Comparative effect of the three talc deposits in detoxification of Cr(VI) from wastewater

Environmental application of three different talc deposits toward the removal of hexavalent chromium ions Cr(VI) from aqueous solution as simulated polluted water was investigated. Three talc samples (T1, T2 and T3) were used from Wadi Atshan area, Eastern desert, Egypt. The affecting parameters, such as pH, contact time, solution pH and the dosage, were studied during the progress of the adsorption process of chromium (IV). The results showed that a contact time of 65 min for T3, 80 min for T2, 85 min for T1 under optimum condition at pH 7 at 25 °C. The adsorption capacity for the removal of Cr(VI) using the three samples T1, T2 and T3 was evaluated to be 78%, 86% and 97%, respectively, under optimized conditions utilizing 1.0 g/L of the adsorbent. Characterization of the three talc samples was performed using XRF, XRD and FTIR analyses in order to assess the physicochemical properties of the adsorbents. This approach provided new class of adsorbent as highly efficient materials for Cr(VI) removal based on talc deposits which possess some privileges such as availability of the natural resources that makes the process to be low cost and simple.


Introduction
The accumulation of Cr (VI) in water resources and its possible effects on human health had been investigated (Salman and Elnazer 2020). It was revealed that the increase in contamination of Cr(VI) ions in the water can threaten the human health ranging from skin irritation to DNA damages and cancer development, depending on dose, exposure level, and duration (Tumolo et al. 2020). Chromium ions exist naturally in two main oxidation states Cr (III) ions and Cr (VI) ions and related ion forms depending on pH values. Cr(VI) compounds appear to be much more toxic systemically than Cr(III) compounds, given similar amounts (Salman and Elnazer 2020;Tumolo et al. 2020;Costa et al. 1997;Zhitkovich 2011). The major factor governing the toxicity of Cr(VI) is the solubility (Salman and Elnazer 2020;Tumolo et al. 2020). Although mechanisms of biological interaction are uncertain, this variation in toxicity may be related to the ease with which Cr(VI) can pass through cell membranes with subsequent intracellular reduction to reactive intermediates (Chiu et al. 2010;Ray 2016). As a result, the mitigation of the hazardous effect accompanied by the presence of Cr(VI) in wastewater gained attentions. Accordingly, the most approaches for the removal of chromium from water included reduction of Cr(VI) to Cr(III) (Nur-E-Alam et al. 2020;Mitra et al. 2017), precipitation (Stylianou et al. 2018;Xie et al. 2017) ion exchange (Verma and Sarka 2020;Gode and Pehlivan 2005) and solvent extraction (Venkateswaran and Palanivelu 2004;Agrawal et al. 2008). However, most of these methods required high energy for operation and some of them were ineffective (Nur-E-Alam et al. 2020;Mitra et al. 2017;Stylianou et al. 2018;Xie et al. 2017;Verma and Sarka 2020;Gode, and Pehlivan 2005;Venkateswaran and Palanivelu 2004;Agrawal et al. 2008). Adsorption method is considered as the most effective and widely used technique due to high removal efficiency and simplicity (Kokab et al. 2021 water such as activated carbon (AC) (Selvia et al 2001;Xu et al.2021), activated alumina (Marzouk et al. 2011), chitosan polymer (Bishnoi,et al. 2004), zeolite (Hena 2010), low-cost bio-adsorbents such as olive, leaves and wool (Sampaio et al. 2015), saw dust (Zakaria et al. 2009), rice husk (Bishnoi,et al. 2004), wheat bran (Singh et al. 2009), bentonite (Castro-Castro et al. 2020, metal oxides such as ferric hydroxide (Altundogan 2005), and nanostructured adsorbents (Huang et al. 2015).
The use of complex minerals such as talc for the adsorption process of chromium (VI) had been investigated in few numbers of reports (Ossman et al. 2014).
Talc deposits are largely distributed in the central and southern Eastern Desert of Egypt. Generally, they are associated with metavolcano-sedimentary rocks of island arc association or associated with ophiolitic ultramafic rocks. They occur as pure talc deposits or associated with carbonates forming talc-carbonate rocks. In the Eastern Desert, talc deposits of low-grade metamorphism are fine-grained with massive to weakly foliated, but they are coarse-grained with moderately to strongly schistose in higher-grade metamorphism. Economically significant talc deposits of Eastern Desert are already being mined in the Darhib, Atshan and Wadi Allaqi regions. Egypt produced 12,924 tons of talc in 2011 and about 172,181 tons in 2015 (Gahlan et al 2021).
The talc samples used in the present study are collected from Wadi Atshan area which is located in the Hamata region, 18 km west the Red Sea Coast (Fig. 1a). The Atshan talc deposit is located in the southern Eastern Desert and is one of the largest talc deposits in Egypt. Wadi Atshan talc is located at the intersection of latitude 24 o 18′ N and longitude 35 o 20′ E. The main Neoproterozoic rock types exposed in this area include serpentinites, metavolcanics and gray granite (Fig. 1b). These rocks were subjected to general metamorphism which leads to formation of talc deposits. The talc deposits of Wadi Atshan occur as several massive large masses (Fig. 2a) or as sheared masses located along fault planes and shear zones (Fig. 2b). Small lenses of sulfides are observed within the talc bodies as segregation from the original rocks due to metamorphism.
Recently, the talc was used as clarifier for wastewater (Grafia et al 2014). Also, talc powder was used as adsorbents for removal of Cr(VI) from wastewater. The experiments were carried out at 25 °C and pH 4, and the adsorbent dose was evaluated to be 1.5 g/L to reveal the % removal of Cr(VI) was 77% after 1 h (Ossman et al. 2014). As a result, this work deals with investigation of the potential efficiency

