Preparation of L-Arginine-Modified Silica-Coated Magnetite Nanoparticles for Au ( III ) Adsorption

L-arginine-modified silica-coated magnetite nanoparticles (Fe3O4/SiO2-GPTMS-Arg) have been synthesized by sol-gel process for adsorption of Au(III) ion in aqueous solution. Modification of L-arginine on silica coated magnetite through a coupling agent of 3-glycidoxypropyl-trimethoxysilane (GPTMS) was performed in a various mole ratio of GPTMS:Arg 1:0; 1:1; 1:2 and 1:3. The products of Fe3O4/SiO2-GPTMS-Arg were characterized with XRD, FTIR, EDX, TGA, and Kjeldahl methods. The results showed that based on characterization data Fe3O4/SiO2-GPTMS-Arg has been successfully synthesized with the optimum mole ratio of 1:2. The optimum adsorption of Au(III) occurs at pH 3 and contact time of 60 min. The adsorption capacity followed Langmuir isotherm model was found 0.638 mmol.g-1 for the Fe3O4/SiO2-GPTMS-Arg 1:2. Fe3O4/SiO2-GPTMS-Arg nanoparticles show a potential adsorbent for an effective Au(III) ion removal.


INTRODUCTION
In the era of modern technology today, gold is not only used for fixing, but also for any application such as household appliances, electronics, cosmetics, biosensors and for the treatment of cancer cells 1,2 .In industrial processes and manufacturing various electronic equipment, stages of coating, washing, chemical and mechanical polishing are necessary.At the stage of washing or rinsing waste water containing precious metal and hazardous ions is often generated 3 .Even thought the waste water produced only containing metal ions in low concentrations, those may be harmful to human health and other living creatures.Therefore those are necessary to be managed and removed from the waste water.
A variety of techniques for wastewater treatment and separation of precious metal ions have been reported, such as chemical precipitation 4 , ion exchange 5 , adsorption 6,7 , membrane filtration 8 , and electrochemical 7,9 .Among of the above techniques, adsorption has been widely used since it is proven to be more effective and economical 10 because the design and operation are flexible.The adsorption is a reversible process so that the adsorbent can be regenerated through desorption process for reuse 11 .Efforts to develop the adsorbent containing selective functional groups have been conducted 12,13 , but to recover precious metal ions and removal of heavy metal ion are still challenge.This is due to the capacity and selectivity towards targeted metal ions low and high cost 14 .Therefore, efforts to probe and develop a new adsorbent or method still continue.
The use of organic materials, such as organic polymers, cellulose, algae, and yeast biomass as adsorbent has a weakness, such as swelling, very sensitive to chemicals and lower its mechanical stability.In other hands, support inorganic solids, such as clay, zeolite, metal oxides and silica are preferable due to relatively resistant in term of physicochemistry and good selectivity.Silica gel has attracted the attention of researchers, because of high chemical and mechanical stabilities and low swelling 15 .In addition, the silica containing a lot of silanol groups on the surface makes easy to be modified with the organic functional group.
Modification of silica gel with amine groups has been reported to be highly efficient in removing of Cu(II), Ni(II), Pb(II), Cd(II), Zn(II) and Hg(II) ions in aqueous medium 16,17 .Aminoguanidine groups have been used as a modifier of adsorbent from the tannin extract of persimmon for recovery of Au(III) ion in acidic media 18 .Therefore, ligands with amine functional groups are very effective to adsorb precious metal ions 19 .Modification of the silica gel with the active site of mercapto (-SH) and an amine (NH 2 ) has been reported by Nuryono et al. 20 for Au(III) ion adsorption.The hybrid of amino-silica adsorbed the Au(III) ion in similar amount (98.77%) to that of mercapto-silica (96.67%) at the pH 4. In that condition, the amine groups of the amino-silica hybrid surface are protonated to form cationic groups -NH 3 + and interact with anionic species of Au(III) ion, AuCl 4 -.
The interaction between Au(III) ion with amino-silica hybrid might happen through ionic interactions.While the mercapto-silica hybrid, the S atoms donate the electron pair to Au(III) ions to form complexes using covalent interactions.
Nowadays, magnetic separation is considered as an effective method for the recovery precious metals and removal of heavy metal ions.Magnetite nanoparticles (Fe 3 O 4 ) modified with functional groups such as dendrimer 21 , dimethylamine 22 , dimercaptosuccinic acid 23 , and 3-aminopropyltrietoksisilan 24 has been tested to be capable of removing heavy metal ions due to the high adsorption ability, time is short and separation of the adsorbent from the solution is easy because it uses an external magnetic field.The active site which contains a nitrogen or a sulfur atom is selectively high for the precious metal ion according to the hard soft acid base (HSAB) theory 25 .The magnetite nanoparticles which modified by thiourea (SC(NH 2 ) 2 ) 26 have been reported to be capable of separating and recovering precious metal ions from acid solution.
L-arginine molecules have a flexible chain of guanidine groups with pKa of 12.5. 27Guanidine (-NHCNHNH 2 ) groups with a positive charge in acidic or neutral conditions leads to adsorb AuCl 4 -anion from species of Au(III) ion 18 .In this research, we have prepared L-arginine modified silica coated on magnetite nanoparticles surface using a linker of 3-glycidoxypropyl-trimethoxysilane (GPTMS) in the sol-gel system.The effect of a mole ratio of L-arginine to GPTMS on the chemical stability, the content of nitrogen and the adsorption properties for adsorption of Au(III) ions in aqueous solution has been evaluated.

