Selective Removal of Toxic Metals like Copper and Arsenic from Drinking Water Using Phenol-Formaldehyde Type Chelating Resins

The concentration of different toxic metals has increased beyond environmentally and ecologically permissible levels due to the increase in industrial activity. More than 100 million people of Bangladesh and West Bengal in India are affected by drinking ground water contaminated with arsenic and some parts of India is also affected by poisoning effect of copper, cadmium and fluoride. Different methods have been evolved to reduce the arsenic concentration in drinking water to a maximum permissible level of 10 μg/L where as various methods are also available to separate copper from drinking water. Of the proven methods available today, removal of arsenic by polymeric ion exchangers has been most effective. While chelating ion exchange resins having specific chelating groups attached to a polymer have found extensive use in sorption and pre concentration of Cu ions. Both the methods are coupled here to separate and preconcentrate toxic metal cation Cu and metal anion arsenate(AsO4 ) at the same time. We have prepared a series of low-cost polymeric resins, which are very efficient in removing copper ion from drinking water and after coordinating with copper ion they act as polymeric ligand exchanger, which are efficiently removing arsenate from drinking water. For this purpose Schiff bases were prepared by condensing o-phenylenediamine with o-, m-, and p-hydroxybenzaldehydes. Condensing these phenolic Schiff bases with formaldehyde afforded the chelating resins in high yields. These resins are loaded with Cu, Ni, and Fe ions. The resins and the polychelates are highly insoluble in water. In powdered form the metal ion-loaded resins are found to very efficiently remove arsenate ion from water at neutral pH. Resins loaded with optimum amount of Cu ion is more effective in removing arsenate ions compared to those with Fe ion, apparently because Cu is a stronger Lewis acid than Fe. Various parameters influencing the removal of the arsenate ion from drinking water to a concentration level below 20 μg/L are studied.


Introduction
The concentration of toxic metal ions like Cu(II), Cd(II), As(III), As(V), Pb(II), Ni(II) and U(VI) etc. has increased beyond environmentally and ecologically sustainable levels due to natural phenomenon as well as anthropogenic impact.It has resulted in severe contamination of ground and surface water.The poisoning effect of toxic metals from contaminated drinking water has evolved as one of the major health hazards in the 21 st century 1,2 .The adverse health effects caused by copper, mercury and arsenic poisoning are far more catastrophic than any other natural calamity through out the world in recent times.Especially in the developing countries, water and soil degradation generated by industrial effluents has been a serious issue 3 .An estimated 120 million people are at risk of poisoning effect of arsenic in Bengal Delta (parts of Bangladesh, Nepal, and West Bengal), Taiwan, the USA, Chile, and Argentina.Many people are also suffering from different health hazards due to poisoning effect of copper, lead, and mercury 4 .Extraction of these metal ions is a tedious process as they are associated with a variety of complex species present in the natural aquatic systems.Again copper, lead, nickel, cadmium and uranium are present as cations such as Cu 2+ , Pb 2+ , Cd 2+ and UO 2+ in ground and surface water while arsenic is present as anions like AsO 4 -and AsO 3 -.Therefore different methods and mechanism are required to separate them from water.
Chelating ion exchange resins having specific chelating groups attached to a polymer have found extensive use in sorption and pre concentration of metal cations [5][6][7][8] .The Schiff bases having multiple coordination sites are known to form complexes with transition metal ion readily [9][10][11] present in a polymeric matrix they are expected to show affinity selectivity towards the metal ions at an appropriate pH.We have reported the synthesis, characterization and capacity studies of a number of phenol-formaldehyde type resins containing different Schiff base moiety [12][13][14][15][16] .These resins were found to react readily with several metal cations 17,18 .It is also a challenge to evolve a cost effective arsenic removal technology which could reduce arsenic concentration below 10 µg/L from ground water.In this respect a lot of research has been carried out and technologies developed.Many technologies have been evolved for arsenic removal from ground water.Of the proven methods available today, polymeric ligand exchanger (PLE) is best and most innovative available technology (BAT) for removal of As(V).
In this paper, we have tried to combined both the above idea to separate and preconcentrate toxic metal cation Cu 2+ and metal anion arsenate(AsO 4 -) at the same time.Because the ground water in some part of India contaminated with both copper and arsenic 19 .Here, we are report the synthesis of three phenol-formaldehyde type resins containing o-phenylenediamine functionalities and their metal ion uptake capacity towards metal ion such as Cu 2+ , Ni 2+ and Fe 3+ .Copper and iron polychelate of the resins are used as polymeric ligand exchanger to remove arsenate from drinking water.A comparative study was done between the copper and iron PLE.