General
Minerals identification of Atshan talc was done on thin sections using a Polarizing Nikon Microscope and confirmed by the X-ray diffraction (XRD) and an electron probe microanalyzer. XRD analyses were carried out at the Egyptian Mineral Resources Authority (Dokki, Egypt) using a PANalytical X-ray diffraction equipment model X'Pert PRO with a Secondary Monochromator with Cu radiation (λ = 1.542 Å) at 45 K.V., 35 M.A. Microprobe analyses were performed using a JEOL JXA-8500F electron microprobe at the Geo-Analytical Lab, Washington State University, USA. Operating conditions were 15 kV accelerating voltage, 20 nA beam current and 3 µm beam diameter. Suitable synthetic and natural standards were applied for calibration. The FTIR pattern was recorded with a Nicolet 6700 infrared spectrophotometer over the 400 cm −1 to 4000 cm −1 wave-number range. X-ray fluorescence (XRF) was recorded on an Axios Sequential WD_XRF Spectrometer, PAnalytical 2005 in the National Research Center Laboratories, Dokki, Cairo, Egypt. K 2 Cr 2 O 7 was purchased from Sigma-Aldrich Company, Germany, and were used without further purification.

Petrography of Wadi Atshan talc
Talc of Wadi Atshan is fine-grained rock exhibiting a schistose texture and ranges in color from brownish yellow to greenish gray. It is composed essentially of talc mineral (~ 90-95%) with minor amounts of carbonates, serpentine, quartz, chlorite and opaques. Talc mostly occurred as fine dense microcrystalline fibrous, shreds, and platy aggregates with parallel arrangement (Fig. 3a). Rarely, coarse-to medium-grained flakes of talc are observed in the studied talc samples. Carbonates occur as sparse patches and veinlets embedded in a very fine talc matrix that sometimes stained by yellow limonitic materials. Few rock fragments of serpentinite are observed within the talc (Fig. 3b). Opaque minerals occur as fine anhedral disseminated crystals of Crspinel and Fe-Ti oxides. They have experienced brittle brecciation. Chlorite occurs as anhedral specks or as thin film around Cr-spinel. Quartz occurs as scattered anhedral finegrained crystals or as veinlets cutting throughout the rock.

Preparation of adsorbents and adsorption studies
Three mineral composites of talc powder were used for removal of Cr(VI). The talc was collected from local sources. The materials were then crushed and sieved into different sizes ranging from 2 mm to 100 µm. Then the talc composites were washed with distilled water, filtered and dried ay 60 °C in an oven for 1 h order to remove any surface impurities. Then the materials were stored in a desiccator.

Batch adsorption experiments for Cr(VI) removal
The three talc minerals T1, T2 or T3, each in a different experiment (0.1, 0.5, 1, 2.5 and 5), were thoroughly mixed with 100 mL of Cr(VI) solution (0.5 g), and the suspensions were stirred at room temperature. 1.0 mL of sample was collected from the supernatant at predetermined time intervals of 5 to 120 min. These samples were then centrifuged for 5 min, and the clear solutions were analyzed for the residual hexavalent chromium concentration according to Dong et al. procedure (Dong et al. 2011).

