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

Raw and modi ﬁ ed bentonite has been used to develop effective sorbents to remove phosphate from aqueous solution. Acid thermoactivation, Rewoquate, Irasoft, calcium, Fe and Al were employed to treat the bentonite. Results show that samples adsorption capacity for phosphate is in the order of, unmodi ﬁ ed bentonite ¼ acid thermoactivation < Rewoquate < calcium ≅ Irasoft < Fe < Al ≅ Fe-Al. The phosphate adsorption with Fe-Al-bentonite (FAB) modi ﬁ cation was more than 99% and the phosphate removal reached the peak value in the initial 30 min. The phosphate adsorption of FAB was pH independent in the range of 2 – 10. The common coexisting ions in wastewater have no effect on the phosphate adsorption. The phosphate adsorption results were very well ﬁ tted in the Freundlich and Langmuir isotherm model and the maximum adsorption capacity was 8.33 mg P/g at pH 6.5 for 1 hour, which was better than similar modi ﬁ ed bentonite with low time and Fe-Al consumption. FAB was characterized by scanning electron microscopy, X-ray diffraction, X-ray ﬂ uorescence and Fourier transform infrared. Therefore, the results con ﬁ rm that FAB is a selective phosphate sorbent and environmentally friendly for its potential application for phosphate removal from wastewater.


INTRODUCTION
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 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. ). 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 ( 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.

Materials
The bentonite used for this study was purchased from the Semnan-Iran bentonite mine and NH 3 , FeCl 3 .6H 2 O, AlCl 3 .6H 2 O, CaCl 2 and KH 2 PO 4 , 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).

Preparation of pillared bentonites by Fe/Al and Ca
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) (1): where C i and C f are the blank and final concentrations of phosphate in the sample solution (mg/L) after adsorption, respectively.

Selectivity of phosphate adsorption
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 W C and the residual concentration of phosphate was analyzed after centrifugation.

Phosphate adsorption from wastewater
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).

RESULTS AND DISCUSSION
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.    Al and Fe hydroxide can be formed in alkaline solutions.

Comparison of different methods for modification
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.

Effect of chemical and physical parameters on phosphate adsorption
Ultimately an adsorbent was prepared under these con- For confidence of complete phosphate adsorption, a contact time of 1 hour was selected for future tests. 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 ¼ 2 which is probably due to formation of weak adsorbent H 3 PO 4 as a predominant species in phosphate solution (Namasivayam & Sangeetha ). The ligandexchange mechanism is much less dependent on pH than the electrostatic mechanism; therefore ligand exchange is the dominant mechanism for phosphate sorption in this study

Adsorption isotherm
Freundlich and Langmuir adsorption isotherm models were applied to study the adsorption capacity of FAB for the phosphate removal, at the optimum conditions, from water. The most important multi-site sorption isotherm for heterogeneous surfaces is the Freundlich isotherm (Equation (2)) (Triantafyllidis et al. ).  (Figure 4(a)).
The Langmuir isotherm is expressed by Equation (3) C e q e ¼ 1 where C e is the concentration of phosphate in a solution (mg/L) at equilibrium, q max is the Langmuir constant related to adsorption capacity (mg/g), and K L is a constant related to phosphate binding energy on an adsorbent (L/mg)  ). The linear relationship between C e /q e vs C e shows the adsorption mechanism is obeyed from the Langmuir isotherm (Figure 4(b)). Isotherm constants for phosphate adsorption are reported in Table 2.
where b is the Langmuir constant and C 0 is the initial phosphate concentration (mg/L). The value of R L indicates the type of isotherm to be unfavorable (R L > 1), linear (R L ¼ 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.   Deviation q max to ratio Fe/Al (column 3/2).
that the proposed modified bentonite with lower Al/Fe and time consumption has better adsorption efficiency (q max / (Al/Fe) ratio ¼ 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.

Characterization of raw and modified adsorbent
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)

Effect of coexisting anions
The effect of common coexisting ions in wastewater such as chloride, sulfate, nitrate and bicarbonate in two

CONCLUSIONS
Results show that phosphate adsorption by raw sorbent ¼ 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.