Synthesis of Schiff base, resin and polychelates of resin
The Schiff base monomers o-, m-, p-hydroxybenzaldehyde-ethylenediamine (o-, m-, p-HBo-PD) were synthesized by reacting 0.01 mol of o-phenylenediamine (o-PD) with 0.02 mol of o-, m-, p-hydroxybenzaldehyde dissolved in 20 mL of ethanol in presence of 0.5 g of anhydrous sodium acetate.The mixture was refluxed for 1h at 60 0 C. The Schiff base o-HBo-PD was formed within 10 minutes of reaction.The formation of m-and p-HB-o-PD required refluxing the mixture for 1 h.The contents were poured into ice cold water, allowed to stand for one hour, filtered and dried at 70 o C. The color of the isolated Schiff bases o-, m-, p-HB-o-PD were yellow, metallic grey and yellowish brown respectively (Figure 1).They were thoroughly washed in water, ethanol and air-dried.In the further steps, the Schiff bases were condensed separately with formaldehyde (HCHO) solutions in 1:2 molar ratios in ethanolic medium using few drops of 0.01 M NaOH solutions as catalyst and the mixture was refluxed in oil bath at 120-130 o C for 2 h (Figure 2).The insoluble resin was filtered, washed repeatedly with distilled water and dried at 70 0 C. To prepare the polychelates, to 100 mg of the dry resin (100 mesh, ASTM) suspended over methanol, 10 mL of metal salt (0.15 M) in water was added.The mixture was stirred for 2 h at 40 0 C. It was filtered, washed in distilled water followed by petroleum ether and dried at 70 0 C.

Procedure for metal ion uptake experiments
The metal ions uptake studies were done employing batch techniques.In the batch technique, a suspension of the resin in the metal solution of known volume and concentration was taken in stopper glass bottles (100 mL) and shaken for a definite time period at the shaking rate of 200 rpm.The pH of the solution was adjusted using suitable buffer.The resin was filtered off, and thoroughly washed with demineralized water.The metal ion concentration in the filtrate and washing were estimated colorimetrically using neocuprion method for Cu(II), thiocyanate method for Fe(III) and dimethylglyoxime method for Ni(II) after proper dilution, if necessary 20 .

Desorption and reuse
Desorption of the metal ion was also carried out from the resin column.After loading the metal ion solutions onto the resin column at appropriate pH, the selected eluting agent was run through the column by regulating the flow with the stopcock of the column.The desorption ratio (%) was calculated using the following expression.

Desorption ratio
Quantity of metal ions desorbed to the eluting medium Quantity of metal ions adsorbed onto the sorbent (%) = × 100 The adsorption-desorption cycle was repeated at least three times with the same resin to obtain a reliable result

Arsenate adsorption studies
To 10 mL of the arsenate solution ([AsO 4 3-] = 200 µg /L), 100 mg of the copper polychelate of 100 mesh was added and shaken for a fixed time period in stoppered conical flasks at 30 o C. The contents of the flask were filtered off and the resin was thoroughly washed in demineralized water.The metal ion concentration in the filtrate and the washings was determined by a two-channel atomic absorption/flame emission spectrophotometer.To determine the optimum conditions for efficient uptake of arsenate ions by the copper polychelate, various parameters like contact time and pH were varied.Doubly deionized water was used through all the experiments.

