Adsorption performance of packed bed column for the removal of perchlorate using modified activated carbon

Accepted Manuscript Title: Adsorption Performance of Packed Bed Column for the Removal of Perchlorate Using Modified Activated Carbon Authors: Radhika R., Jayalatha T., Rekha Krishnan G., Salu Jacob, Rajeev R., Benny K. George PII: DOI: Reference:

S0957-5820(18)30147-2 https://doi.org/10.1016/j.psep.2018.04.026 PSEP 1370

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

22-5-2017 26-4-2018 30-4-2018

Please cite this article as: R., Radhika, T., Jayalatha, G., Rekha Krishnan, Jacob, Salu, R., Rajeev, George, Benny K., Adsorption Performance of Packed Bed Column for the Removal of Perchlorate Using Modified Activated Carbon.Process Safety and Environment Protection https://doi.org/10.1016/j.psep.2018.04.026 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.

Adsorption Performance of Packed Bed Column for the Removal of Perchlorate Using Modified Activated Carbon Radhika R*., Jayalatha T., Rekha Krishnan G., Salu Jacob, Rajeev R., Benny K George Analytical and Spectroscopy Division, Analytical Spectroscopy and Ceramics Group, Propallents Polymers Chemicals and Materials Entity,

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Vikram Sarabhai Space Centre, Thiruvananthapuram- 695 022, India

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* e-mail id: [email protected]

Highlights  Perchlorate removal efficiency evaluated for a packed bed column using granular activated

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carbon modified with HCl.

Adsorption behavior studied using Thomas, Yoon-Nelson and Adam-Bohart models.

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First order kinetics followed, tends towards the second order as the initial concentration and

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The chemical regeneration using HCl most effective than thermal method with good

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

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flow rate increased. It followed first order kinetics at bed heights studied.

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efficiency, yield and processability.

Abstract

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The adsorption performance of packed bed column using coconut shell based activated carbon for the removal of perchlorate from water was investigated. The influence of parameters like inlet ion

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concentration, flow rate and bed height on the breakthrough curves and adsorption performance were studied. The results indicated that the adsorption efficiency increased with increase in the initial concentration and the bed height, decreased with increase in the flow rate which in turn resulted in a shorter saturation time. It also revealed that the throughput volume of the aqueous solution increased

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with increase in bed height owing to the availability of more adsorption sites. The adsorption kinetics was analysed using three kinetic models viz. Adam- Bohart, Thomas and Yoon-Nelson models. The maximum adsorption capacity increased with increase in flow rate and initial ion concentration but decreased with increase in bed height. The perchlorate uptake data was also analyzed for first and second order kinetics. The regeneration of spent activated carbon was systematically investigated by thermal and chemical regeneration methods under different operating conditions. 1

Keywords— perchlorate, activated carbon, adsorption capacity, breakthrough curve, column studies, regeneration

1. Introduction Perchlorate and its salts are mainly used in solid propellants for rockets, for manufacturing matches, explosives and fireworks. Perchlorate has received tremendous attention in the recent years, mainly

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due to the challenges faced by the drinking water industry regarding its treatment. The main health concern regarding perchlorate is its ability to interfere with iodine uptake by the thyroid gland resulting in decreased production of the thyroid hormones [Asha et. al. 2009, Rangesh et al. 2009,

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Urbansky et. al. 1998]. The toxicity of perchlorate was revised in 1990 by USEPA after it was discovered that it could be potentially more toxic than was previously thought. USEPA has included perchlorate in the Contaminant Candidate List (CCL) with a reference dose which corresponds to a

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drinking water level of parts per billion (ppb) only. Due to uncertainties in perchlorate toxicity the

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recommended levels vary between 1 & 20 ppb in several states in USA [Urbansky et. al. 1999, Roy et. al. 2012, Mahmudov et. al. 2010, Long et.al., 2012]. The occurrence and toxic nature of

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perchlorate with its unusual physical and chemical properties make it challenging subject to study.

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A significant amount of research has been performed to evaluate treatment alternatives for perchlorate remediation in drinking water [Yang et. al. 2007]. Current technologies available for perchlorate

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remediation in drinking water can be classified into destruction or removal technologies. In the case of destruction technologies, perchlorate is reduced to harmless products. The destruction techniques are

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classified into biological treatment, chemical reduction, and electrochemical reduction [Bardiya et. al. 2011, Venkatesan et. al. 2011, Nadaraja et. al. 2013]. The major removal technologies comprise of adsorption, ion-exchange and membrane filtration [Nadaraja et. al. 2013, Darracq et. al. 2014]. Many

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of the water purifiers use different types or a combination of resins for water treatment. It also removes many organic contaminants viz., nitrophenols, phenols, etc., [Manuel et. al. 2006, Jia et al. 2002, Victor et.al. 2016]. Ion exchange (IX) has been considered as one of the best available technologies for

