Polyhydroxamic acid functionalized sorbent for

Chemical Engineering Journal 326 (2017) 318–328

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Polyhydroxamic acid functionalized sorbent for effective removal of chromium from ground water and chromic acid cleaning bath Samina H. Shaikh, Sanjukta A. Kumar ⇑ Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

A novel sorbent for removal of ‘Cr’

from ground water and industrial effluent. Highly toxic Cr(VI) reduces to relatively non-toxic Cr(III) by PHA. Sorption capacity was 322 and 176 mg g1 for Cr(III) and Cr(VI) respectively. Recovered chromium compound from chromium sorbed PHA.

a r t i c l e

i n f o

Article history: Received 5 April 2017 Received in revised form 23 May 2017 Accepted 24 May 2017 Available online 26 May 2017 Keywords: Solid phase extraction Polyhydroxamic acid Chromium Chromic acid bath Chromium recover

a b s t r a c t Polyhydroxamic acid sorbent was synthesized by polymerization of acrylamide using N, N0 -methylenebis-acrylamide as crosslinker and ammonium per sulphate as a thermal initiator. The sorbent was characterized for composition, thermal stability, functional group and morphology by elemental analysis, thermal analysis, infrared spectroscopy and secondary electron microscopy respectively. The synthesized sorbent was evaluated for the removal of trivalent and hexavalent chromium ion from aqueous solution. Experimental parameters such as pH, equilibration time, solute concentration and temperature were optimized. It showed a sorption capacity of 322 mg g1 for Cr (III) and 176 mg g1 for Cr (VI) at pH 4. The sorbent reduces the more toxic Cr (VI) to relatively less toxic Cr (III) during the sorption process as confirmed by X-ray photoelectron spectroscopy. The sorption phenomena followed Langmuir model and the sorption reaction was found to be pseudo–second order confirming chemisorption. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Advancement of technology results an increase in industrialization and consequently heavy metal pollution in environment. Heavy metal pollution in the environment is of major concern due to their bioaccumulation and high toxicity. Because of this heavy metals are a threat to living system and environment. Therefore, their removal from environment especially wastewater have received much attention nowadays [1–3]. For the conservation of ⇑ Corresponding author. E-mail address: [email protected] (S.A. Kumar). http://dx.doi.org/10.1016/j.cej.2017.05.151 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

biodiversity, it is necessary to remove such contaminants from the waste streams before releasing them into the environment. Chromium is one of the most toxic heavy metal. It primarily exist in +3 and +6 oxidation state in aqueous solutions. Trivalent chromium is an essential element in trace amounts for different metabolic processes in mammals while hexavalent chromium compounds are toxic and carcinogenic due to their high solubility and mobility in water [4]. In aqueous solution Cr (VI) predomi2 2 nantly exists as HCrO 4 , CrO4 , Cr2O7 species [5]. Acute exposure to Cr (VI) causes dermatitis, nausea, internal hemorrhage, diarrhea, respiratory problem (asthma). Chronic inhalation of Cr (VI) compounds increases the risk of lung cancer. It targets the respiratory

