Bentonite surface modification and characterization for high selective phosphate adsorption from aqueous media and its application for wastewater treatments

Removal and recovery of phosphorus compounds from wastewater has an important role in managing environmental and economic concerns, such as eutrophication of surface waters and depletion of phosphorus resources. Also, in most countries, stringent legal standards are followed for the phosphorus discharge range in water and wastewater. Several techniques have been widely investigated for phosphorus removal from wastewater including chemical precipitation, and physical and biological processes (Clark et al. 1997; de-Bashan & Bashan 2004; Mulkerrins et al. 2004).

Among these methods, phosphate removal from aqueous solutions by adsorption has recently been given more attention, due to severe effluent standards, the high efficiency of this method and also the phosphate-loaded adsorbents that can be used in agriculture as fertilizer or soil conditioner (Zhang et al. 2009). Adsorption is an environmentally friendly, simple, feasible and economical method for phosphate removal from wastewater, leading to practical recycling and reuse of phosphorus.

Different materials have been used for phosphate adsorption that can be divided into two kinds: non-clay (such as natural, synthesis and waste materials) and clay adsorbent. The key problem in the phosphorus adsorption method is finding an efficient, available and low-cost sorbent. Clays are low-cost and easily available minerals that, due to their individual properties, are used as general sorbents in water and wastewater treatments (Atia 2008; Chen et al. 2008; Alkaram et al. 2009; Guo et al. 2009; Haghseresht et al. 2009; Koswojo et al. 2010; Gu et al. 2011; Wang et al. 2013; Siti et al. 2014). Clays are used as a natural scavenger for cationic and anionic pollutants through ion exchange or adsorption or both (Moharami & Jalali 2013). Various natural and modified clays have been used for phosphate adsorption, e.g. vesuvianite and lanthanum-doped vesuvianite (Li et al. 2009), natural bentonite, kaolinite and zeolite (Moharami & Jalali 2013), bentonite-hematite (Dimirkou et al. 2002), lanthanum-modified bentonite (Phoslock) (Haghseresht et al. 2009), bentonite modified with Fe polycations and cetyltrimethylammonium bromide (Ma & Zhu 2006), and hydroxy-aluminum, hydroxy-iron and hydroxy-iron-aluminum pillared bentonites (Yan et al. 2010). Notably, some of the adsorbents are vulnerable under a broad range of pH values (Dimirkou et al. 2002; Haghseresht et al. 2009; Yan et al. 2010) and this may be problematic for their application in wastewater treatment which may also need a long contact time (Li et al. 2009; Yan et al. 2010). Also natural bentonite has low adsorption capacity and needs a long contact time for phosphate adsorption (Moharami & Jalali 2013).

In this study, organic and inorganic bentonite was used for phosphate removal from aqueous solutions in a batch test. Several studies used bentonite as a phosphate sorbent from water and wastewater, but this study investigated different methods for bentonite modification and also optimized preparation. Four inorganic pillared bentonites, namely Ca-, Al-, Fe- and Fe-Al-bentonite (FAB), two organic bentonites, modified by Irasoft-T18 and Rewoquate-WE18 surfactants, and an acidthermoactivation bentonite were prepared and have been used in experiments. Ultimately, after selection of best modification process and parameters, their structures and mineralogy were thoroughly characterized.

The bentonite used for this study was purchased from the Semnan-Iran bentonite mine and NH₃, FeCl₃.6H₂O, AlCl₃.6H₂O, CaCl₂ and KH₂PO₄, were all of analytical grade, obtained from Reidel-de Haenand Merk Company from Germany. Irasoft-T18 and Rewoquate-WE18 surfactant were purchased from NiliPadideh Chemistry, Iran and Gold-Schmidt, Germany, respectively. Sorbents for all stages were ground to 35 meshes (0.5 mm).

A total of 25 mL of pillaring solutions (Fe and/or Al) were added to 10 g of bentonite, stirred for 10 min and then remained for different times (1, 2, 4, 8, 16, 24 h) to check its adsorption capability at room temperature (25°C). The results showed that after 2 h the adsorption ability remained constant, so 4 h was used for further experiments. Then the mixture was filtered, and 50 mL ammonia solution 20 mM was added and remained for 30 min at room temperature. The mixture was filtered again and washed with 100 mL dilute ammonia solution 2 mM and deionized water, and dried at 70–75°C. The dried product was ground to 35 meshes, and kept for adsorption experiments.