Preparation of L-Arginine-Modified Silica-Coated Magnetite Nanoparticles (Fe 3 O 4 /SiO 2 -GPTMS-Arg)
In a beaker polyethylene, magnetite 0.4 g was added with 10 mL of distilled water and sonicated for 5 minutes.After that, the suspension was added 12 mL of 2.8 M Na 2 SiO 3 solution and sonicated.After the suspension stirred mechanically for 30 minutes, it was added 1.13 mL of GPTMS and stirred it about 30 minutes.Furthermore, the suspension was added with 5 mmol L-arginine in order the mole ratio of GPTMS to L-arginine 1:1, stirred for 60 minutes and 2 M HCl added dropwise until pH 7 and formed a gel.After the gel left in one night, gel washed with aqua mineral and flushed with ethanol three times, and dried at 65°C in the oven until its weight of constant (See Fig 1).Analogue work was conducted varying the mole ratio of GPTMS to L-arginine according to Table 1.

Characterization
The functional groups in samples were identified with FTIR spectrophotometer (Shimadzu FTIR 8400S) with KBr pellet technique in wave number 400-4000 cm -1 .Powder XRD pattern of samples was compiled on Bruker D8 Advance 206276 (Source used radiation of Cu-Ka (l 0.154 nm) on 40 kV voltage and 30 mA current.X-ray diffraction patterns were scanned at a 2è range of 3-80° in scan speed 5°/minutes.The nitrogen content of samples was counted with energy dispersive X-ray (EDX Carl Zeiss 9 EVO MA 10 series 1454) and Kjeldahl method.TGA was carried out on Mettler Toledo with a rate of heating 20°C per minute, at temperature 40-800°C.Absorption atomic spectrophotometer Analyst 100 Perkin Elmer apparatus was used to define the content of metal ion in solution.Niobium magnet was used for the magnetic separation.The measurement of the pH solution was conducted with a pH meter (pH/ion 510 Eutech Oakton).
The chemical stability was tested by mixing 25 mg of sample with 25 mL HCl solution 1 M and shaking for 3 hours.After that, it was left overnight at a room temperature.The filtrate was divided from the mixture by an external magnetic force and dissolved iron was determined by atomic absorption spectroscopy method 28,29 .

