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Adsorption of Gold onto I3-aminopropyltriethoxysilane Grafted Coconut Pith.

Byline: Muhammad Usman, Javaid Akhtar and Javed Iqbal

Summary: This study was carried out to investigate adsorption kinetic and adsorption thermodynamics of Au(III) ions onto I3-aminopropyltriethoxysilane grafted coconut pith. The results from equilibrium adsorption were fitted in various adsorption isotherm models such as Langmuir, Freundlich, Temkin and Dubinin-Radushkevich and the best fit for the experimental data was Langmuir isotherm. The maximum adsorption capacity for virgin coconut pith (VCP) and the grafted coconut pith (GCP) were 256.41 and 285.59 mg/g, respectively. The kinetic data was verified using pseudo-first-order, pseudo-second-order, elovich equation and intraparticle diffusion model. The correlation results suggested that the pseudo-second-order model fits the experimental data well.

A thermodynamic study revealed the endothermic nature of reaction due to positive enthalpy (IHo) values and negative values of Gibbs free energy (IGo) describes the spontaneity of adsorption process. The regenerability of VCP and GCP adsorbents were investigated with NaOH (1.0 M).

Keywords: Coconut pith; Isotherm models; Thermodynamics; Kinetic models; Gold.

Introduction

Gold belonging to precious metal group has certain characteristics such as: high electrical conductivity, chemical stability and excellent resistance to corrosion, which makes it very attractive for its application in various industries; moreover, it has been extensively used in jewelry and also in medical equipments [1]. Therefore, in recent years, there has been an increase in gold concentrations from various sources such as: wastewater streams of various industries like mining [2, 3], ore and metal processing [4], electrical and electronic manufacturing [5, 6] and e-waste recycling [7, 8]. The major contribution in the electrical and electronic waste (e-waste) is from printed circuit board (PCB) containing several precious metals (i.e., gold, silver and palladium) as exhibited in Table-1 [9]. Billions of people round the globe are using cellular phones as fast communication device.

The life span of these devices is decreasing due to the most users upgrade to new technology, design and style. In 2005, it was estimated that 500 million mobile phones weighing 250,000t were ready for disposal [10].

Table-1: Analysis of metals in PCB [9].

###Intrinsic value of PCB

###Component###Wt.%###valuea (APS/kg)

###(APS/kg of PCB)###%

###Gold###0.025###142000.00###3.55###65.4

###Palladium###0.010###6200.00###0.62###11.4

###Silver###0.100###250.00###0.25###4.6

###Copper###16.0###3.30###0.53###9.7

###Aluminum###5.0###1.20###0.06###1.1

###Iron###5.0###0.10###0.01###0.1

###Tin###3.0###8.10###0.24###4.5

###Lead###2.0###1.30###0.03###0.5

###Nickel###1.0###13.20###0.13###2.4

###Zinc###1.0###1.20###0.01###0.2

Thus, there is an urgent need of recycling waste mobile phones to protect the environment and resource conservation [11]. It is indicated that e-waste contains large amounts of precious metals from their respective ores and can be considered as a secondary source of these valuable metals [12,13]. However, it requires an efficient system to recover, reuse and recycle gold due to its limited and continuously depleting sources [14]. There are various technologies engaged for the gold recovery such as: hydrometallurgical (ion exchange resin [15], solvent extraction [16] and precipitation [17]) and pyro-metallurgical [7] (incineration and smelting in a furnace), however, the application of these processes has some drawback, which are high cost, labor and time extensive. Therefore, these processes were not seen as efficient processes for metal recovery and the search continues for a process that can replace these techniques and should be effective, economical and environmentally benign.

Biosorption process in seen as an alternative process in which biomass based adsorbents is applied for the separation and recovery of precious metals. This process draws attention due cost and simple operation [18-20]. An industrial crop produces huge quantities of cheap material during their reaping and processing of food crops [21]. A tragic situation rises as most of the lignocelluloses are eliminated by burning, which is even banned in developing areas and considered a threat to environment [22]. Different types of adsorbents have been used for Au(III) adsorption, including chemically modified chitosan [23], lignophenol gel [24], rice husk carbon [25], alfalfa [26], egg shell membrane [27], sulfur derivatives of chitosan [28], calcium alginate beads [29], dealginated seaweed waste [30] and immobilized fungal biomass [31].

However, the emphasize in to utilize low-cost and naturally abundant materials for the recovery of gold from its source i.e., electronic and electroplating factories wastewater [1].

