Specific Adsorption of Aspartic Acid on Iron (III) and Nickel (II) Oxides

The dependences of zeta – potential and adsorption of aspartic acid on NiO and Fe 2 O 3 oxides were investigated as a function of aspartic acid concentration, pH and adsorption time using microelectrophoresis and potentiometric titration methods. Shifting of (cid:618)(cid:583) IEP (the isoelectric point or IEP) towards the acid zone for Fe 2 O 3 and towards the base zone for NiO in the solutions of aspartic acid in comparison with HCl shows the presence of specific adsorption of aspartic acid anion and cation forms on the surfaces of Fe 2 O 3 and NiO oxides, respectively. The complexation process of Fe( (cid:594) ) ions with aspartic acid in the bulk solution and on the oxide surface was examined by spectrophotometer. It was determined that Fe( (cid:594) ) ions form complexes with aspartic acid in the bulk solution.


Introduction
Currently, a lot of investigations have been devoted to the study of colloid-chemical mechanisms of the process run in the dispersion systems containing biologically active substances (BAS). It is caused by the significant role which such systems play in the vital activity of plant and animal organisms. BAS are widely used in such spheres as medicine, pharmacology, agriculture and food industry. Study of interactions in the given systems contributes greatly to the development of the theory of the BAS specific adsorption on the solid surfaces [1][2][3][4]. One of the most important BAS are proteins. Proteins are complex natural polymers containing different aminoacids, which determine the character of proteins interactions with a surface.Therefore, the study of the specific adsorption mechanism of the primary aminoacids on the model systems depending on their character and acid-base properties is of vital interest.
Metal oxides can be used as model systems because their electrosurface properties within the aqueous solutions of simple electrolytes are well known.
The main interest has Fe 2 O 3 oxide as iron is a part of blood cells therefore it is called "life metal". Another major problem arises due to removing toxic compounds from the organisms, such as nickel and its oxides. For example interaction of nickel with aminoacids leads to severe allergic reactions in the humane organism.
The aim of the given study is the experimental investigation of the specific adsorption of the primary BAS: aspartic acid, glycine, asparagines on the surface of nickel (II) and iron (III) oxides depending on the composition of the aqueous phase (concentration of aminoacids, ) and a time of adsorption.
Despite many works studying disperse systems containing BAS its mechanism of interactions with a solid surface is still not clarified. In this work electrokinetic and adsorptive properties of iron (III) and nickel (II) oxides in aspartic acid aqueous solutions are studied. Also complexation processes in the system aspartic acid (AA) + Fe 3+ are under investigation.

Experimental
All the chemicals and other reagents used during the study were of analytical grade. For preparation of solutions and experiments deionized water was used. Powders of NiO, Fe 2 O 3 oxides and aqueous solutions of aminoacids: glycine, and L -aspartic acid and simple electrolytes (HCl, KOH, KCl) were used as the objects. L -aspartic acid concentrations were used such as 1 10 -2 , 5 10 -3 , 1 10 -3 , 5 10 -4 , 1 10 -4 , 10 -5 , 1 10 -5 mol/l. Concentrated L-aspartic acid solutions were prepared using the exact weight and more diluted solutions were made by the progressive dilution method. pH was changed by adding HCl or KOH, respectively. The following pH values in aqueous L -aspartic acid solutions were used 3,0; 3,5; 4,0; 4,5; 5,0; 5,5; 6,0. Concentrations prepared were checked using electroconductivity meter and titration methods.
Adsorption of the aminoacids was determined according to change of their concentrations before and after adsorption. Concentration of aminoacids was measured using titration with alkali in the presence of phenolphthalein indicator; for the study of complexation -method of isomolecular series and spectrophotometry were used. Investigation of the oxide adsorption properties was carried out using powders with the particles 0.5-10.0 micrometers. Specific surfaces were determined using BET method and were 3.3 m 2 /g and 2.8 m 2 /g for NiO and Fe 2 O 3 , respectively.