Adsorption of Au(III)
Adsorption of Au(III) on Fe 3 O 4 /SiO 2 -GPTMS-Arg was conducted by the effect of pH, adsorption time and concentration of Au(III) ions.Adsorbent 10 mg was added to 10 mL of Au(III) solution in a batch system of a polyethylene bottle at a room temperature (302 K).The mixture was shaken at 350 rpm for certain time and the adsorbent was separated with an external magnet.The concentration of Au(III) ion not adsorbed in the supernatant was measured by Atomic Absorption Spectrophotometer (AAS).A number of Au(III) ions adsorbed was calculated by the difference between the initial and final concentration of Au(III) in the supernatant using Eq. ( 1) q is Au(III) ions adsorbed (mmol.g - ); C 0 and C e are initial and final concentrations of Au(III) (mmol.L -1 ), respectively.V is volume (L) of the Au(III) solution and W is the adsorbent mass (g).
The pH of medium was controlled by 0.1 M HCl and/or 0.1 M NaOH solution to reach 1-5, the effect of concentration of Au(III) ion was carried on 25-250 mg L -1 and the variation of contact time was observed at the range of 5-180 minutes.The models of Langmuir and Freundlich isotherm were applied to evaluate data of the effects of initial concentration and the models of some kinetics reaction are used to verify the appropriate adsorption rate constant.
The exper iment of desor ption was conducted by using thiourea solution in 1 M HCl-HNO 3 .The firstly, 200 mg of adsorbent was added with 50 mL 100 mg.L -1 Au(III) at optimum pH for 60 minutes.After the mixture was shaken and separated with a magnet, the supernatant was analyzed by AAS to decide the content of Au(III) not absorbed.The adsorbent loaded with Au(III) was washed by aquamineral and dried at 65°C to get the weight of constant and stored in a desiccator overnight before desorption process.The adsorbent loaded with Au(III) was added with 10 mL of H 2 O, and then was shaken for 10 min and supernatant was separated with an external magnet.The supernatant was analyzed by AAS to decide the content of Au(III) that leached with H 2 O.The adsorbent-loaded was added with 10 mL of 0.5 M thiourea solution in 1M HCl.Then the mixture was shaken for 8 h and separated with a magnet, and the Au(III) ion released was measured its concentration.The desorption was repeated three times using the same eluent.Desorption data of Au(III) was calculated with Eq 2 30 : ...( 2)

Characteristic of Fe 3 O 4 /SiO 2 -GPTMS-Arg Functional groups
The FT-IR spectra of silica-coated magnetite nanoparticles with and without L-arginine modification are showed in Fig 2 .All spectra have a peak at 568.96 cm -1 .The peak is ascribed to Fe-O mode of magnetite 31 .Peaks 3446.56 cm -1 and 1645.17cm "1 are associated the O-H stretching and bending vibration, respectively (Fig 2a).The characteristic peak of silica is observed at around of 1100 cm "1 related to the Si-O(Si-O-Si) asymmetric stretching vibration 32 .In all spectra, the peak at 796.55 cm -1 is observed, assigned to symmetric stretching of siloxane Si-O(Si-O-Si), while the band around 460.96 cm -1 corresponds to the Si-O-Si or O-Si-O bending modes 33 .This suggests that silica has been coated on the magnetite surface 34  Peak around 2939.31 cm -1 is appointed to the stretching vibration of C-H asymmetric 32 , from GPTMS and L-arginine.The peak at 1645.17 cm "1 (Fig 2a) is related the O-H (Si-OH) bending vibration (due to condensation of GPTMS), while the peak at 1654.81, 1662.52, and 1656.74cm -1 (Fig 2b-d) are related to the N-H bending vibration 18,35 .It indicates that the coupling agent of GPTMS and modifier of L-arginine have been modified on silica-coated magnetite nanoparticles surface.

Crystalline Structure
The X-ray diffraction patter n of the L-arginine modified silica coating on magnetite nanoparticles surface synthesized with various mole ratios of GPTMS to L-arginine are presented in Fig. 3.The observation of X-ray diffraction pattern for four samples seems characteristic peaks of magnetite confirmed by JCPDS 19-0629 with the index field (220), (311), (400), ( 511) and (440) 36 , and hence it denote that the cubic crystal phase of magnetite is stand after coating with L-arginine-modified silica 37 .The presence of SiO 2 amorphous phase is showed by specific 2q at 22 degrees 38 (Fig. 3b, c, d, and e) according to JCPDS No. 46-1045.

Chemical stability
Magnetite powder is readily dissolved in acidic solution 39 .Silica coating may stabilize magnetite from an acidic medium 34,39,40 .In this work, the magnetite nanoparticles were coated with silica and added a modifier of L-arginine.The samples of Fe 3 O 4 /SiO 2 -GPTMS-Arg obtained were mixed with hydrochloric acid 1 M for 1 day.Iron leached from the sample was determined with AAS.The result showed that the acid solution leached iron from the coated magnetite samples in a range of 0.235-0.344mmol g -1 .It is much lower than the amount of iron leached from uncoated magnetite (1.09 mmol g -1 ).

