Journal of Materials Research and Technology Journal of Materials Research and Technology
Short Communication
Biocomposite application for the phosphate ions removal in aqueous medium
Haq Nawaz Bhattia, Javeria Hayata, Munawar Iqbalb,, , Saima Noreena, Sadia Nawazc
a Environmental and Material Chemistry Laboratory, Department of Chemistry, University of Agriculture, Faisalabad, Pakistan
b Department of Chemistry, The University of Lahore, Lahore, Pakistan
c Department of Microbiology and Immunology, Arabian Gulf University, Manama, Bahran
Received 16 March 2017, Accepted 23 August 2017
Abstract

Mango stone biocomposite efficiency for the removal of phosphate ions (PO43−) from aqueous solution was investigated as a function of pH (2–8), biocomposite dose (0.05–0.40g/100mL solution), contact time (5–120min), initial PO43− ions concentration (20–800mg/L) and temperature (33–61°C). Maximum PO43− ions removal was achieved at pH 2, biocomposite dose 0.3g, contact time 90min and initial PO43− ions concentration 200mg/L. At optimized conditions, up to 95mg/g PO43− adsorption was achieved. Biocomposite pre-treatment with surfactants (SDS, Tween-80, C-TAB, VIM and Surf excel) were also investigated and it was observed that surfactants pre-treatments decreased the adsorption capacity of the biocomposite. Thermodynamic study (ΔG0, ΔH0 and ΔS0) revealed that PO43− adsorption process onto biocomposite was spontaneous and endothermic in nature. Adsorption data fitted well to the Freundlich isotherm and pseudo-second-order kinetic model. Adsorbed PO43− was successfully desorbed using 1.0M NaOH solution. Results revealed that biocomposite adsorbed PO43−, which could possibly be used for the adsorption of PO43− efficiently from wastewater.

Keywords
Biocomposite, Phosphate ions, Modification, Modeling, Desorption
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1Introduction

The waste discharged from agricultural, industrial and domestic domains contain significant amount of PO43− ions which pollute the aquatic environment [1]. The PO43− ions > 2μM in water is harmful and is enough to disturb the natural food chain by stimulating the growth of algae (eutrophication) and also decrease dissolved oxygen (DO), which not only is lethal to aquatic organisms, but also change the quality of water reservoirs [2–4]. Hence, there is a need to develop efficient wastewater treatment technologies that eliminate the micro-pollutants from wastewater and PO43− ions are not easy to remove through the conventional techniques. Different physical, chemical and biological methods have been developed for the treatment of wastewater containing PO43− ions. The biological treatments is difficult to operation because aerobic and anaerobic conditions have to main for efficient treatment of wastewater and osmosis and electro dialysis have been developed for PO43− ions removal which are efficient, but these methods are costly [5]. Also, these methods also generate sludge, which cause secondary pollution. Under the current scenario of pollution [6–20], there is a need to develop green and eco-friendly methods for the remediation of pollutants. Biosorption is regarded as efficient remediation tool and is equally efficient for the removal of organic and inorganic pollutants [3,4,6,13,21–31]. More recently, researchers developed adsorbents, which are cheap, easily available and do not need special maintenance during operational conditions [32]. In this regard, biocomposite based on agro-industrial by-products such as peanut hulls, rice husk, orange peel and cotton sticks are proved to be highly efficient, recyclable and cost effective. Number of studies have been conducted and biocomposites have shown high potential for elimination of metal ions, inorganic ions, dye and other organic compounds from wastewater [33–38].

In view of efficiency of biocomposite for the removal of micro-pollutants, mango stone biocomposite was prepared and used for the removal of PO43− ions. The pH, biocomposite dose, contact time, initial PO43− ions concentration and temperature were optimized for maximum ions removal. Moreover, effect of surfactant pre-treatment of biocomposite on adsorption capacity was also explored. Adsorption data was modeled using different kinetic and isotherms models. Thermodynamic was also computed in order to evaluate the PO43− ions adsorption nature using biocomposite.

2Material and methods2.1Chemical and reagents

The reagents and chemical used were of analytical grade, i.e., potassium dihydrogen orthophosphate (KH2PO4), ferric chloride (FeCl3), potassium borohydride (KBH4), ethanol, sodium hydroxide (NaOH) and hydrochloric acid (HCl) were purchased from Sigma–Aldrich. Ultra-pure water with a resistivity of 18.2cm from Milli-Q system (Millipore) was used for the preparation of solution throughout the study.