For Ca modification, 500 mL CaCl₂ (0.5 M) with 50 g bentonite was shaken for 1 h and centrifuged (this stage was repeated three times). Then the product was washed with 500 mL distilled water (twice) and finally with 200 mL ethanol, stirred for 1 h and centrifuged. The final product was dried at 70–75°C in an oven.

Two grams of Irasoft or Rewoquate surfactant was dissolved in 50 mL ethanol and acetone solution (50% V/V). Fifty grams of bentonite was added to the prepared solution and shaken for 30 min, then dried at 50–55°C. The dried product was ground to 35 meshes and used for adsorption experiments.

Also for preparation of acidthermoactivation bentonites, 250 mL, HCl (1 mol/L) was added to 30 g bentonite and shaken on a hotplate at 80°C for 4 h, the centrifuged product was washed with distilled water and dried at 70–75°C.

The cation exchange capacity (CEC) values were determined by the ammonium acetate method (Bashour & Sayegh 2007). X-ray powder diffraction patterns were obtained using an X-ray diffractometer D8 advance of Bruker (Germany) with an X-ray tube anode Cu of wavelength 1.5406 (Cu Kα). The X-ray diffraction (XRD) patterns were obtained from 3 to 60°. Major elements were determined using an X-ray fluorescence (XRF) spectrometry analyzer model S4 Pioneer (Bruker, Germany).

Fourier transform infrared (FTIR) spectra were obtained in the transmission mode on a Jasco 680 Plus Fourier transform IR spectrometer. Samples were mixed with KBr in an agate mortar with ratio W/W 1:30, then powdered to prepare the KBr pellets. The adsorption bands in the range of 400–4,000 cm⁻¹ were recorded. The bentonite samples were coated with Au under vacuum in an argon atmosphere for scanning electron microscopy (SEM) by Philips series XL30 Scanning Electron Microscope (The Netherlands).

The effect of common coexisting ions in wastewater such as chloride, sulfate, nitrate and bicarbonate on the adsorption of phosphate was investigated by adding solutions containing sodium chloride, sodium sulfate, sodium nitrate and sodium carbonate at two concentration levels. The pH of solutions was adjusted close to 6.5 by HCl and 2 g of FAB was added to the 100 mL mixed solutions. Then they were agitated at 125 rpm for 1 h at 25°C and the residual concentration of phosphate was analyzed after centrifugation.

The efficiency of modified bentonite for phosphate removal from urban wastewater was tested using a batch experiment (2 g adsorbent, 100 mL wastewater, 125 rpm, 1 h).

Important parameters such as the cost of material, the adsorbate selectivity and stability of the sorbent in the environmental conditions (pH range and temperature) must be considered in selection of adsorbent material.

It was believed that a structural change in the clay surface, to improve a specific surface area and also decrease negative charges of the adsorbent surface after treatment with acid, had an important role in increasing adsorption capacities of modified clay for adsorption of different anions, such as phosphate, from solution (Ye et al. 2006). Ye et al. (2006) reported phosphate adsorption on acid-thermal treated > acid treated > unmodified palygorskites (Ye et al. 2006). However in this study it was found that acidthermoactivation of modified bentonite is not generally effective for phosphate adsorption due to decreasing of Al and Fe ions and also surface area in a concentration of 1 M HCl (Ajemba & Onukwuli 2012; Bendou & Amrani 2014).

Al and Fe-bearing minerals are the most effective minerals for P sorption and the sorption is credited to the presence of Al-OH and Fe-OH functional groups on the mineral surface (Kasama et al. 2004).

Clay modification with hydroxyl Fe or Al species readily increased the phosphate adsorption capacity of clays (Borgnino et al. 2010). Modifying surface clay minerals with Al/Fe can form the Fe/Al-OH and/or Fe/Al-H₂O groups (which are active sorption sites for phosphates in solution) which could enhance phosphate adsorption capacity of clays by ligand exchange with OH groups (Zhu et al. 2007; Zhu et al. 2009a).