The content of nitrogen element
L-arginine as a modifier to the silica coated magnetite contains amine groups which may react well with Au(III) in solution.Based on the FTIR spectra in Fig. 2 seem that coated magnetite contains amine group -NH from L-arginine.In addition, we also confirmed the presence of nitrogen element from L-arginine using the EDX and Kjeldahl methods 41 and the result is expressed in Table 4.The content of nitrogen in sample synthesized with a mole ratio of GPTMS to Arginine 1:2 and 1: 3 analyzed with EDX and Kjeldahl methods are relatively similar and higher than in sample synthesized with the ratio 1:1.However, the content of nitrogen based on the theoretical calculation is much greater than result from the experiment.This difference occurs probably since not all arginines are bonded on the surface.The presence of nitrogen in all samples supports the success of modification of arginine on silica coated magnetite.

Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was conducted on four samples of magnetite coated with silica-arginine at temperatures from 40-800°C, with the heating rate of 20°C per minute.Fig. 4 shows that The first stage, degradation occurs at a temperature of 40-100°C, the second stage of 100-250°C, the third stage of 250-500°C, and the fourth stage of 500-800°C.In Fig. 4 is observed that all four samples of coated magnetite lost the weight at a temperature of 40-100 °C (<100 °C).The weight loss is related to the removal of water molecules adsorbed physically, which removal of water molecules is continued at the temperature of 100-220°C 42,43 .Fig. 4 shows that the weight loss in the second stage at that temperature range (100-250°C), for Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:0, 1:1, 1:2, 1:3 is 3.279; 3.384; 4.306 and 4.707%, respectively.
At the temperature range of 250-500°C, TGA curves of Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:0 the weight loss is 9.208%.The weight loss is linked to the decay of the organic part of a silica network, as reported Shajesh et al. 43 .Another reason of the weight loss is a dehydration and dehydroxylation reaction of silanol (vicinal, geminal, and combinations) in the silica 42,44 .Meanwhile, Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:1, 1:2, and 1:3 at the temperature range of 250-500°C loosed the weight of 7.570, 18.549, and 17.577 %, respectively.In that temperature, the decomposition of organic parts 43,45 , including L-arginine bound to the silica network produces CO and CH 4 .At higher temperature than 500°C NH 3 is released.At a temperature range of 250-500 °C show that the percentage of the weight loss is higher for L-argininemodified silica-coated magnetite sample prepared with greater mole ratio.This phenomenon can be explained that smaller size nanoparticles have the larger surface area so that it can bind arginine on the surface in larger number, and the percentage of weight loss is greater 46,47 .At a temperature range of 500-800°C (in the fourth stage), the weight loss is related to the silanol dehydroxylation, especially silanol in geminal position 42 .Thermal decomposition of all sample at this temperature are decreasing, this shows that all the decomposition stopped.In addition, at temperature 500°C, the magnetite is changed into hematite 48 .

Adsorption properties Effect of medium pH on the Au(III) ion adsorption
The experimental result to study the effect of pH on is presented in Fig. 5 showing that for all investigated adsorbents adsorption of Au(III) reaches optimum at pH 3. In the highly acidic condition (at pH 1-2), the adsorption is very low and   at higher pH (above 3), the adsorption decreases.Adsorbents Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:1, 1:2, and 1:3 show relatively similar adsorption capability (61.95, 67.45, and 66.88 mg g -1 , respectively).Fe 3 O 4 / SiO 2 -GPTMS-Arg 1:0 gives very low adsorption capability.It is associated with the active site in the magnetic adsorbent surface, without L-arginine, the probable active site playing a role in adsorption is only -OH of GPTMS.Au(III) in solution at pH 2-3 is dominant as a stable complex ion [AuCl 4 ] -and at increased pH, chloride ion in the complex [AuCl 4 ] -is replaced by OH -ions.At a pH of 4-9 in [AuCl 3 OH] - [AuCl 2 OH 2 ] -[AuClOH 3 ] -are formed and at pH higher than 9 the [AuOH 4 ] -form dominates 49 .In the acidic condition amino groups (NH 2 ) of arginine on silicacoated magnetite nanoparticles surface undergo protonation to generate positively charged groups (-NH 3 + ).Those groups may interacts electrostatically with AuCl 4 -, a species of Au(III) in acidity.As seen in Fig. 5 the adsorption of Au(III) at pH 3 is observed maximum.The interaction between the surface of Fe 3 O 4 /SiO 2 -GPTMS-Arg and Au(III) metal ions in acidic conditions is illustrated hypothetically in Fig. 6.

