Accepted Manuscript Title: Sulphuric acid doped poly diaminopyridine/graphene composite to remove high concentration of toxic Cr(VI) Author: Diptiman Dinda Shyamal Kumar Saha PII: DOI: Reference:

S0304-3894(15)00154-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.02.065 HAZMAT 16637

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

31-10-2014 17-2-2015 25-2-2015

Please cite this article as: Diptiman Dinda, Shyamal Kumar Saha, Sulphuric acid doped poly diaminopyridine/graphene composite to remove high concentration of toxic Cr(VI), Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.02.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sulphuric acid doped poly diaminopyridine/graphene composite to remove high concentration of toxic Cr(VI)

Diptiman Dinda and Shyamal Kumar Saha*  Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India *Corresponding author. Tel.: +91 33 2473 4971 (Ext 227). Fax: + 91 33 24732805. E-mail address: [email protected]  

GRAPHICAL ABSTRACT: 

Highlights:  Sulphuric acid doped diaminopyridine polymers are synthesized on GO surface.  Shows high adsorption capacity of 609.76 mg g-1 compared to other nano materials.  Capable to remove high concentration of Cr(VI) (500 mg L-1) from water quickly.  Shows advancement in removal efficiency at higher pH conditions.  Exhibits good recycling ability with ~92% removal efficiency after fifth cycle. 1   

Abstract: Sulphuric acid doped diaminopyridine polymers are synthesized in situ on graphene oxide surface via mutual oxidation-reduction technique. Exploiting large and highly porous surface, we have used this polymer composite as an adsorbent to remove high concentration of toxic Cr(VI) from water. It shows very high adsorption capacity (609.76 mg g-1) during removal process. The composite takes only 100 min to remove high concentration of 500 mg L-1 Cr(VI) from water. Interesting features for this material is the enhancement of removal efficiency at lower acidic condition due to the formation of acid doped emeraldine salt during polymerization. XPS and AAS measurements reveal that our prepared material mainly follows reduction mechanism at higher acidic condition while anions exchange mechanism at lower acidic condition during the removal experiments. Good recycling ability with ~92% removal efficiency after fifth cycle is also noticed for this material. Easy preparation, superior stability in acidic condition, remarkable removal efficiency and excellent recycling ability make this polymer composite an efficient material for modern filtration units in waste water purification.

Keywords: Graphene, sulphuric acid doping, in situ polymerization, removal of toxic Cr(VI), high adsorption capacity and good recycling ability.

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1. Introduction With the advancement of technology, water pollution is a great concern nowadays to our civilization. Different toxic heavy metals from various industries are mixed with ground water and contaminate it [1]. Among these, Cr(VI) is one of the most pronounced and dangerous water pollutant as its different compounds e. g. Cr2O72-, CrO42-, HCrO4- are highly soluble in water and cause harmful effects on human body due to its strong oxidizing power [2]. Therefore, it is highly desirable to develop some effective routes to remove Cr(VI) from contaminated water to prevent the deleterious impact of Cr(VI) on the ecosystem and public health. As Cr(III) is not so harmful to our health [3], it is the best way to convert Cr(VI) to less toxic Cr(III) and separate it from water solution. To date, different techniques have been applied to remove toxic metals among which adsorption technique is most effective because of its several advantages like easy process, high efficiency and good recycling ability [4-8]. After the revolutionary discovery of conducting polyacetylene, a new window of conducting polymer is opened. Now these materials have attracted a lot of attentions because of their potential applications in versatile areas. Their unique electronic properties, flexibility, simple doping-dedoping chemistry and excellent stability in aqueous medium make them very popular in opto-electronic, sensing, antistatic coating and also for applications in water purification [913]. Though some aromatic amines based polymers show adsorption properties towards toxic Cr(VI) metal but do not possess very large specific surface area resulting quite low removal efficiencies for those toxic metals [14-17]. Graphene, a monolayer of carbon atoms with honey comb structure is the most suitable material for this adsorption purpose as it possesses very high specific surface area and excellent stability

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in aqueous medium [18-21]. Functionalization of graphene oxide is an effective way to remove different toxic metals including Cr(VI) [22-25]. From the literature, we can see that some graphene based materials are used to remove Cr(VI), but they are mainly effective for low concentration [26-28]. To make more effective in high concentration as well as time consumption limit for modern filtration units, we have designed a new graphene based polymer compound by simple oxidation reduction process. In the present work, we have polymerized 2, 6-diamino pyridine for the first time on graphene oxide surface by mutual oxidation reduction mechanism to prepare poly 2, 6-diaminopyridine/graphene (G-PDAP) composite. Large specific surface area and highly porous surface increase its adsorption capacity to 609.76 mg g-1. Now, we can remove high concentration of Cr(VI) 500 mg L-1 within 4 h and 50 mgL-1 Cr(VI) in 30 min only. With increasing adsorbent dose we can remove 500 mg L-1 Cr(VI) in quick 100 min. Generally, removal efficiency for Cr(VI) decreases drastically with increase in pH of the solution. While, we have achieved to overcome this problem with our prepared sulphate ion doped polymer composite. It shows ~71, 59 and 46% removal efficiency at pH 5, 7 and 9 respectively. At lower acidic condition, facile anions exchange between mobile dopant sulphate and chromate ions helps to increase Cr(VI) removal efficiency. Good recycling ability with ~92 % removal efficiency after fifth cycle is also achieved for this material. We believe, this study will provide a good impact for waste water purification particularly in tannery industrial area.