Metal ion uptake studies Effect of contact time
The Cu(II), Ni(II) and Fe(III) solution were treated with the resin at natural pH of the solutions.The contact time was varied between 5 min to 24 h.The saturation time was obtained by plotting the percentage of metal ion against the contact time variation, keeping the initial metal ion concentration fixed (200 µg./10 mL.).In all the three resins, the rate of Cu(II) adsorption is higher than that of Ni(II) and Fe(III) (Table 1).Several authors have noted the higher adsorption of Cu(II) over other metal ions 21,22 It is also found that o-HB-o-PD-HCHO is the most efficient resin for all the metal ions.It takes 80, 49.9, 48.8% of Cu(II), NI(II), Fe(III) respectively at 24 h in natural pH of the solution.The order of adsorption efficiency of the resin is o-HB-o-PD-HCHO >> m-HB-o-PD-HCHO > p-HB-o-PD-HCHO.It could be explained on the basis that due to structural features of the above resins, the azomethine nitrogen and/or phenolic oxygen forms stable complexes on the above order.Because as we move from ortho to para complex the distance between azomethine nitrogen and phenolic oxygen increases.

Effect of pH
The effect of the reaction medium on the extent of adsorption of Cu(II) and Ni(II) was studied using buffers in the pH range of 3.42-5.89for Cu 2+ , 3.42-8.9for Ni(II) and 3.42-9.0for Fe(III) (Figure 3a-c).The metal ions are precipitated as hydroxides beyond the above pH ranges.The ease of coordination of the phenoxide ion over that of phenolic OH group and the enhanced basicity of the C = N nitrogen at higher pH are some factors for the resins to uptake high percentage of metal ions at higher pH.Because in lower pH, the coordinating groups get protonated 23 .Dev and Rao have reported enhanced adsorption of metal ions with increase 24 in the pH.In our case, the optimum pH of the adsorption of Cu(II), Ni(II) and Fe(III) ions were 5.89, 8.9 and 9.0 respectively 24,25 .It is also observed that the metal ion uptake percentage was higher in case of o-HB-o-PD-HCHO resin than the other two resins.However, all the three resins showed higher affinity towards Cu(II) than Ni(II) and Fe(III) .

Effect of metal ion concentration
The effect of metal ion concentration on the uptake behavior of the resins was studied in the metal ion concentration range 50-500 µ g/mL.It was observed that with increase in concentration of the metal ion, the amount of adsorption also increased up to a certain stage, after which there was no further increase in amount of metal ion adsorption.Many authors reported similar observations 26 .This could be attributed to the saturation of the available coordinating sites in the resin with the metal ion.The adsorption coefficient, k ad , of the resins for the uptake of Cu(II) was computed from Freundlich adsorption isotherm.log (x/m) =log k ad + 1/n log C (1) where, 'C' is the concentration of the metal ion in mmol, 'm' is the weight of the resin in gram, 'x' is the metal ion adsorbed by the resin in mmol and 'n' is a constant.For all the resins the value of k ad and 'n' were found out and presented in Table 2. High k ad values were observed in all cases, which indicated that the equilibrium for metal ion adsorption was attained at a fast rate.Blasius and coworkers 28 have reported the adsorption constant for Mo 6+ and W 6+ and slow adsorption rate of the metal ion was associated with low k ad values 27,28 .

Effect of added salt
Effect of the cations like Na + , K + and Mg 2+ on the adsorption behavior of the resin towards Cu(II) was observed.It is done by treating 100 mg of all the resins with 200 µg per 10 mL of the Cu(II) in presence of the above alkali and alkaline earth metal ion solution at the natural pH of the solution for 24 h.It was observed that the presence of alkali and alkaline earth metal ions and the accompanying anion have negligible effect on the adsorption behavior of the resins (Table 3).Hence, the resin could be used to remove Cu(II) ion from saline and non-saline water rich in these above cations.Hodgkin and Eibl 29 prepared a Cu 2+ -selective (sirorez -Cu ) from phenolformaldehyde and piparazine and the selective capacity for Cu 2+ was studied in the pH range 3-10.5.They observed that, the alkali and alkaline earth metals were not retained by the resin in this range.Dev and Rao 24 also reported the same observation as we are report in this paper.