Talc deposits for the Cr(VI) removal
Potassium dichromate K 2 Cr 2 O 7 (0.5 g) was dissolved in 100 ml of distilled water. pH, contact time and dose effect of each talc deposit were studied.

Results and discussion
Talc is a mineral that belongs to the phyllosilicates, corresponding to the chemical formula for hydrous magnesium silicate with formula Mg 3 Si 4 O 10 (OH) 2 . The structural unit consists of three sheets, i.e., octahedral-coordinated magnesium hydroxide group sandwiched between two layers of tetrahedrally linked silica layers (Marzbani et al. 2016). Herein in this study, three deposits of talc, namely T1, T2 and T3, were collected and their adsorption efficiencies toward Cr(VI) ions removal were investigated. These deposits were thoroughly characterized to study their structure, morphology and composition by using techniques, such as XRD and FTIR.

XRF analysis
The results of the quantitative analysis of the three talc samples using XRF measurement are shown in Table 1.  Concentrations of major oxides in three samples from the Wadi Atshan talc ore bodies are shown in Table 1. The studied talc samples are characterized by significant amounts of SiO 2 (55.78-64.46 wt%) and .28 wt%).

Microprobe analyses
Talc, carbonates and chlorite are analyzed in the Atshan talc using the electron microprobe technique. The representative chemical analyses of talc are given in Table 2 (Table 3). MgO is the major oxide (43.42-45.48 wt%) in the analyzed carbonates with minor amounts of FeO (1.98-3.79 wt%), CaO (< 0.7 wt%) and MnO (< 0.32 wt%). The total of the analyzed carbonates is low because CO 2 is not counted.
Few specks of chlorite were analyzed by the electron microprobe in the Atshan talc, and their analyses are given in Table 4. The major oxides of the analyzed chlorites are SiO 2 , MgO, Al 2 O 3 and FeO. The chlorites have more MgO than FeO and are classified as clinochlore using the classification scheme (Hey 1954).
From the results, the three materials can be expressed as the following chemical compositions (Fig. 5).

FTIR analysis
FTIR spectrum is illustrated in Fig. 3 for three talc deposits indicating various groups and bands in accordance with their respective wavenumbers (cm −1 ) showing the complex nature of the adsorbent. In Fig. 6a, the FTIR spectrum of talc sample (T1) (99% talc) showed absorption bands located at 3675 cm −1 arising from the surface OH groups linked to Si (Si-OH) and Mg (MgOH). The peak at 1667 assigned for H-O-H bending. The observed peaks around 1017and 464 cm -1 were assigned to the out-of-plane symmetric Si-O-Si groups of talc and 669 cm −1 assigned to Si-O-Mg (Farmer and Russel 1967;Prado et al. 2015]. The results expressed on pure talc with chemical formula (Mg 3 Si 4 O 10 (OH) 2 ). In Fig. 6b, the FTIR spectrum of sample (T2) showed the peaks around 3677, 3660, 3644 cm −1 attributed to the hydroxyl groups attached to Mg-OH, Fe-OH, Si(Si-OH). The peaks at 1668 refer to the H-O-H bending. The strong bands at 1010 and 460 cm −1 represent the Si-O-Si groups of the tetrahedral sheet. The results indicated that the sample is talc combined with clinochlore with chemical formula of (Mg 3 Si 4 O 10 (OH) 2 + (Mg,Fe 2+ ) 5 Al(Si 3 Al)O 10 (OH) 8 ). Figure 6c shows the vibrations in the bands of the FTIR spectrum for sample (T3), where wave numbers of 3676, 3660, 3644, 3420 cm −1 assigned for the hydroxyl group linked to Mg-OH, FeOH-, Si-OH. The peaks at 995 and 440 cm −1 referred to the siloxane group (Si-O-Si) stretching vibrational, while the band at 669 cm −1 is assigned to the Si-O-Mg bond (Farmer and Russel 1967).

The batch adsorption study
The maximum adsorption at optimum dose with respect to contact time for T1, T2 and T3 was investigated by studying the affecting parameters such as pH, contact time and the adsorbent dosage to determine the adsorption capacity of each talc deposit.