2.2Biomass collection

Mango stone was collected from student market, University of Agriculture Faisalabad, Faisalabad, Pakistan. The mango stone extensively washed with water to remove particulate matter and dust and dried in open air followed by oven drying at 60°C until constant weight. The dried mass was grinded (Moulinex, France), sieved (25mm) and used for biocomposite preparation.

2.3Biocomposite preparation

Mango stone powder (10g) was mixed with 250mL of FeCl3, then, KBH4 solution (∼250mL) was added drop wise with constant slow stirring. Here, KBH4 acts as a reducing agent for the conversion of FeCl3 to zero valent iron. After another 30min stirring, the mixture was filtered; residue was extensively washed with ethanol (4 time, to remove un-reacted ions). Finally, the solid mass obtained was dried in oven at 60°C for 24h, grinded and passed through siever of 300μ (OCT-DIGITAL 4527-01).

2.4Biocomposites pre-treatment

The biocomposite was treated with different surfactants i.e., SDS, Tween-80, C-TAB, VIM and Surf excel. All surfactants solution (5%) was prepared and agitated with biocomposite for 1h at 120rpm in orbital shaker at 30°C. After stipulated time period, the biocomposite was washed thoroughly with water and dried in an oven at 60°C overnight [39] and used for adsorption process. Similar adsorption conditions were adopted both un-treated and surfactants pre-treated biocomposites.

2.5Biosorption procedure

Stock solution of KH2PO4 (1000mg/L) was prepared in distilled water and working concentrations were prepared by dilution. The pH of the solution was adjusted using 0.1M NaOH and HCl solution. For batch biosorption experiments, pH 2–8, biocomposite dose 0.05–0.4g/100mL of solution, contact time 5–120min, initial PO43− ions concentration 20–800mg/L and temperature 33–60°C were investigated at fixed shaking speed of 120rpm. Adsorption experiments were conducted in 250mL flask taking 100mL of PO43− ions solution both for native biomass, surfactant pre-treated and un-treated biocomposites. After mixing of adsorbent with PO43− ions solution and pH adjustment, flasks were covered with aluminum foil and set at 120rpm in temperature controlled incubator shaker. After stipulated time period, the adsorbent was separated by filtration and residual PO43− ions concentration was measured spectrophotometrically (CE Cecil 7200, UK) [40]. All experiments were carried out in triplicate, data was averaged and the adsorption capacity (mg/g) was estimated as shown in Eq. (1).

where C0 is the initial PO43− ions concentration (mg/L), Ce is the equilibrium PO43− ions concentration (mg/L), V is the volume of the solution (L) and W is the mass of the biocomposite (g).

2.6Desorption study

Desorption of PO43− ions was carried out using 0.1–1.0M NaOH solution. The loaded biocomposite was mixed with NaOH solution and stirred at 120rpm for 2h. The biocomposite was separated from solution and PO43− ions concentration was measured and desorption percentage was estimated using relation shown in Eq. (2)[29]. Where, qd (%) is the percentage of PO43− ions desorbed. D and R are representing the PO43− ions adsorbed and desorbed, respectively.

2.7Kinetics and isotherms molding

The kinetic models, pseudo-first-order [41] and pseudo-second-order [42] were applied to investigate the reaction rate, mass transport and rate controlling step for the adsorption of PO43− ions on to biocomposite. The equilibrium experimental data was fitted using Langmuir [43] and Freundlich [44] and Harkins-Jurra [45] isotherms.

2.8Statistical analysis

The adsorption experiments were performed in triplicate and data was reported as mean±SD. The regression coefficients (R2) values of isotherms and kinetics models were calculated using statistical functions of Microsoft Excel (version Office XP, Microsoft Corporation, USA).

