arsenic species such as HAsO 4 2and H 2 AsO 4 -, were exchanged by OH. Thus, the As released from dissolution of magnesium and calcium arsenate is more than As stabilized on the Fe-zeolite. In case of Fe-bentonite, the amount of arsenate stabilized by iron, is more than the As released from magnesium and calcium arsenate. Probably the lamellar structure, expandable layer and high interlayer space of bentonite allowed that the As easily bonded with iron.

As, Cd and Pb Uptake by Sunflower
Adding of Fe-zeolite to the contaminated soil, As concentrations in plant (roots and shoots) increased, whereas Cd and Pb concentration decreased. In soils enriched with Fe-bentonite concentration of As, Cd and Pb in plant (roots and shoots) reduced, ( Table 4). The calculation of the shoot/root ratio shows a slight penetration of arsenic in the shoot (0.01-0.03) and significantly higher values for Pb (0.08-0.15). The highest values of the shoot/root ratios are observed for Cd (0.69-0.80) ( Table 4). With application of sorbents, this index decreased for As, Pb and Cd. Results obtained in Table 3 and 4 indicated that Fe-bentonite was the most effective treatment rather than Fe-zeolite.

Conclusion
In a contaminated soil, efficiency of two sorbents for reducing of bioavailability of Cd, As and Pb were evaluated. Application of Fe-zeolite and Fe-bentonite decreased bioavailability of Pb and Cd in soil. Fe-bentonite stabilized water-soluble As, but Fe-zeolite increased water-soluble As in soil. In final, the additions of 6% (w/w) Fe-bentonite have a most effective treatment as indicated by the reductions of water-soluble As, and DTPA-TEA extractable Cd and Pb (53.6, 9 and 34.8% respectively) in the soil.