2. Experimental 2.1. Materials 2, 6-diamino pyridine (DAP) is purchased from Sigma-Aldrich. 1, 5-diphenyl carbazide (DPC), Phosphoric acid (85 wt %), acetone, nitric acid (70 wt %), hydrogen peroxide (30%), methanol

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(99%), sulphuric acid (98 wt %), ammonia soln. (25%), potassium dichromate, ammonium persulfate (APS) are purchased from Merck Chemicals. Ultrafine graphite powders (Loba), sodium nitrate (Merck), sodium chloride (Merck), sodium hydroxide (SFDL), potassium permanganate (Merck) are also used. All the chemicals are of analytical-grade and used as it is without purification. 2.2. Synthesis of G_PDAP composite In the first step, we have synthesized graphene oxide (GO) from graphite powder using modified Hummer’s method [29]. Then, we take 10 ml of as prepared GO solution and dilute to 100 ml with distilled water in a 250 ml beaker. The solution is sonicated for 1 h in an ultrasonic bath to form a homogeneous solution. 50 mM DAP solution taken in 1 M H2SO4 is added to this GO solution drop wise and cooled in an ice bath. Then, 0.1 M APS solution in H2SO4 is added to it keeping the temperature at 0 ºC. Immediate blackening of the solution is observed as a result of polymerization of DAP molecules. The system is kept for 24 h with constant stirring at room temperature. Finally, the deep black solution is filtered, washed with distilled water and ethanol for several times until the excess APS is removed. The black solid is dried in vacuum oven at 60 °

C for 2 days to get G-PDAP composite. The synthesis scheme is given in Scheme 1.

2.3. Cr(VI) removal experiment Four standard solutions with Cr(VI) concentrations 50, 100, 200 and 500 mg L-1 are prepared dissolving solid Potassium dichromate (K2Cr2O7) in distilled water. The pH of the solution is adjusted to 1 for all the adsorption experiments at room temperature. For all the experiments, 50 ml of Cr(VI) solution is taken in a 250 ml beaker and 50 mg of adsorbent (G-PDAP) is added. 5   

The solutions are then stirred for different time intervals depending on Cr(VI) concentrations. 3 ml from each stirred solution is collected at different time intervals. 0.20 ml of DPC in acetone and 0.1 ml aqueous solution of H3PO4 are mixed with it. The final mixture is kept for 10 minutes to appear red purple color and Cr(VI) concentration is measured using UV-Vis spectrophotometer at wavelength 540 nm. It is to be mentioned that for higher Cr(VI) concentrations (100, 200 and 500 mg L-1), the mother solution (3 ml) is diluted accordingly to compare its removal efficiency [24]. The effect of adsorbent dose is also carried out using 500 mg L-1 Cr(VI) solution for different adsorbent doses at pH 1. The pH dependent removal efficiency is investigated at different pH values (1, 3, 5 and 7) with 500 mg L-1 Cr(VI) solution. Total Cr concentrations are determined by atomic absorption spectrometer (AAS) at different pH levels after each adsorption experiments to quantify the maximum removal capacity of G-PDAP. Cr(III) concentrations are also determined for those solutions . It is to be mentioned that each Cr(VI) removal experiment is conducted for five consecutive times to get the reproducible results within an error of 0.99 in pseudo-second-order is 12   

much better than that for pseudo-first-order (R2 < 0.84). The extracted parameters as obtained from the fitting procedure using the pseudo-second and first order equations are summarized in Table-1. 3.5. Adsorption isotherm Equilibrium adsorption studies are conducted at the concentration ranges varying from 50 to 500 mg L-1 at room temperature with a fixed adsorbent dose of 1 g L-1. To determine the surface property of the adsorbent and the mode of interaction between Cr(VI) ions with the G-PDAP, the experimental data are analyzed with the help of Langmuir and Freundlich isotherm models. From Fig 6(b) and S3, we can see that the correlation coefficient (R2) is higher for Langmuir isotherm model (>0.99) than that of Freundlich model (>0.95). This result reinforces that Langmuir isotherm is more useful to explain the adsorption of Cr(VI) on the polymer adsorbent as it follows the monolayer adsorption mode rather than multilayer. We can also conclude that the adsorption of Cr(VI) occurs at specific homogeneous sites on the adsorbent surface. We have also calculated the maximum adsorption capacity (Qm) and binding affinity (KL) of the adsorbent G-PDAP. The maximum adsorption capacity is 609.76 mg g-1 which is very close to our experimental adsorption capacity. It is to be mentioned that in the present case adsorption capacity is enhanced around 57% compared to our previous results based on monomer composite. The obtained value of KL (0.075) is also in the range of 0 < KL