Phosphate may partly be adsorbed by forming binuclear complexes or by an electrostatic force mechanism. Also, it can form P-bearing materials, which is accompanied by dissolution of Fe/Al-bearing minerals which is important in the P sorption at low pH media or high P concentration (Kasama et al. 2004; Zhu et al. 2009a).

Phosphate adsorption by clays can be readily increased by their modification with Fe or Al hydroxylated species (Borgnino et al. 2010). Therefore, Fe-Al treatment was selected for bentonite modification, in which the increased absorption capacity could be attributed to decreasing crystallinity of Al and Fe oxides in the FAB during aging and drying, causing higher sorption capacities due to higher reactivity of surface functional groups (Zhu et al. 2009a). This result is compatible with studies that displayed the modification on montmorillonite phosphate adsorption (Zhu et al. 2009a). However, another study about modification of bentonite showed a reverse order: Al- > Fe- > Fe-Al-Bentonite (Yan et al. 2010) which could be due to a different modification method on the surface of sorbent. Therefore in this study, modification by Al-Fe was selected for bentonite modification and different modification parameters were studied.

Also, the results of an investigation into the effect of the ratio of Fe and Al on phosphate adsorption are shown in Table 1. The results showed that a negligible difference exists between the concentration ratios of one and two times the CEC. In this study, due to probable precipitation of Al and Fe on the surface of bentonite, and/or formation of hydroxyl Al and hydroxyl Fe cations in the interlayer spaces of clays, and also their adsorption on the external planar surfaces and/or at the edges (Zhu et al. 2009a), confidently a total concentration of two times the CEC (1.5 Al and 0.5 Fe with adsorption percent greater than 99%) was selected for modification.

Contact time between a solution containing Al-Fe as the modifiers and bentonite is one of the physical parameters that may affect the adsorption of phosphate, but the results show that this parameter had negligible effect on phosphate adsorption.

Al and Fe hydroxide can be formed in alkaline solutions. Therefore, the effect of ammonia concentration for modification of bentonite was investigated. The results show that ammonia concentration has no effect on phosphate adsorption. This has shown that pH of bentonite is significant for formation Al and/or Fe hydroxides.

Ultimately an adsorbent was prepared under these conditions: contact time of 4 h for pillaring solution, Fe/Al up to 2 × CEC (1.5 Al + 0.5 Fe) and without adjustment of pH, and then the effective chemical and physical parameters in phosphate adsorption were investigated.

An investigation into the effect of contact time on the phosphate removal shows that more than 96% removal is achieved in the initial minutes and removal reaches a maximum value of 99.9% after 30 min. Electrostatic sorption on the outer surfaces and at pore entrances of the adsorbent and the direct ion exchange process can elucidate this rapid sorption. Also, phosphate can be adsorbed on the inner surfaces of deep pores by diffusion into the sorbent matrix through meso- and micropores (Zhu et al. 2009a). For confidence of complete phosphate adsorption, a contact time of 1 hour was selected for future tests.

Adsorption is affected by the surface charge of adsorbent and the surface charge is influenced by the pH of the solution (Jeon & Yeom 2009). To investigate the influence of pH on the phosphate adsorption, experiments were carried out by adding 1 g of the adsorbent sample into a 100 mL Erlenmeyer flask, containing 50 mL of 25 mg P/L phosphate solution at constant pH. Experiments were performed in binary repeat and together with control solution. The pH was adjusted by adding drops of 0.1 mol/L HCl or NaOH.

The results demonstrate the phosphate removal was evidently independent of pH with the greatest adsorption occurring under a wide pH range (3–10). Only slight decreases can be seen at a pH = 2 which is probably due to formation of weak adsorbent H₃PO₄ as a predominant species in phosphate solution (Namasivayam & Sangeetha 2004). The ligand-exchange mechanism is much less dependent on pH than the electrostatic mechanism; therefore ligand exchange is the dominant mechanism for phosphate sorption in this study (Zhu et al. 2009a). pH is an important factor for some synthetic and nano adsorbents (Liu et al. 2011; Long et al. 2011; Hamdi & Srasra 2012; Su et al. 2015) but the important property of the developed sorbent is its capability to be used for wastewater treatment in various pH conditions. Efficient phosphate adsorption on La-modified tourmaline was achieved at a wide pH range (Li et al. 2015).