In this study, an organosilane, I3-aminopropyltriethoxysilane (I3-APS) with the chemical formula (NH2(CH2)3Si(OC2H5)3) was applied as coupling agent with the general formula R-Si-X3, where R denotes the organic moiety and X denotes an oxyalkyl group [32]. The modification effect of I3-aminopropyltriethoxysilane (I3-APS) on coconut pith was investigated for the uptake of gold. The kinetic and isotherm models were applied on experimental data to elaborate the adsorption mechanism. The regenerability study was conducted by suitable eluting agent (NaOH) to enhance reusability and attractiveness of the adsorption process.

Experimental

Materials and Reagents

The biomass (coconut pith) used in the experiment was purchased from the TandH sdn. Bhd, Johor Malaysia. The sodium hydroxide (NaOH pellets) used in desorption studies was acquired from Merck and nitric acid (HNO3) was bought from sigma-Aldrich. Ethanol and toluene were purchased from sigma Aldrich. Silane coupling agent (I3-APS) was purchased from Power Chemical Corporation (China). Gold (III) chloride (AuCl3) of purity 99% was purchased from sigma-Aldrich. All the chemicals were analytical grade, except I3-APS, which was industrial grade.

Preparation of Adsorbent

Raw coconut pith was first grounded, sieved to obtain the desired particle size (0.075-0.150 mm) and later on washed to remove coloration and dirt particles. In this study, dewaxing was used as a pretreatment process to remove low molecular weight substance such as: waxes, therpenes and other impurities from coconut pith. The coconut pith was soxhelt-extracted with toluene-ethanol mixture (2:1 v/v) for 48 hours and dried in an oven at 50 AdegC for overnight and regarded as virgin coconut pith (VCP). Moreover, chemical modification of VCP was executed by grafting I3-APS (0.1 M) solution. In the first step, a miscible mixture of ethanol and water (80:20) was stirred and then I3-APS (alkaline pH) was added to the above prepared mixture, which will lead to the hydrolysis of silane. After some period of time, VCP (0.5 g) was added to be modified by I3-APS. The mixture was stirred for 2 hours (500 rpm) followed by drying at 60 AdegC for overnight and regarded as grafted coconut pith (GCP).

Adsorption Experiments

Batch adsorption experiments were conducted by placing adsorbent (0.05 g) in 100 mL flask containing 50 mL (10 to 500 mg/L) of Au(III) at pH 4.00. The bottles were shaken at 200 rpm for 2 days using a mechanical shaker to achieve equilibrium. The mixture was filtered by nylon syringe filter 0.80 um to remove adsorbent particles. After filtration, the concentration of Au(III) was determined by atomic absorption spectrophotometer (AAS) (Model Perkin-Elmer precisely HGA 900). The amount of Au(III) adsorbed at equilibrium qe (mg/g) was calculated by equation 1:

(Equation)

Where Co and Ce (mg/L) are the initial and the equilibrium concentrations of Au (III). V is the volume of gold solution (L) and m is the mass of adsorbent used (g). Kinetic studies were done by mixing 0.05 g of VCP and GCP with 50 mL of Au(III) solution (50 mg/L, pH = 4.00) and taking samples at different time intervals (1-2880 min). The amount of Au(III) adsorbed at time t, qt (mg/g) was calculated by equation 2:

(Equation)

Where Co and Ct are the initial and final concentration of Au (III) at time t, respectively. Three batch adsorption cycles were conducted to study durability and regenerability of the adsorbents. This was performed by mixing adsorbents (0.250 g) with 250 mL (250 mg/L) of Au(III) solution. The spent adsorbents (VCP and GCP) were filtered and rinsed with deionized water to remove traces of gold and dried in an oven at 50 AdegC for overnight. After that, washing was performed with eluting agent, NaOH (1.0M). The mixture was shaken at 200 rpm for 2 days, followed by filtering to separate adsorbent from the alkali solution. The adsorbent was dried in an oven at 50 AdegC and executed again an adsorption cycle. The experimental scheme for the preparation of adsorbent and the adsorption experiments for the uptake of Au(III) ions is shown in Fig. 1.