NiO and Fe 2 O 3 are crystal polydispersed powders with particle size from 0.1 m till 40.0 m. In order to avoid surface contaminants the oxides were rinsed by plenty of distilled water till electroconductivity and pH of the rinsing water remained constant and then the oxides were dried in an oven at 105 .
Electrokinetic potential, zeta-potential ( ) was measured using method of microelectrophoresis. The radius of the used particles were ranging from 0.5 m till 1.0 m obtained by fractional sedimentation. Zeta-potential values were calculated according to Helmholtz-Smoluchowski equation with boundary condition such as ae× >>1, where ae is an inverse thickness of the double electric layer (DEL), is a radius of particles.
Adsorptive and electrokinetic properties of NiO and Fe 2 O 3 were studied as function of concentration and pH of aminoacids solutions and adsorption time. To show a role of aspartic acid, electrokinetic experiments were carried out for NiO and Fe 2 O 3 in solutions of simple electrolytes such as HCl, KOH, KCl.
The complexation processes of Fe 3+ ions with aspartic acid were investigated using spectrophotometry. Spectrophotometric investigations were carried out using spectrophotometer CF-26 produced by LOMO. Dependences of optical density on the length of transmitted light wave within the range = 200-600 nm were measured in the quartz cuvettes with thickness 10 mm for aspartic acid pure solutions, solutions of FeCl 3 and compositions of AA+FeCl 3 at different concentrations of AA and electrolytes. Spectra of the systems under investigations were compared with initial ones. Shifting of spectral peaks indicated the presence of complexation.

Results and Discussions
Presence in the aminoacids two different groups such as acid (carboxyl, -COO -) and base (amino, 2 NH ) groups leads to their acid-base properties and their ability of anion or cation forms sorption. That is why above mentioned aminoacids were selected for the present work.
In aqueous solutions of aminoacids different physical (aggregation) and chemical (chemical condensation) processes take place. These processes can influence on the adsorption equilibrium. Because of that the electrokinetic potential of Fe 2 O 3 (NiO) in the aqueous solutions of AA was measured as a function of an adsorption time (t c ). On the basis of the obtained re sults time t c = 24 hours which corresponds = f(t c ) is constant was selected. In Fig. 1 zeta-potential of Fe 2 O 3 versus pH at different AA concentrations is shown. Shift of pH iep towards the acid zone in the presence of the aminoacid (Fig. 1, curve 2, 3) compare with pH iep in the aqueous solutions of HCl (Fig. 1, curve 4) is presented. This fact tells about a specific sorption of AA anion form. Aspartic acid is a weaker acid than HCl (Table 1), the concentration of H + ions is less, consequently H + ions adsorption is less and zeta-potential is less.  Zeta-potential value for Fe 2 O 3 can decrease because of the domination of specific adsorptionwith a positive charged surface and due to specific interactions with a formation of surface complexes. Specific adsorption of aminoacids is possible due to donor-acceptor bonds formation between a metal atom on the oxide surface and ligand groups of the aminoacid molecules. The ligand groups are such as: -NH 2 , -COOH, -COO -( Table 2). Accordinganion forms of AA. Anion forms of AA can be adsorbed on Fe 2 O 3 due to electrostatic interactions to the literature [5,6] -NH 2 i s a stronge r lig and th an -COOH a n d -C O Ogroups. Analysing dependences Fig. 1 we take into account AA ion composition in aqueous solutions at working pH. The data are presented in Table 2. It is know from the literature that aspartic acid can exist in two anion forms: Both anions can be adsorbed specifically due to donor-acceptor bonds formation with iron crystal net. The anion I can form a donor-acceptor bond using the oxygen atom of the carboxyl group while the anion II can do this using the nitrogen atom of the amino group and the oxygen atom of the carboxyl group. It should be noted that in the anion II nitrogen bonds of 3 NH group are completely saturated and the amino group can not be a ligand. Despite that the probability of AA cation specific adsorption on the carboxyl group should be estimated. The role of this cation on adsorption can not be significant as it is a counter ion and if this cation was adsorbed specifically zeta-potential values of Fe 2 O 3 would have been higher in the AA solutions than in HCl solutions. This fact was not proved by the experiments.