Adsorption isotherm of Au(III)
The adsorption isotherms of Au(III) on four magnetic adsorbents were tested with the effect of initial concentrations of Au(III) (25-250 mg/L) at pH 3. The adsorption experimental data were evaluated by Langmuir and Freundlich isotherm models 34,50 expressed in mathematical equation (Eq. 3 and 4).where q e and C e are the amount Au(III) adsorbed and concentration of Au(III) at equilibrium, q m is adsorption capacity and K L is the constant of Langmuir relating to the binding sites affinity.The value of q m obtained from slope and K L found from the intercept of a plot of C e /q e vs C e. K F .The constant of Freundlich is related to the adsorption capacity and n is Freundlich exponent linked to the intensity of adsorption.The values of K F and n are obtained from slope and intercept of plot log q e vs log C e .The adsorption process according to Langmuir isotherm is assumed as monolayer and occurs on the sites of the homogenous surface.While the Freundlich isotherm assumed that the adsorption is held on the sites of the heterogeneous surface.

Adsorption kinetics
Study on the kinetics of Au(III) adsorption was conducted by interacting with 50 mg L -1 Au(III) solution at various contact times (5-180 min), pH 3 and room temperature (302 K).The adsorption data is presented in Fig. 8.It is observed that in the first five minutes adsorption occurs rapidly, followed by slow inclining and lastly reached equilibrium after 60 minutes.It was associated with a lot number of sites on the coated magnetite adsorbent surface available for Au(III) in the early reaction and finalized with equilibrium after saturation.
The reaction kinetics models of pseudofirst-order and pseudo-second-order were applied to evaluate the adsorption of Au (III) on the adsorbents, two common models for adsorption of metal ions on a solid adsorbent.The reaction kinetics models of pseudo-first-order and pseudo-second-order are stated in Eq (5) and ( 6), respectively 52,53 .6) q e and q t are the amounts of adsorbed metal ion at equilibrium and anytime, respectively (mmol.g -1 ), and k 1 (min -1 ) and k 2 (g.mmol -1 .min - ) are the rate constants of pseudo-first-order and pseudosecond-order.The rate constants can be determined from the linear curve plot of (ln q e -q t ) against t and t/ q t against t to pseudo-first order and pseudo-second  order reactions, respectively.The result of calculation is presented in Table 6, showing that the adsorption kinetics model of pseudo-second order is more suitable with the higher linear coefficient (R 2 ) value (0.999) than pseudo-first-order one.This shows that the adsorption system involves the exchange of electrons between the magnetic adsorbent and Au(III) ion in aqueous solution 37,53 .

Desorption
The Au(III) ion adsorbed on Fe 3 O 4 /SiO 2 -GPTMS-Arg was recovered by sequence desorption experiments.Desorption of precious metals by using a thiourea solution, HCl, thiourea-HCl, HNO 3 , thiourea-HNO 3 has been reported by previous researchers 22,26,30,54,55 .In this work, desorption of Au(III) adsorbed on Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:2 (25.068 mg/g) was carried out within the different composition of eluent at a room temperature as presented in Table 7.Each fraction (10 mL) of desorption, the concentration of Au(III) desorbed was determined with AAS.As shown in Table 7, desorption using 1 M thiourea-0.5 M HCl gives the highest desorption percentage (75.2997%).However, this result has not satisfied yet and the investigation is still going on to reach 100 % of recovery.

CONCLUSION
In conclusion, L-arginine-modified silicacoated magnetite nanoparticles (Fe 3 O 4 /SiO 2 -GPTMS-Arg) have been successfully synthesized using 3-glycidoxypropyl trimethoxysilane as the coupling agent via a sol-gel process with the optimum mole ratio of GPTMS to arginine of 1:2.Adsorption of Au(III) on Fe 3 O 4 /SiO 2 -GPTMS-Arg 1:2 occurred optimally at pH 3, followed the model of pseudo-second-order with a rate constant of 1.77 g. mmol -1 .min - and fitted to the model of Langmuir isotherm with the capacity of 0.64 mmol.g -1 .The further investigation to find the best desorption technique in which Au(III) may be leached from adsorbent 100 % is still going on, and it is expected that magnetic material produced may be useful for recovery of precious metals from both industrial waste water and precious metal mining samples.
. From Fig 2(b-d) can be seen peaks at 3415.70, 3443.06 and 3411.84 cm -1 ascribing the presence of amino groups.Those peaks are apparent broader than that of Fig 2a due to overlapping with O-H stretching vibration.