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perchlorate removal [Gingras et. al. 2002], and many commercial IX resins can offer high perchlorate sorption capacity. However, the regeneration efficiency of the IX resins has been found to be prohibitively poor. Apart from this, perchlorate is merely transferred to a waste brine stream which must itself be treated or disposed off [Bae et al. 2010, Bae et al. 2014, Xiong et al. 2007]. Hence Adsorption using various adsorbents like layered double hydroxide (LDH), organo clay minerals [Xiea et al. 2011, Xuemei, et. al. 2010, Yang et. al. 2012,Yang et. al. 2013, Young et.al. 2013, Lin et. al. 2013, Bagherifam et al. 2014, Yajie et.al. 2014] and activated carbon (AC) is the most attractive 2

method for the removal of perchlorate. Adsorption process can be considered as either chemisorption or physisorption [Lv et. al. 2014, Lin et al. 2013]. In physisorption the impurities are held on the surface of the carbon by weak van der Waals forces while in chemisorption, the forces are relatively strong and adsorption occurs at active sites on the surface. The biggest advantage of using granular activated carbon (GAC) is that it is widely used in the drinking water treatment industry and it would be easy to retrofit to target perchlorate in the water [Chen et. al. 2005, Xu et. al. 2011, Lin et. al. 2013]. Although virgin GAC has not been found to be very effective for perchlorate adsorption, modification

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of GAC had made it comparable to other treatment technologies [Utrilla et.al., 2011]. Surface tailoring of activated carbon with ammonia and cationic surfactants have resulted in significant increase in

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perchlorate removal capacity [Lin et. al. 2013, Byrne et al. 2014, Rovshan et. al. 2015, Xu et. al. 2011, Chen et. al. 2005]. The effect of surfactant type and surfactant loading on activated carbon for perchlorate removal were also studied by several researchers and significant improvement in perchlorate removal over plain activated carbon was achieved [Xu et al. 2011, Jang et.al., 2009, Chen

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et. al. 2005].

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Adsorption is an equilibrium process wherein a consistent danger of adsorbate being released into

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ambient environment on disposal or storage can take place [Irfan et.al. 2013, Andrey et.al. 2001]. Hence spent adsorbent needs to be stabilized after being discarded. At times adsorbate may be a

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resource that needs to be recovered or concentrated. Under such circumstances regeneration of adsorbent facilitates recovery of resources for reutilization and minimizes the demand for virgin

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adsorbents. Thus regeneration assumes essential importance for techno economical viability, environmental and energy benefits [Irfan et.al. 2013, Salvador et.al. 1996].Many methods of

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activated carbon regeneration [Mark et.al. 1992, Richard et.al. 2000] are currently being researched which include, thermal regeneration [Alvarez et.al. 2004, Sabio et.al. 2004, Samonin et.al. 2013], steam regeneration [Zhengyong et.al. 2009,Duan et.al. 2012, Irfan et.al. 2013] pressure swing

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regeneration, vacuum regeneration, micro wave regeneration [Duan et.al. 2012, Ondon et.al. 2014], ultrasound regeneration [Sarra et.al. 2015], chemical regeneration [Huiping et.al. 2002, Samonin et.al. 2013], oxidative regeneration [Cinthia et.al., 2010.], ozone regeneration [Alvarez et.al. 2004],

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bio-regeneration [Wenhui et.al. 2012]. Apart from these regeneration methods combined methods have been also explored e.g. thermo chemical regeneration [Sheintuch et.al. 1999], electro-chemical [Roberto et.al. 1994], etc. This work deals with the development of a cost effective, efficient and user-friendly technique for the removal of perchlorate from water using chemically modified activated carbon. GA C modified by acid and base treatment were used for the removal of perchlorate from water and found that maximum adsorption capacity was obtained for HCl and H3PO4 modified GAC [Rekha et. al. 2017] 3

Column study and breakthrough analysis were carried out using GAC-HCl to get an understanding of the effects of inlet flow rate, adsorbent mass, initial concentration and column aspect ratio on the adsorption of perchlorate in a fixed bed column. Fixed bed operation is influenced by equilibrium, kinetic and hydraulic factors [Moumita et. al. 2013, Sekhula et. al. 2012, Lavinia et. al. 2015, Sushanta et. al. 2014, Nwabanne et. al. 2012, Kulkarni et. al. 2015]. Thomas, Yoon-Nelson and Adam- Bohart models were used to analyze the column performance for the removal of perchlorate from water. An inference on the perclorate uptake kinetics was also done.

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Regeneration studies were performed to compare the reactivation of spent GAC using thermal method under different atmosphere and chemical method. The perchlorate adsorption capacities of

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the regenerated GAC obtained were compared with the non-exhausted GAC-HCl.