S.H. Shaikh, S.A. Kumar / Chemical Engineering Journal 326 (2017) 318–328

system, kidney, liver, skin and eye. Effluents from various industries such as chrome plating, chromate manufacturing, leather tanning, paints and dyeing, automobiles petroleum refining etc. contain Cr (VI) in large concentrations. Chemical laboratories use chromic acid for cleaning of soiled glass and Teflon wares. In literature, it was mentioned that when glassware becomes unduly clouded or dirty or contains coagulated organic matter, it must be cleansed with chromic acid cleaning solution [6]. Chromic acid oxidizes most residues and eats away a very thin layer of the glass surface, leaving a new, clean surface. However the safe disposal of used chromic acid is very much essential since Cr (VI) is a potential carcinogen. According to World Health Organization and United State Environmental Protection Agency the maximum permissible limit of total chromium in drinking water are 0.05 mg L1 and 0.1 mg L1 respectively [7,8]. Therefore, removal of chromium from waste water is essential before disposal. Various methods are available for removal of heavy metals from aqueous streams, such as chemical precipitation, solid phase extraction (SPE), solvent extraction, ion-exchange chromatography, membrane filtration, reverse osmosis and electro deposition [9,10,11]. Chemical precipitation requires large amount of chemicals and generate solid sludge which constitutes a solid waste disposal problem and other methods are relatively expensive. Among these methods, SPE is preferred for remediation of metal due to its ease of operation, reusability of the sorbent and option to scale up. The basic principle of solid phase extraction is the sorption of target ion on solid sorbent and thereby removal from solution. Organic resins with suitable functional groups for binding of specific cations/anions are most widely used sorbents in various remediation processes as well as in drinking water purification process. Researchers have anchored/immobilized chelating ligands on solid support such as saw dust [12] natural polysaccharide starch [13], graphene oxide [14,15,16,17] etc. and used them as solid phase sorbents. The ligands on the surface of the sorbent forms a complex with the target metal ion thereby separating it from the solution. The development and modification of such sorbent materials for selective and efficient removal of metal ion is an active area of research recently [18,19]. The present work was aimed at preparation and chemical modification of polyacrylamide (PAM) to polyhydroxamic acid (PHA) for the removal of trivalent and hexavalent chromium. The general formula of hydroxamic acid is R-CO-NH-OH (R = alkyl or aryl) having a keto and enol tautomerism. Hydroxamic acid, being a bidentate ligand forms a stable chelate with heavy metal ions [20]. The synthesized polyacrylamide and polyhydroxamic acid were characterized for composition, thermal stability, functional group and morphology by elemental analysis, thermal analysis, infrared spectroscopy and scanning electron microscopy. Presence of hydroxamic acid group was also confirmed by visual colorimetric test. The effect of different parameter such as solution pH, equilibrium time, solute concentration and temperature was evaluated in batch experiments. The sorbent reduces highly toxic Cr (VI) to relatively less toxic Cr (III) during the sorption process as indicated by color of Cr(VI) loaded sorbent and confirmed by X-ray photoelectron spectroscopy. Interference of sodium, potassium, calcium and magnesium, the common cations in ground water, on the sorption process was evaluated. The sorption phenomena followed Langmuir model and the sorption reaction was found to be pseudo–second order confirming chemisorptions. A sorption capacity of 322 mg g1 for Cr (III) and 176 mg g1 for Cr (VI) was obtained at pH 4 in batch mode. The evaluated resin was applied to ground water and used solution of chromic acid bath for removal of chromium. On one hand, it is important to develop novel and innovative sorbent materials for removal of toxic metals from environment, on the other hand the