Separation
To know the adsorption efficiency of the resins in a competitive environment where both Cu(II) and Fe(III) ions are present, two set of experiments were carried out.In the first set of experiment 10 mL of solution containing 200 µg each of Cu(II) and Fe(III) was treated with 100 mg of the resins at varying pH (Table 4).It was found that in the pH range 3.42-4.63all the resins exclusively adsorbs Cu 2+ ion while in the pH range 4.63-5.89all the resins adsorbs high percentage of Cu 2+ with many fold increase in the k d value along with a small amount of Fe 3+ and most efficient among all the resin is o-HB-o-PD-HCHO.Mendez and Pillai 30 have reported a resin which is highly selective for Cu 2+ over UO 2 2+ and VO 2+ above pH 3. In the second set of experiment, 10 mL of the solution containing a fixed amount of Cu 2+ (200 µg/ 10 mL) and varying amounts of Fe 3+ ( 100-400 µg/ 10 mL) was treated with 100 mg of the resins at affixed pH for 24 h (Table 5).It was seen that at pH 5.89, all resins take up Cu 2+ quantitatively and the adsorption of Fe 3+ was negligible.Again the most efficient resin is o-HB-o-PD-HCHO.Therefore it can be concluded that in a competitive environment where both Cu(II) and Fe(III) are present these resins are quantitatively adsorbs Cu(II) over Fe(III) and forms effective metal complex with Cu(II) ions.

Arsenate adsorption studies
Literature survey shows that most of the work on metal-loaded polymers used for separation of arsenate has been done with Fe(III), but these sorbents cannot be effectively used for drinking water treatment [31][32][33] .Because all observation shows that only at low pH As(V) can be removed.Again, because of the weak Lewis acid characteristic of ferric ions, the amount of Fe 3+ loaded was low.Moreover, the loaded iron was nearly completely stripped off the hosting resin during regeneration and reloading of Fe 3+ was necessary after each cycle of operation.Realizing the critical drawbacks of Fe 3+ ions, Raman and Sengupta 34 prepared a PLE by loading Cu 2+ onto a weak base chelating resin (known as DOW 2N) with 2picolylamine groups.Since Cu 2+ is a much stronger Lewis acid than Fe 3+ , according to the Irving and Williams 35 order.So a much greater metal-loading capacity was observed.The copper loaded DOW 2N showed orders of magnitude greater selectivity for arsenate and selenate in the presence of competing sulfate ions than commercial SBA resins 36 .Here we used the Cu(II) polychelate of the most efficient resin o-HB-o-PD-HCHO as polymeric ligand exchanger (PLE) and studied the arsenate adsorption capacity of it.Also compared the result with the arsenate adsorption capacity of the Fe(III) polychelate of the same resin.The metal polychelates were synthesized using the metal nitrate salts.We have also observed similar result as Raman and Sengupta 34 which is discussed below.

Equilibrium time
To determine the equilibrium time for the adsorption of arsenate ions 100 mg, 100 mesh of the polychelates o-HB-o-PD-HCHO -Cu(II), o-HB-o-PD-HCHO -Fe(III) were treated with metal salt solutions (2 µg/10 mL) at the 7.0 pH of the reaction mixture.The contact time was varied between 5 min to 24 h.Comparing the arsenate adsorption capacity of the PLEs, it is observed that the arsenic uptake efficiency of o-HB-o-PD-HCHO-Cu(II) polychelate is significantly higher than that of o-HB-o-PD-HCHO-Fe(III).The former PLE is able to take 80.2% of arsenate ion at 24 h in natural pH of the solution while the later only adsorbs 52.8% of arsenate (Table 6).The reason for such an observation could be attributed to the concurrent Lewis acidbase interactions between arsenate and the immobilized Cu 2+ ions at the sorbent-sorbate interface.Under the experimental conditions, monohydrogen arsenate (HAsO 4

2-
) is considered as predominant arsenate species.HAsO 4 2-is a divalently charged, bidentate ligand and stronger lewis base.Consequently, interactions between arsenate and the immobilized Cu 2+ ions involve both LAB interaction (or inner-sphere complexation) and ion pairing (or electrostatic interactions) (Figure 4).It is noteworthy that LAB interaction also enhances the electrostatic interactions between arsenate and the loaded Cu 2+ ions.This is because the inner-sphere complexation occurs over a much shorter distance than outer-sphere complexation, and the electrostatic interactions within the much shortened distance are much stronger in accord with the Coulomb's law.But due to weak acid strength of Fe 3+ o-HB-o-PD-HCHO -Fe(III) has comparatively lower arsenate adsorption capacity than o-HB-o-PD-HCHO-Cu(II).
It was also observed that competing ions like sulphate has negligible effect on the arsenate adsorption.Though sulfate is also a divalently charged ligand, it is a much weaker Lewis base than the arsenate.Therefore interactions between sulfate and the Cu 2+ ions is predominantly ion paring.Therefore o-HB-o-PD-HCHO-Cu(II) offered much greater affinity for arsenate over sulfate.Again in case of SBA resins, the commercial SBA resins 36 , take up anions predominately through electrostatic interactions, i.e., the ligand strength of an anion does not play a role in sorption affinity.Therefore, SBA resins are not selective for arsenate.Similar observations were reported by Zhao et al. 36