Results and Discussion

Adsorption Isotherm

The study on adsorption isotherm describes the relation between the adsorbate concentration in the bulk and the adsorbed amount on the interface [33]. The isotherm study is conducted by fitting experimental data to different isotherm models (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich) and selecting the suitable model for design purposes [34]. The Langmuir adsorption model considers the various phenomenon such as: adsorption is occurring on a monolayer alone, all binding sites are similar and can adsorb only one atom per active site, and the rate of adsorption of one molecule to a binding site have no effect from its adjacent site occupancy [34-36]. The mathematical expression of Langmuir model is described by equation 3:

(Equation)

Where qe (mg/g) and Ce (mg/L) is the equilibrium metal concentration on the adsorbent and in the solution, respectively. While, qmax is the maximum amount of the metal adsorbed per gram of adsorbent (mg/g) and KL (constant) is solely related to the affinity of the binding sites (L/mg). Langmuir equation in the linearized form is described by equation 4:

(Equation)

Where Co is the highest initial metal concentration of adsorbate (mg/L) and KL (L/mg) is Langmuir constant. RL indicates the shape of isotherm to be either unfavorable (RL > 1), favorable (0 < RL 0.99), whereas, the low correlation coefficients (R2 0.8 then the below equation 15 is applied.

Bt = - 0.04977 - ln (1 - F) ----------(15)

And if the value of F < 0.8 then the below equation 16 is applied

(Equation)

The slop (B) of the plot (Bt vs. t) indicates pore diffusion coefficient (D2) from the equation 17.

(Equation)

The values of D2 and R2 for VCP and GCP adsorbents are listed in Table-4. The comparison of film diffusion (D1) and pore diffusion (D2) coefficients indicates that pore diffusion is faster than the film diffusion coefficients and similar results were reported by Johari et al., [58] and Onal et al., [59]. Furthermore, chemical structure of I3-APS and the modification effect of this aminofunctional silane coupling agent on the surface of coconut pith and adsorption of Au(III) ions is shown in Fig. 5.

Adsorption thermodynamics

The study of the influence of temperature on adsorption of Au(III) indicated an increase of adsorption capacity with the rise of temperature, which indicates endothermic nature of Au (III) adsorption on VCP and GCP adsorbents (Fig. 6). Temperature played a key role to enhance surface activity of the active binding sites and kinetic energy of solute [60].

The set of thermodynamic parameters, such as, Gibbs free energy (IGo), enthalpy (IHo) and entropy (ISo) were determined by the following Equations at different temperatures [34,55]:

(Equations)

Where thermodynamic parameters (IHo and ISo) were determined from the slope and the intercept of the linear plot (lnqe/Ce versus 1/T) (Fig. 7) and then values of IGo were calculated. Therefore, negative values of IGo describes the spontaneity of Au(III) adsorption process. However, positive values of IHo and ISo suggested the endothermic nature of the reaction and increased randomness at the solid/solution interface for Au(III) adsorption (Table-5) [34,61].

The higher entropy values for GCP adsorbent compared to VCP depicted the higher randomness of modified adsorbent.

Table-5: Thermodynamic data for VCP and GCP adsorbents.

###IH###IS###I-G (kJ/mole)

###(kJ/mole)###(kJ/mole.K)

###303###313###323 K###333

###K###K###K

VCP###0.02059###0.07361###22.28###23.01###23.75###24.49

GCP###0.07252###0.25814###78.10###80.68###83.26###85.84

The reusability of the adsorbents is an important feature to make the adsorption process attractive and economical. In the present study, adsorption-desorption batch operations were conducted to exam the reusability of adsorbents for Au (III) ions. The results indicated that both adsorbents could be repeatedly recycled for 3-4 cycles with only a slight decrease in their initial adsorption capacities (Fig. 8). Moreover, modified adsorbent (GCP) exhibited a higher performance in its reusability compared to VCP adsorbent.

Conclusion

In this work, coconut pith adsorbents exhibited an excellent affinity for Au(III) ions, especially modified coconut pith (GCP), which showed higher uptake capacity and reusability. The characterization of the adsorbents before and after adsorption cycles explained the morphology and structure of adsorbents in detail. The experimental data was well fitted by Langmuir isotherm, giving exhibiting a maximum adsorption capacity of 285.59 mg/g for GCP at 30 AdegC. Adsorption kinetics was well described by the pseudo-second order model. A thermodynamic study revealed the endothermic nature of Au (III) adsorption on VCP and GCP, which fitted well with the temperature study. Regenerability study for VCP and GCP showed a little loss in uptake capacity even after three cycles.

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Author:Usman, Muhammad; Akhtar, Javaid; Iqbal, Javed
Publication:Journal of the Chemical Society of Pakistan
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Date:Dec 31, 2017
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