Effect of pH
The pH effect on the adsorption of Cr(VI) was studied. Firstly, control experiments were performed without adding adsorbent in aqueous solution of Cr(VI) (5000 mg/L), and the pH was varied at each experiment at different values of pH (1, 3, 5, 7 and 10) for 60 min. It was revealed that the % removal was zero. Other series of experiments were carried out by adding the dose of adsorbent (100 mg/L) at different values of pH (1-10) with 60 min contact time. As shown in (Fig. 7), it was indicated that the % removal of Cr(VI) was decreased at pH = 1, 3 and 5. However by increasing the values of pH to 7 till 10, the % removal was increased and remained at contestant value ranged from 52%, 56% and 78% by using T1, T2 or T3, respectively. The explanation for this phenomenon is as follows: when the pH shifted from acidic to neutral and alkaline (i.e., pH = 7-10), low concentration of H + ions was observed, and the surface of the sorbent possessed high negative charges, and thus the sites became easily available for adsorption of Cr(VI). Therefore, the suitable pH of solution for maximum removal of chromium and mercury ions is 7 (Fig. 7).  Fig. 8 shows the effect of adsorption time on Cr(VI) with comparative data for the tested adsorbents T1, T2 and T3 (100 mg/L). The amount of adsorbed Cr(VI) increased with contact time for the three adsorbents used, and equilibrium was remained within 65 min for T3, 80 min for T2 and 85 min for T1 to achieve removal efficiency 82% for T3, 63% for T2, and 56% for T1 (Fig. 8). From these results, it was determined that T3 deposit showed higher adsorption capacity for Cr(VI) than T1 and T2, because of its higher pore volume and the probable coordination with the metal hydroxyls presented in the mineral.

Effect of adsorbent dosage
The effect of adsorbent dosage on the adsorption of a solution of 100 mL of Cr(VI) (5000 mg/L) by stirring at pH with 0.1, 0.5, 1, 2.5 and 5 g/L of adsorbent for 1 h at 25 °C. Figure 6 shows the effects of varied dosage of adsorbents. It was found that the % removal for T1 was increased from 52 to 78% as the adsorbent dosage was increased. In the case of T2 deposit, the % removal was increased from 56 to 86%, while in the case of T3 deposit, the % removal was increased from 78 to 97%. This emphasis on the can the fact that as the dosage of adsorbent increases, the contact surface is being increased and the adsorption process can be enhanced until reaching adsorbate-adsorbent equilibrium. It was determined that, at the dose 1 g/L for each talc deposit, an equilibrium was retained and similar % removal was observed. It was demonstrated thatT3 showed higher efficiency resulting from the higher surface area exposure for adsorption and lower mass transfer resistance due to the available porous structure and volume (Fig. 9).

Adsorption isotherms
The Cr(VI) ions as adsorbed-talc samples as adsorbates were plotted versus the equilibrium concentration to determine the efficiency of the adsorption process and the mechanism of the adsorption system. Three isothermal adsorption models have been investigated for the technicality of Cr(VI) adsorption on the T3 sample (Table 5) (Oladoja et al. 2008).
In the case of Langmuir (LH) isotherm, the regression coefficient value (R2 = 0.99) that was slightly greater than that of Dubinin-Kaganer-Radushkevich (DKR) (R 2 = 0.97). These results indicate well-fitting of the adsorption of chromium ions on talc deposit (T3) with LH. It was postulated that the active surface of the talc sample is homogeneous and the adsorbed chromium ion monolayer could be created upon equilibrium (Hutson and Yang 1997). The calculated sorption capacity for chromium ions on T3 sample was calculated (qe = 249.907 mg/g).

Conclusion
Three talc deposits T1, T2 and T3 were collected from Wadi Atshan, Egypt, and were fully characterized. These samples were applied as adsorbents for removing hexavalent chromium ions from aqueous solution. Optimum conditions were investigated through the evaluation of adsorption process under different parameters, namely pH, contact time and amount of adsorbents. It was determined that talc containing 4% clinochore (T3-sample) exhibited potential adsorption capacity more than pure talc (T1-sample) and talc contaminated with 2% clinochlore (T2-sample).
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Conflict of interests The authors have declared no conflict of interest (no competing interests).
Ethical approval This study did not use any kind of human participants or human data, which require any kind of approval.

Consent to participate Not applicable.
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