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management of solid waste generated post remediation is also essential. Polyhydroxamic acid is a biocompatible as well as biodegradable material and hence can be used safely for landfill. Alternately the material can be incinerated and chromium, recovered as chromium oxide can be reused. 2. Experimental 2.1. Reagents and standards Acryl amide, N, N0 -methylene bis acryl amide, N, N, N0 , N0 -tetra methylethylenediamine, hydroxylamine hydrochloride and sodium hydroxide used for making PHA, chloride salts of sodium, potassium, calcium and magnesium and sodium sulfate used during interference study were of AR grade. Required stock standard solutions of Cr (III) and Cr (VI) were prepared from AR grade chromium nitrate and potassium dichromate respectively. Calibration standards were prepared by serial dilution of stock solutions using 1% nitric acid. Nano pure water measuring 18.2 M X cm1 of specific resistance, collected from a Millipore system was used throughout the experiments. Nitric acid and hydrochloric acid used were of supra pure grade procured from E. Merck. pH values of experimental solutions were adjusted using either dilute HCl or dilute NaOH. 2.2. Instruments used for measurements A Continuum source Flame Atomic Absorption Spectrometer (CSAAS 300, Analytik Jena, Germany) was used for the determination of chromium. All the pH measurements were carried out using a Mehtrom 780 pH meter. The functional groups on sorbent surface were confirmed by attenuated total reflection fourier transformed infrared spectrometer using a FTIR spectrometer (Bruker ALPHA-P). Elemental analysis was carried out using an elemental analyzer (Euro vector EA 3000). Thermal stability of sorbent was recored by a TGA/DSC analyzer (Mettler Toledo TGA/DSC analyzer). The surface morphologies of polyacryalmide and polyhydroxamic acid were observed using a scanning electron microscope (SEM-SERON INC South Korea, Model ATS 2100). The presence of ’Cr’ on PHA after sorption of chromium was confirmed by energy dispersive X-ray fluorescence spectrometer (EX-3600 M Xenemetrix EDXRF spectrometer). The oxidation state of ’Cr’ on Cr(VI) loaded PHA was confirmed by X-ray photoelectron spectroscopy (PHOIBOS 100/150 analyzer). All spectrophotometric measurements were carried out using a spectrophometer (DH-2000-BAL, Ocean Optics). The X-ray diffraction spectra were obtained using portable XRD instrument (Inxitu XRD). For measurement of total chromium concentration in the solution before and after equilibration with PHA the CSAAS instrument was calibrated with a series of aqueous standard solution with chromium concentration of 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mg L1 prepared from the stock solution. The concentration of chromium in sample solution was determined using the calibration plot. The spectrophotometric determination of Cr(VI) concentration was done by using diphenylcarbazide as a Cr(VI) specific colorimetric reagent. The spectrophotometer was calibrated with aqueous standard of Cr(VI) with 0.4, 0.6, 0.8 and 1.0 mg L1 concentration by measuring the absorbance at 540 nm. The concentration of chromium in sample solution was determined by fitting the sample absorbance in the calibration plot. 2.3. Synthesis and characterization of PHA The method available in literature [21] was modified suitably and followed for synthesis of PHA. Acrylamide (CH2CHCONH2) and crosslinker, N, N’–methylene – bis – acrylamide (CH2CHCO

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NHCH2NHCOCHCH2) was dissolved in deionised water with constant stirring to get a clear solution. Ammonium persulfate was added to this solution as a thermal initiator to initiate the polymerization. The resultant mixture was heated at 60 °C for about 15 min to initiate the polymerization. The bulk polymer thus obtained was crushed, and washed with water to remove the excess/un-polymerized reagents. This is termed as polyacrylamide (PAM). PAM was further treated with hydroxylamine hydrochloride solution for 30 min followed by adjusting the pH of the solution to 12 using sodium hydroxide solution. The mixture was left overnight and neutralized using 3 N HCl. Sorbent thus obtained was washed with water till free from acid, air dried and termed as polyhydroxamic acid (PHA). This PHA was characterized by C, H, N analysis and FTIR spectroscopy. Presence of hydroxamic acid group was also confirmed by visual colorimetric test. Surface morphology was established by SEM-EDX. Thermal stability of the sorbent was established by thermogravimetric analysis.

to 25 mL of 5 mg L1 Cr (III) and Cr (VI) solutions separately. After continuous equilibration with stirring using a magnetic stirrer for a predetermined time interval, the sorbent samples were taken out and residual metalion concentrations were determined. Since Cr (VI) exist as anion in aqueous medium, tolerance of Cl1, a common anion found in ground water and SO2 4 , the major constituent of chromic acid bath was also studied by equilibrating 25 mL of 5 mg L1 Cr (III) and Cr (VI) solutions with 100 mg L1, 500 mg L1 and 1000 mg L1 of these ions.