2. Experimental 2.1 Materials

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The GAC used in the experiments was coconut shell based with particle size 0.4 mm to 1 mm

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obtained from Active Char Products Pvt Ltd., Kochi. The following chemicals used for the study

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were purchased from Merck, India and of ACS grade: sodium chloride, hydrochloric acid, sulphuric acid, phosphoric acid, nitric acid, acetic acid and formic acid. Tartaric acid, oxalic acid and citric

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acid of GR grade were purchased from Sigma Aldrich. Ammonium perchlorate used for this study was prepared in house by the electrolytic oxidation of sodium chlorate to perchlorate followed by

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double decomposition with ammonium chloride solution at 100°C and the ammonium perchlorate formed is separated by cooling the reaction mixture to 5°C. Deionized (DI) water was used for

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preparing analytical standards of perchlorate solutions. The DI water was produced by a Millipore MilliPac 40 system, and it hosted an electrical resistivity of ≥ 18 MΩ/cm2.

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2.2 Preparation and efficiency evaluation of modified GAC GAC was stirred with 1M of preferred inorganic/organic acid/base; the excess acid/base was washed with water and air dried. The acid/base treated GAC was stirred with 100 mL of 1000 mg/L perchlorate solution for 1 hour and evaluated perchlorate adsorption efficiency by Ion

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Chromatography (IC). Dionex model ICS 2000 IC equipped with AS16 column, ASRS 300 Suppressor and conductivity detector was employed for the analysis of perchlorate using 35 mM NaOH as eluent with a flow rate of 1 mL/min. Chromeleon chromatographic software was used for the data analysis of chromatograms by peak area / peak height methods. 2.3 Characterisation of Activated Carbon

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The physical charaterisation of GAC and treated GAC (GAC-HCl) was done by following ASTM methods given in Table 1. The moisture content in the sample was estimated by drying in an air oven at 105°C and the difference in weight gave the moisture content. The measurement of the extent of the pore surface developed within the matrix of the carbon block was done by using Quantachrome NOVA 1200 surface area and pore size analyzer. The specific surface area was determined by nitrogen adsorption of the sample by utilizing BET approach. The sample was preheated at 150 °C in vacuum condition and nitrogen adsorption was performed at 77 K. The surface and morphological

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study of samples were carried out by using Carl Zeiss, Supra 55 model field emission scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDX)which identifies

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the elemental composition of materials. 2.4 Column Experiments

A glass column of 1cm internal diameter and 65 cm length was used for the fixed bed column studies. The activated carbon having 0.4 mm to 1 mm particle size range was used. The activated

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carbon was packed in the column with a layer of glass wool at the bottom. Bed heights of 10 cm (3 g

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of GAC-HCl), 30 cm (10 g of GAC-HCl) and 50 cm (17 g of GAC-HCl) were used. The flow rates

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ranging of 1 to 15 mL/min were used while initial ion concentrations varied from 50, 100 and 200 mg/L. The effluent samples were collected at specified intervals and analyzed for the residual

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perchlorate concentration using IC. Column studies were terminated when the column reached exhaustion. The column studies are carried out at room temperature for practical purpose.

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2.5 Kinetic models

In this work, three models were used namely (1) Thomas, (2) Yoon-Nelson and (3) Adam- Bohart

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models for kinetic studies [Lavinia et.al. 2015, Moumita et. al. 2013, Sekhula et. al. 2012, Sushanta et. al. 2014, Nwabanne et. al. 2012]. The Thomas model, also known as the bed-depth-service-time (BDST) model, ignores both the intraparticle (solid) mass transfer resistance and the external (fluid-

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film) resistance directly. This means that the rate of adsorption is controlled by the surface reaction between the adsorbate and the unused capacity of the adsorbent. Its derivation assumes Langmuir kinetics of adsorption- desorption and without any axial dispersion. The expression by Thomas for

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an adsorption column is given as follows: ln[( C0⁄Ce) − 1] =

KTq0M KTC0V − … … … … … . (1) Q Q

where Ce, Co = the effluent and initial solute concentrations (mg/L), qo = the maximum adsorption capacity (mg/g), M = the total mass of the adsorbent (g), Q = volumetric flow rate (mL/min), V= the throughput volume (mL) and KT = the Thomas rate constant (mL/min/mg).