According Table 2 anion I concentration increases from 11% till 99%. at different pH (3 ÷ 6). In case of spe cific adsorption of the anion I, ze tapotential values should increase towards the base zone in Fig. 1. At such high concentrations of specifically adsorbed anion I at pH>4 the oxide surface should be charged negatively. However from Fig. 1 values of zeta-potential do not change accordingly above estimations. In addition according to the experimental data obtained and above estimations of the ligands no the cation nor the zwitter ion can not adsorb specifically. In conclusion interactions of the anion I with the positive charge iron oxide surface are due to Coulomb forces and the anion I is an opposite charge ion.
According to the above conclusions that an anion is adsorbed and from both anions the behaviour of the anion I can not explain the fact of specific adsorption we can conclude that the anion II is responsible for the specific character of AA adsorption on the surface of iron oxide. The donor acceptor bond can be fixed on the amino group. Increase of IEP with increase of the aminoacid concentration is in good agreement with the above conception. Fig. 2 shows that in all cases zeta-potential values of Fe 2 O 3 in AA solutions (curve 1) are positive and less than in the HCl solutions (curve 2). Influence of H + ions adsorption on zeta-potential at different AA concentrations is presented in curve 3. Values of zeta-potential at AA concentrations C AA =1·10 -2 10 -4 mol/l in the curve 1 are less than in the curve 3, which can be explained by the leading role of AA anions in the specific adsorption with increase of AA concentrations. In Fig. 3 dependences = f( ) for the system NiO -aqueous AA solutions are shown. Shift of pH IEP in the presence of the aminoacid compare with pH IEP in HCl solutions has a direction from acid zone towards base zone. It is necessary to point out that for NiO shift of pH IEP compare with Fe 2 O 3 is in an opposite direction. It testifies a specific adsorption of AA cation form on the surface of NiO. Indication of AA cation specific adsorption is proved by the direction of zeta-potential values shift and relatively high zeta-potential values for AA curves and HCl curves in the region of positive zeta -potential values at any fixed pH values. According to Table 2 these is only one cation that is why these is no a question about a nature of the specifically adsorbed cation, however these is a question about a localisation place of the donoracceptor bond. Fig. 3. Dependences of zeta-potential of NiO particles versus pH in the aqueous solutions: 1 -10 -4 mol/l AA, 2 -10 -3 mol/l AA, 3 -HCl.
As noticed above the amino group can not be a ligand. Consequently in this case a coordination bond formed using the carboxyl group has a leading role.
During the analysis of the obtained data on NiO it is interesting to notice that IEP is higher for the solution with less AA concentration than for the more concentrated AA. This fact can be explained by the adsorption of AA cation form. In general cation and anion forms of AA can adsorb on the studied oxides specifically. Shift of IEP toward acid or base zone shows a specific adsorption of an anion or a cation. An increase of AA concentration gives an increase of an anion form in the solution. According to the experimental data influence of an anion form increases with an increase of AA concentration.