3Results and discussion3.1Optimization of process variables

The screening study was done between native and biocomposite biosorbent for PO43− ions removal from aqueous solution and results are shown in Fig. 1. Result indicated that biocomposite had possessed high potential for elimination of PO43− ions from wastewater as compared to native biomass. The optimization of process variables for the adsorption of PO43− ions was done for biocomposite due to its higher efficiency. Initial pH plays an important role in biosorption of ions by affecting the charge on the surface of biosorbent, degree of ionization of functional groups and solution chemistry [46]. Removal of PO43− ions using biocomposite was studied at a range of pH 2.0–8.0. The pH profile for PO43− ions removal was studied using dosage of 0.1g, initial PO43− ions concentration of 50mg/L at 33°C for the contact time of 90min and results are shown in Fig. 2A. From results, it was observed that the maximum biosorption capacity (60.1mg/g) was achieved at pH 2 and after increasing pH, reduction in biosorption capacity was observed. The optimum pH was found to be 2. The higher removal of PO43− at lower pH might be due to production of more positive active sites on biosorbent surface, which favors the biosorption because of creating electrostatic interaction among PO43− and biosorbent surface [47]. Kose and Kivanc [48] found the same trend in PO43− ions adsorption process using calcined waste egg shell. Previous studies also showed at the adsorption of PO43− ions on to activated rice husk, fruit juice residue rice husk and fruit juice residue was highly pH dependent and adsorptions of PO43− ions were maximum at pH 6 and by further increasing the pH beyond this values the adsorption capacities of PO43− ions decreased and author correlated this behavior of PO43− ions adsorption with pHPZC[49]. Similarly, La-modified clinoptilolite showed maximum adsorption in the pH range of 5.0–8.0 [50] and authors correlated this behavior with the formation of hydroxides. So far, under alkaline condition (pH>8), the increased hydroxide ions would compete with PO43− ions for the adsorption and resultantly, complexation of PO43− ions suppressed and adsorbent capacity reduced.

Fig. 1.
(0.04MB).

Phosphate ions adsorption comparison of native and biocomposite adsorbents (values are mean±SD of triplicate adsorption experiments).

Fig. 2.
(0.11MB).

(A) Effect of pH (2–8) on the adsorption of phosphate ions on to biocomposite, (B) effect of adsorbent dose (0.05–0.40g/L) on the adsorption of phosphate ions on to biocomposite (values are mean±SD of triplicate adsorption experiments).

The effect of biocomposite dose (0.05–0.4g) on biosorption of PO43− was studied at optimum pH 2, 50mg/L of initial PO43− ions concentration and temperature 33°C. The results are shown in Fig. 2B. Results indicated that the adsorption capacity increased from 40.77 to 78.99mg/g by increasing dose from 0.05 to 0.3g/100mL of solution and beyond this dose, the adsorption did not increase and maximum removal was achieved at 0.3g/100mL of solution. The enhanced adsorption with biocomposite dose revealed the availability of active binding sites for PO43− ions and the reduction beyond certain dose was due to overlapping of biocomposite particles, which reduced the contact between PO43− ions and functional groups [51]. Similar results have been reported previously for the biosorption of PO43− on to mine wastes [52].

Equilibrium time is an important parameter for designing of an economical system for the treatment of wastewater. The effect of contact time on adsorption was studied in the range of 5–120min. The equilibrium was achieved with in 90min at pH 2.0, biocomposite dosage 0.3g, initial PO43− ions concentration 50mg/L and 33°C (Fig. 3A). The PO43− ions adsorption was very fast initially which was due to accessibility of high number of vacant active binding sites. Later, the removal capacity of biosorbent was slowed down due to coverage of active sites and then equilibrium was reached with 90min of contact time and at the time of 80.5mg/g adsorption was observed. After achieving equilibrium, the effect of contact time was insignificant and this trend was in line with already reported studies [53]. Das et al. [54] also found the same trend in PO43− ions removal from aqueous solution using double layered hydroxides. The initial PO43− ions concentration gives information about driving force to reduce the diffusion mass transport resistance between adsorbate and biocomposite [29]. Adsorption capacity of biocomposite for the removal of PO43− ions was studied in the initial concentration range of 20, 40, 60, 80, 100, 150, 200, 300 (mg/L) and results thus obtained are shown in Fig. 3B. Results showed the direct relation between biosorption capacity and initial PO43− ions concentration, the PO43− ions adsorption increased by increasing the concentration of PO43− ions. The maximum removal of PO43− ions was observed at 200mg/L initial concentration. The highest amount of PO43− ions taken up by biocomposite was 95mg/g. The higher PO43− ions uptake might be because of more probability of collision between PO43− ions and biosorbent at higher initial PO43− ions concentration. After coverage of all active binding sites, there was no increment in biosorption capacity was observed by increasing more initial PO43− ions concentration [55]. Similar behavior was observed in a comparative adsorption study of PO43− ions using different agricultural wastes [56].

Fig. 3.
(0.11MB).