Effect of pH
As in any ion exchange process, the PLE's selectivity for various competing ligands can be strongly influenced by solution pH.Solution pH can affect the PLE's As uptake in two different aspects.First, solution pH governs the speciation of arsenate, resulting in arsenate species (H 3 AsO 4 , H2AsO4 -, HAsO 4 2-, and AsO 4 3-) of different ionic charges and ligand strength.Second, the hydroxyl anions become aggressively formidable competitors for the ligand exchange sites as solution pH goes up.
The effect of pH of the reaction medium on the extent of adsorption was studied extensively.The extent of adsorption of the PLEs was investigated using buffers in the pH range 3.42-8.8.It is observed that with increase in pH the arsenate adsorption capacity o-HB-o-PD-HCHO-Cu(II) increased till pH 7.0 and then decreased but for o-HB-o-PD-HCHO -Fe(III) with increase in pH the arsenate adsorption capacity decreased (Table 7).
Sharp declination was observed in case of o-HB-o-PD-HCHO -Cu(II) after the pH range 8.0. Figure 5b indicates that the optimal arsenate uptake occurs in the pH range of 6.0-8.0, with the peak uptake being at pH 7.0.At pH 4 or >10 there was virtually no uptake of arsenate observed.It is also interesting that As uptake started increasing at pH 4.0 almost in proportion to the increasing ).However, the As uptake dropped sharply as pH exceeded 8.0.Based on both ligand strength and ionic charge, the adsorbability of various arsenate species follows the sequence H 3 AsO 4 < H 2 AsO 4 -< HAsO 4 2-<AsO 4

3
. At pH<4 the much less adsorbable H 2 AsO 4 -or H 3 AsO 4 is the predominant arsenate species, which cannot stand the competition of divalently charged sulfate anions.As a result, no As uptake is likely in the low pH range as observed in Figure 5(a-b).The fact that the As uptake appears to be in proportion to the formation of HAsO 4 2-in the pH range of 4.0 -7.0 agrees with the notion that to overcome the competition from sulfate, arsenate must be converted to the more adsorbable HAsO 4 2-species.

Conclusion
It can be concluded that the phenolic Schiff base resins containing o-phenylenediamine are very efficient for uptake of various cations of heavy metals like copper, nickel and iron etc. Again the copper polychelates of o-HB-o-PD-HCHO can be used as a polymeric ligand exchanger for effective separation of arsenate from drinking water.So by the help of the resin and its polychelate simultaneously the toxic cation Cu(II) and toxic anion arsenate can be separated from drinking water.Hence the resins and polychelates of the phenolic Schiff bases are very useful in combating poisoning due to toxic metal ions, such as, copper and arsenic, etc.

Figure 3 Figure 3 Figure 3 (
Figure 3(a).Uptake of Cu(II) ions by the resins with increasing pH.pH pH pH

Table 1 .
Effect of contact time.

Table 2 .
Effect of variation of metal ion concentration on adsorption behavior of resins andFreundlich adsorption isotherm data.Metal ion: Cu(II), Resin quantity: 100 mg, Sorbent size: 100 mesh, Temperature: 30 o C,

Table 3 .
Effect of added salt on the adsorption of Cu(II) ion by the resin.

Table 5 .
Separation of Cu(II) from a mixture of Cu(II) and Fe(III) with increasing Fe(III) concentration at fixed varying pH.Resin quantity: 100 mg, Sorbent size: 100 mesh, Temperature: 30 o C, Contact time: 24 h, pH:5.89 by performed similar experiments over DOW 3N-Cu PLE .

Table 6 .
Effect of contact time for arsenate adsorption studies.