In Fig. 4 dependences = f(-lgC AA ) for NiO -AA aqueous solutions are presented. To show the role of AA ions the curve 3 in Fig. 3 is plotted the same way as in Fig. 1. From Fig. 4 it is obvious the specific character of AA cations adsorption (the curve 3 is below the curve 1) and the additional influence of the anion II specific adsorption is confirmed to the rapprochement of the curves 3 and 1 at high AA concentrations. Experimental results on AA adsorption on Fe 2 O 3 surface are shown in Fig. 5. From Fig. 5 it is obvious that in the system Fe 2 O 3 -AA aqueous solutions value is: -monotonically increase in the region where t c changes from 0 to 60 minutes and stays constant at 60< t c <3000 minutes; -further these is a region of a non monotonical complex change of = f(t c ). We assume that the non monotonical way of dependences = f(t c ) at t c >3000 minutes is due to long processes of Fe (III) cation hydrolyses on the iron oxide surface, which leads to changes of the cation and anion forms of AA in the solution. The Fe (III) hydrolysis is proved by the monotonical dependence = f( t c ) for the system NiO -AA aqueous solutions shown in Fig. 6 as Ni (II) cation does not hydrolyse at <7 [7]. Analysing the obtained data we concluded that the time t c (0-3000 minutes) corresponds the adsorption of monomolecular forms from the true AA solutions and reaching a local equilibrium. At the time t c > 3000 minutes the complex molecular formations (associations) and colloid particles (aggregates) form in the bulk solution. Fig. 6. Dependences -t c for the system AA -NiO: 1 -10 -2 mol/l; 2 -10 -3 mol/l; 3 -10 -3 mol/l; 4 -5 10 -4 mol/l.
Since aminoacids aggregates formation the adsorption is transferred into the adagulation. The adagulation is a process governed by other roles than adsorption. The fact of associates and aggregates formation is found in the literature [8]. The investigation of AC adsorption isotherm presented in Fig. 7 for Fe 2 O 3 and NiO at t c = 24 hours showed that NiO adsorbs aspartic acid in large quantities compare with NiO at all concentrations. This connects with Fe (III) cation hydrolysis. Experimental curves at t c = 24 hours have a character similar for Langmuir adsorption isotherm. At this consideration in the studied region of AA concentrations a monolayer formation of AA takes place on the oxide surfaces. From Langmuir equation of adsorption the average surface oxide area of AA molecule (S 0 ) was calculated (for Fe 2 O 3 , S 0 = 8,3 10 -20 m 2 and for NiO, S 0 = 6,4 10 -20 m 2 ). A small value of S 0 for AA (close to the geometric dimensions of the molecule [9]) proves the fact that AA molecules are dehydrated and adsorbed specifically on the oxide surfaces. Spectrophotometric study of the complex formation processes in the system A + FeCl 3 was performed at different pH. Maximum of the electron C (mol / l) spectra adsorption in the solution of FeCl 3 = 0.1 mol/l) at pH = 2.0 is at = 402 nm, but in the presence of AA it is shifted towards = 390 nm. It is important that at pH = 2 these is only non hydrolysed cation Fe 3+ in the solution. This shift proves the complex formation of Fe (III) ions with AA in the solution. At increase pH up to 5.1 the picks become broader and their maximum are shifted into long wave part of the spectra as for FeCl 3 solution, as for FeCl 3 A. This connects with hydrocomplex formation of Fe (III) in FeCl 3 solution with increase of and formation of complex complexes of Fe (III) carrying hydroxide groups as ligands and AA for the system FeCl 3 A. Change of electron spectra at different pH confirms the fact of the ligand exchange.

Conclusions
Experimental results showed that aspartic acid adsorbs specifically. An anion plays a leading role in the specific adsorption on the surface of Fe 2 O 3 but a cation is on NiO. The obtained data were analysed according to the coordination theory. According to the obtained results and data from the literature about an amino acid state in a bulk volume, specific adsorption a counter anion forms due to a donor-acceptor bond formation on the amino group and a counter cation on the carboxyl group. Complex formation of Fe (III) with aspartic acid in the aqueous solutions is confirmed by spectrophotometry. The analyse of these spectres showed the presence of maximum shifts for the appropriate cations in the presence of aspartic acid, which confirm the fact of complex formation between Fe (III) cation and aspartic acid. It should note that a possibility of a ligand exchange of AA and OHfor these systems is important for studying specific adsorption of AA on the metal oxides, as the oxide surfaces are in a significant degree hydrolysed.