(A) Effect of contact time (20–120min) on the adsorption of phosphate ions on to biocomposite, and (B) effect of phosphate ions initial concentration (20–120min) on the adsorption of phosphate ions on to biocomposite (values are mean±SD of triplicate adsorption experiments).

Temperature is critical parameter that describes the feasibility and nature of biosorption process. Fig. 4 shows the removal of PO43− ions from aqueous solution as a function of temperature (33–61°C) using 0.3g of biocomposite, pH 2, initial PO43− ions concentration 50mg/L and shaking speed 120rpm. The optimum temperature was found to be 54°C. Results demonstrated that the biosorption process was endothermic in nature because significant increment in PO43− ions removal was observed as the temperature was increased from 33 to 61°C. The reason behind such increment might be due to more interaction between PO43− ions and biosorbent functional binding active sites because of high kinetic energy at high temperature [57]. Namasivayam and Prathap [58] investigated that the biosorption of PO43− ions by Fe(III)/Cr(III) hydroxide and adsorption was endothermic of PO43− ions.

Fig. 4.
(0.04MB).

Effect of temperature (33–60°C) on the adsorption of phosphate ions on to biocomposite (values are mean±SD of triplicate adsorption experiments).

3.2Effect of surfactants

The effect of surfactants on biosorption capacity of biocomposite for PO43− ions was also investigated, which showed the adsorption capacity decreased with addition of surfactants (CTAB, Tween-80, SDS and two commercial scale detergents vim and Surf Excel). Nonionic surfactant Tween-80 effect decreased the sorption capacity significantly. The results are shown in Fig. 5A. It might be due to the masking of binding sites by interaction with surfactants [59].

Fig. 5.
(0.11MB).

(A) Effect of surfactants pre-treatments on the adsorption of phosphate ions on to biocomposite and (B) desorption of phosphate ions using NaOH (0.2–1.0M) as desorbing agent (values are mean±SD of triplicate adsorption experiments).

3.3Kinetic modeling

The rate of removal of PO43− ions using biocomposite was studied by applying two kinetic models i.e., pseudo-first-order [41] and pseudo-second-order [42]. The linear forms of pseudo-first-order and pseudo-second order kinetic models are given in Eqs. (3) and (4).

The qe and qt represent the value of biosorption capacity at equilibrium and at time t, respectively. The rate constant of pseudo-first-order is represented by k1 (min−1). It was calculated from slope by plotting the log (qeqt) against t.

where qe (mg/g) is the biosorption capacity at equilibrium, while qt (mg/g) is the biosorption capacity at time t. The k2 (g/mgmin) is the pseudo-second-order rate constant. k2 was be calculated from the intercept by plotting t/qt against t. The values of kinetic parameters are given in Table 1. The pseudo-second-order kinetic model was found to be well fitted to the PO43− ions adsorption data. The R2, experimental and calculated biosorption capacity values were more reliable in case of pseudo-second order kinetic model as compared to pseudo-first order kinetic model. So far, the PO43− ions followed second-order kinetic model since the qe value determined experimentally was in agreement with the calculated value. Moreover, the R2 value was also higher in case of second-order kinetic model.

Table 1.

Comparison of rate constants and coefficient of correlation for pseudo-first-order and pseudo-second-order kinetic model.

Pseudo first order kineticExperimental  Pseudo second order kinetic
ModelValue  Model
qe (mg/g)  K1 (1/min)  R2  qe (mg/g)  qe (mg/g)  K2 (1/min)  R2 
5.186  0.0357  0.803  80.5  78.56  0.17  0.98 
3.4Equilibrium modeling

The equilibrium experimental data was fitted into linear forms of three isotherms namely Langmuir, Freundlich and Harkins-Jurra. The mathematical form of Langmuir isotherm [43] is shown in Eq. (5):

where Ce (mol/L) is the PO43− ions concentration at equilibrium, while qe (mg/g) is the biosorption capacity. b is the biosorption binding energy and qmax (mg/g) is monolayer biosorption capacity.

The Freundlich isotherm relation is shown in Eq. (6)[44].

where KF (mg/g) and 1/n are the Freundlich constants and these represented the biosorption capacity and biosorption intensity, respectively, which were calculated from intercept and slope by plotting the graph between log qe and logCe. qe (mg/g) is the amount of PO43− ions biosorbed per unit biocomposite.