2.4. Batch sorption studies

3.1. Characterization of synthesized PHA

Various parameters e.g. pH, time for maximum sorption as well as sorbate concentration were evaluated by batch experiments in aqueous solution for optimizing the sorption of Cr (III) and Cr (VI) using PHA at room temperature (28 ± 1 °C). Uptake studied were carried out by equilibrating 25 mL solution containing predetermined concentration of Cr (III) and (VI) with 50 mg of PHA. The percentage uptake of the metal ions by PHA was calculated using the following equation:

Hydroxamic acid is known to give specific colors with different metal ions. Therefore, a qualitative test was carried out to confirm the conversion of amide group in polyacrylamide to hydroxamic acid group by equilibrating the hydrated PHA-sorbent with a solution of Fe (III). On equilibration with Fe (III), the PHA-sorbent turned dark brown (Fig. 1) instantly confirming the presence of hydroxamic acid groups. The functional group present in PHA was also confirmed by FT-IR analysis. The FTIR spectra of PAM, PHA, Cr(III) sorbed PHA and Cr (VI) sorbed PHA are shown in Fig. 2. The comparison of IR spectra showed that PAM and PHA have a similar backbone. Absorption peak at 3360 and 3180 cm1 corresponds to N–H asymmetric and symmetric stretching vibrations respectively of amide group. Peaks observed at 29602920 cm1 and 1475-1445 cm1 are due stretching and bending vibrations of CH2 group (R-CH2-NHR’). Characteristics absorption peaks for N-H bending at 1108 cm1 and C@O (carbonyl group) stretching at 1650 cm1(usually appears as a doublet in solid state which is a characteristics of amides) are observed in both PAM and PHA. Absorption peak at 960-930 cm1 in PHA is mainly due to N-O stretching vibration of -C@N-OH group (oxime functional group) which is absent in PAM. The IR spectra, thus confirms the conversion of acid amide group of PAM to hydroxamic acid group in PHA. Experiments were carried out as per Section 2.5 for determination of the point of zero charge (pHPZC) of PHA. It was determined to be 3.8 as obtained from plot of DpH (pHinitial – pHfinal) vs pHinitial (Fig. 3). From the plot, it is evident that at pH 3.8 thee charge on PHA surface is zero. Below this pH the surface of PHA acquires positive charge and this pH the surface is negatively charge. Reaction of PAM with hydroxylamine results mainly conversion of acid amide group to hydroxamic acid. However a minor fraction of acid amide group can get hydrolyze to form carboxylic acid

% Uptake ¼ ðC0 CeÞ 100=C0

ð1Þ

where, C0 and Ce are initial and equilibrium concentration of metal ion in mg L1. 2.5. Determination of point of zero charge (pHPZC) For determination of pHPZC value of sorbent pH drift method was applied. For this 25 mL 0.1 M NaCl solutions were adjusted to pH values from 1 to 10 with 0.01 M HCl or 0.01 M NaOH solution. The adjusted pH was recorded as initial pH. These 0.1 M NaCl solutions were equilibrated with 100 mg PHA at room temperature for 48 h. The final pH of solution was recorded after filtration. 2.6. Sorption capacity The metal ion uptake capacity of PHA for Cr (III) and Cr (VI) was determined separately by the batch experiment technique. For this 50 mg of the PHA was equilibrated with 25 mL of 200–4000 mg L 1 Cr (III) and Cr (VI) solutions separately for 24 h. The PHA was then separated from the solutions by filtration and the concentrations of the residual metal ion in the solutions were determined. The amount of metal ion in grams that can be absorbed by one gram of PHA (qe) was then evaluated using the mass balance equation given below:

qe ¼ ðC0 Ce Þ V=w

2.8. Application to real samples The synthesized poly hydroxamic acid was applied successfully for removal of chromium from ground water and used chromic acid bath. 3. Result and discussion

ð2Þ

where, C0 and Ce are initial and equilibrium concentration of metal ion in mg L1. V is the volume of the aqueous phase in L and w is the mass of the PHA sorbent used in gram. 2.7. Tolerance study The chromium sorption experiments were performed in the presence of known concentration of common ground water matrix elements viz; Na, K, Ca and Mg. Three different concentrations of these ions; 100 mg L1, 500 mg L1 and 1000 mg L1 were used for these studies. Appropriate quantities of these ions were added

Fig. 1. (a) PHA and (b) PHA equilibrated with Fe(III).