It was observed that the final pH value of the solution increased with increasing phosphate adsorption. It can be expressed by ligand exchange with the -OH groups on the hydroxylated surface which indicated that hydroxyl ions were released into the solution after uptake of phosphate onto the surface of the adsorbent (Zhu et al. 2007; Ning et al. 2008; Long et al. 2011).

The high correlation coefficient (R²) of the Langmuir and Freundlich model indicate that the experimental data are fitted by both models. The Freundlich constant n, not only indicates the deviation from linearity of the adsorption (heterogeneity factor), but also indicates the intensity of the adsorption (n < 1 means poor adsorption, from 1 to 2 means moderately difficult adsorption, and n > 2 means good adsorption) (Ma et al. 2012). The obtained n value of the adsorbent was 3.85 indicating a good adsorption between FAB and phosphate.

The Langmuir model behavior in this system establishes the strong electrostatic sorbent-adsorbate attraction forces, and also indicates the monolayer coverage for adsorption of phosphate onto the FAB surface (Ma et al. 2012). The affinity of the adsorbent towards the adsorbate can be estimated by measuring K_L in the Langmuir model (a higher value of K_L means a higher adsorption level at a low concentration) (Xiong & Mahmood 2010). The maximum adsorption capacity of phosphate by the Langmuir isotherm was 8.33 mg P/g.

Values of R_L are found to be between 0.33 and 0.78 for phosphate concentrations in the range of 9–982 mg P/L onto the FAB which indicates that adsorption is favorable.

Table 3 shows the comparison of obtained q_max for FAB and other reported modified bentonites. The results show that the proposed modified bentonite with lower Al/Fe and time consumption has better adsorption efficiency (q_max/(Al/Fe) ratio = 10.5). As well, the main disadvantage of phosphate adsorbents is their pH dependency which changes the adsorption capacity, but in this study pH has a weak effect on phosphate adsorption. This shows the practicability and remarkability of the modification method for bentonite.

The CEC of bentonite was 118 meq/100 g. The chemical composition of the raw and modified bentonite was determined by XRF analysis (Table 4). The results show aluminum and iron silicates are the most prevalent compounds. So the Fe/Al polycation could intercalate bentonite through the cation-exchange reaction and form hydroxyl-Fe/Al-pillared bentonite that can increase the surface area and pore volume of bentonite (Chen & Zhu 2009; Gil et al. 2011).

Phosphate adsorption from urban wastewater by modified bentonite exhibited affinity of removal of more than 99% at the concentration of 4.15 mg P/L. Tian et al. (2009) ascribed the reduction in phosphate adsorption from yellow phosphorus industry wastewater by lanthanum/aluminum pillared montmorillonite to the competitive effect of concomitant anions. Also, the phosphate adsorption onto the adsorbent strongly depends on the initial pH (Tian et al. 2009). All these results suggest that the FAB can be used as an adsorbent for phosphate removal from wastewater.

Results show that phosphate adsorption by raw sorbent = acidthermoactivation < Rewoquate < calcium ≅ Irasoft < Fe < Al ≅ Fe-Al treatment. The Al and Fe activation at the ratio 1.5 and 0.5 CEC, respectively, was more effective. Favorable phosphate removal by this bentonite occurs at pH values from 2 to 10. The phosphate adsorption results were very well fitted in the Freundlich and Langmuir isotherm models and the maximum adsorption capacity (q_max) was 8.33 mg P/g for 1 hour, which was better than similar modified bentonite with low time and Fe-Al consumption. Besides, the modified bentonite is a selective phosphate adsorbent and environmentally friendly for its potential application to phosphate removal from wastewater. It may also be suitable for use as a fertilizer and soil conditioner.

The authors wish to thank Isfahan University of Technology (IUT) Research Council and Center of Excellence in Sensor and Green Chemistry for supporting of this work and Professor H. S. Ghaziaskar and H. Shariatmadari from IUT for kind technical comments.