The Harkins-Jurra isotherm linear form is shown in Eq. (7)[45].

where A and B are the Harkins-Jurra isotherm constants. The calculated values of PO43− ions adsorption on to biocomposite for different isotherms are given in Table 2. Among three isotherms, Freundlich showed the best fitness which explained the formation of multilayer of PO43− ions on the surface of biocomposite. The R2 values in case of Freundlich isotherm was considerably higher versus other two isotherms. The qe value determined experimentally was also in agreement with calculated qe value in case of Freundlich isotherm. So far, the Freundlich isotherm showed best fitness for the adsorption of PO43− ions on to the biocomposite. The Freundlich isotherm describes the sorption of PO43− ions was occurred on heterogeneous surfaces and multilayer adsorption was favorable. It assumes that the uptake of adsorbate ion occurs on a heterogeneous adsorbent surface. The Freundlich isotherm also describes an empirical relation that the stronger binding sites were occupied first and binding strength decreased by increasing the degree of binding sites occupation [31].

Table 2.

Langmuir, Freundlich and Harkins-Jura isotherm parameters for phosphate uptake by biocomposite.

LangmuirExp. value  FreundlichHarkins-Jura
q(max) mg/g  B  R2  q(max) mg/g  n  K  R2  A  B  R2 
28.901  0.08258  0.955  95  0.51  91  0.987  25.90  401.65  0.761 
3.5Thermodynamic study

Temperature effect on removal of PO43− ions using biocomposite was studied in the temperature range of 306–334K and changes in standard enthalpy (ΔH°), standard entropy (ΔS°), and Gibbs free energy (ΔG°) were calculated as shown in relations (8) and (9).

where Kd is the coefficient of distribution and it was calculated using the expression Kd=qe/Ce. “T” represents the temperature in Kelvin (K) and R is the universal gas constant. The value of ΔH° (kJ/mol) and ΔS° (kJ/molK) were calculated from slope and intercept by plotting the lnKd against 1/T. From thermodynamic study, it was observed that the process for the removal of PO43− ions using biocomposite was endothermic in nature. The high value of entropy indicating the ordering of system as biosorption process was proceeded. The value of ΔG° (kJ/mol) represented the degree of non-spontaneity of biosorption process and it was reflected energetically favorable because its value was increased with rise of temperature (Table 3).

Table 3.

Thermodynamic parameters for the adsorption of phosphate ions onto biocomposite.

Temperature (K)  ΔG° (kJ/mol)  ΔH° (kJ/mol)  ΔS° (kJ/molK) 
306  −3.7239  45.238  0.13627 
313  −2.4480     
320  −1.4388     
327  −0.6681     
334  0.129     
3.6Desorption study

The biomass used in biosorption should be reused to make the process attractive, efficient and economical. For desorption, selection of efficient eluent is important. Desorption depends on the biosorption process, biosorbate and biosorbent can be recovered and regenerated, respectively. Since the adsorption was efficient at low pH value, so the desorption was studied using basic solution. NaOH (0.2–1.0M) was used for the desorption of PO43− ions [60] from loaded biocomposite. It was noted that PO43− ions was desorbed efficiently at higher concentration of NaOH (Fig. 5B). Desorption of PO43− ions was achieved up to 82% using 1.0M NaOH. Overall, the biocomposite showed promising efficiency for the adsorption and desorption of PO43− ions and could possibly be used for the removal of PO43− ions from industrial wastewater.

4Conclusions

Biocomposite was prepared from mango stone and used for the removal of PO43− ions from aqueous solution. The process variable such as pH (2–8), biocomposite dose (0.05–0.40g/100mL solution), contact time (5–120min), initial PO43− ions concentration (20–800mg/L) and temperature (33–61°C) were optimized for maximum PO43− ions adsorption. The PO43− ions adsorption was achieved up to 95mg/g at pH 2, biocomposite dose 0.3g, contact time 90min and initial PO43− ions concentration 200mg/L. The biocomposite pre-treatments with surfactants (SDS, Tween-80, C-TAB, VIM and Surf excel) decreased the adsorption capacity of the biocomposite. Adsorption data fitted well to the Freundlich isotherm and pseudo-second-order kinetic model. The thermodynamic study revealed that the PO43− ions adsorption process was spontaneous and endothermic in nature. Biocomposite showed promising adsorption efficiency, which could possibly be used for the adsorption of PO43− ions from wastewater and this biocomposite is also extendable to other inorganic ions sequestration from wastewater.

Conflicts of interest

The authors declare no conflicts of interest.

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Corresponding author. (Munawar Iqbal bosalvee@yahoo.com)
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