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Fig. 2. FT-IR spectra of Polyacrylamide (PAM), Polyhydroxamic acid (PHA), Cr (III) loaded PHA and Cr (VI) loaded PHA.

3.2. Thermal stability study Thermal stability of the polyacrylamide and polyhydroxamic acid had been investigated using thermogravimetric analysis (TGA). TGA curves were recorded from room temperature to 900 °C with a heating rate of 30 °C min1 for 30 min. The TGA and DTG spectrum (Fig. 5) of PAM and PHA showed three different mass loss regions. The first region observed at around 86 °C with mass loss of 7.5% which corresponds to loss of moisture from PAM and PHA. Second region appears at 260 to 275 °C with mass loss of 18% which corresponds to dehydration. The last region represents the decomposition of PAM and PHA. The decomposition temperature of PAM and PHA are 414 °C and 367 °C with mass loss of 55% and 40% respectively which shows the less thermal stability of PHA as compared to PAM. The low thermal stability of PHA is one of the advantages to recover back the chromium compound from chromium loaded PHA. Fig. 3. Point of Zero Charge.

3.3. Effect of pH on uptake of Cr (III) and (VI)

Table 1 Percentage C, H, N and O. Sample

%C

%H

%N

%O

a

51.1 51.4 42.7 43.2

7.0 7.1 5.8 6.2

19.6 19.0 16.3 16.0

22.4 22.5 35.2 34.6

PAM b PAM a PHA b PHA a b

Obtained by theoretical calculation. Obtained from instrumental analysis.

group. The hydrolysis of acid amide group to carboxylic acid group was minimized by carrying out the reaction in controlled condition. The % conversion of acid amide to hydroxamic acid was found out from C, H, N analysis. Table 1 represents the data for percentage C, H, N and O obtained by theoretical calculation (considering 100% conversion of acid amide group to hydroxamic acid group) as well as from analysis. The percentage nitrogen content of PAM and PHA confirms >95% conversion of acid amide group to hydroxamic acid group. Fig. 4a and b represents the SEM images of dry PAM and PHA respectively. Pallets of PHA and PAM were made for recording the SEM. The images reveal that both have similar morphology.

The pH of a solution plays an important role in heavy metal sorption as the species of metals ions varies at different pH. The effect of pH on sorption of Cr(III) and Cr(VI) was investigated thoroughly in the pH range of 1–7 and 1–12 respectively. Beyond pH 7 Cr (III) exists as [Cr (OH)3]0 and forms amorphous precipitate and hence its sorption behavior could not be studied. The results were represented graphically in Fig. 6. It was found that both Cr (VI) and Cr (III) showed maximum sorption at pH 4. Cr(VI) was seen to have appreciable sorption even in the lower pH range. This may be because at lower pH the surface of sorbent is positively charged due to strong protonation and as Cr (VI) exists as negatively charged hydrogen chromate (HCrO 4 ) species, its sorption is facilitated. On increasing pH values from 4 to 12, the HCrO 4 gradually converts to the divalent CrO2 and finally to Cr2O2 as per the 4 7 Scheme 1 shown below. At the same time the protonation degree of sorbent surface is also reduced under weak acidic condition, which results in less electrostatic attraction between more negatively charged ions and sorbent surface. Therefore, the overall sorption of Cr (VI) by PHA is higher at lower pH and maximum sorption occurs at pH 4(Fig. 6). A similar pH depended sorption trend was observed in literature [22,23,24]. On the other hand Cr (III) exists in +3 oxidation states at lower pH (