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Yoon and Nelson model is based on the assumption that the rate of decrease in the probability of adsorption for each adsorbate molecule is proportional to the probability of adsorbate adsorption and the probability of adsorbate breakthrough on the adsorbent. This model requires no data about the characteristics of the system such as the type of the adsorbent and the physical properties of the adsorption bed. The linearized form of the Yoon and Nelson model is as follows: ln [Ce⁄(C0-Ce)] = KYN t-  KYN

……………………..(2)

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where KYN is the rate constant (l/min), τ is the time required for 50% adsorbate breakthrough (min) and t is the breakthrough (sampling) time (min), Ce is the concentration of perchlorate in the effluent and C0 is the initial concentration of perchlorate. According to the Yoon–Nelson model, the amount

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of perchlorate ion sorbed in a fixed bed is half of the total perchlorate ion entering the adsorption bed within 2τ period. In this context, for a given bed, the column sorption capacity in the Yoon–Nelson

q(Total) m

=

Cor 1000m

… … … … … . . (3)

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(qo)YN =

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model, (q0)YN can be computed with the following equation:

time required for 50% sorbate breakthrough.

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where C0 is initial concentration (mg/L); r is flow rate (mL/min); m is weight of sorbent (g) and τ is

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Adams and Bohart established a fundamental equation describing the relationship between C e /C0 and t. It was derived on the basis of the assumption that the adsorption rate is proportional to both the

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residual capacity of the adsorbent and the concentration of adsorbate; therefore, the rate of the sorption is proportional to the fraction of sorption capacity still remains on the sorbent [Lavinia et.al.

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2015, Moumita et. al. 2013, Sekhula et. al. 2012, Sushanta et. al. 2014, Nwabanne et. al. 2012]. This model assumes that the adsorption process is continuous and that equilibrium is not attained instantaneously. The Adam-Bohart model is used for the description of the initial part of the

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breakthrough curve. The linearised form of the Adam-Bohart model is expressed as equation (4). ln [Ce⁄(C0-Ce)] = KAB C0t- KAB 𝑁AB (𝑍/F)………………..(4) where, kAB is the kinetic constant (L mg-1 min-1), F is the linear flow rate calculated by dividing the

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flow rate by the column cross-sectional area (cm min-1), Z is the bed depth (cm) of the column, and NAB is the maximum ion adsorption capacity per unit volume of the adsorbent column also known as the saturation concentration (mg L-1). The kinetics of solute uptake on adsorbent was studied by varying the parameters like flow rate, bed height and initial concentration. 2.6 Regeneration of spent GAC

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Regeneration of spent GAC-HCl was attempted by thermal, hot water extraction and chemical regeneration methods. A known weight (20g) of spent GAC-HCl was taken for the batch studies. Thermal regeneration was done by electrically heat treating the spent adsorbent in a tubular furnace at350˚C in air as well as inert atmosphere. In hot water extraction method, the spent GAC-HCl was heated at 70- 90°C with 50 ml DI water on hot plate for 15 minutes. Chemical regeneration was done using sodium chloride (2M) and HCl (2M). The spent GAC-HCl was equilibrated in a mechanical shaker for 1 hour.

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The regeneration efficiency [Abbas et.al. 2008] of the regenerated GAC-HCl was determined by evaluating the perchlorate (100 mg/L) removal capacity (by IC) after drying at 105˚C for 2 hours in

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an air oven. Calculation of regeneration efficiency

The regeneration efficiency (RE) of the AC was calculated as follows: 𝐴𝑟

𝑅𝐸 (%) = (𝐴 ) 𝑋 100

--------------(5)

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where A0 is the original adsorption capacity of the GAC and Ar is the adsorption capacity of the

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regenerated AC.

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3. Results and discussion

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The physical properties of the GAC used for the study were measured by following ASTM test methods. The various parameters studied, the test method numbers, and the results obtained are

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indicated in the Table 1. It is clearly observed on the SEM images given in Figure 1(a) and 1(b) that the smooth surface of virgin GAC was converted to rough surface with increased open pores. This may result in easy access of perchlorate throughout the GAC reactive centres. The increased

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porosity after treatment also reflected in the increased surface area and iodine number of treated

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GAC. The ash or metallic content in GAC was reduced considerable after the acid treatment.

3.1 Adsorption of perchlorate on acid/ base modified GAC GAC was treated with acid and base to improve the adsorption efficiency. It was observed that, acid

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treated GAC shows better adsorption efficiency than base treated and untreated GAC [Rekha et.al. 2017]. On acid treatment the surface area and acidity in GAC is improved which in turn enhances the perchlorate adsorption capacity of GAC-HCl by electrostatic interaction, ion exchange and Vander Waals force of attraction.

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The maximum adsorption capacity of GAC-HCl was evaluated for an adsorbent dose of 1g and varying the adsorbate concentration from 50 mg/L to 7000 mg/L. The variation of the adsorption capacity qo (mg/g) with concentration of perchlorate (mg/L) is given in the Figure 2. there after it became a constant. A maximum adsorption capacity of 36 mg/g was obtained for GACHCl. 3.2 Column Studies 3.2.1 Effect of flow rate on breakthrough curves

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The effect of flow rate for the adsorption of perchlorate onto GAC-HCl at flow rates of 1, 5, 10 and 15mL/min, with an initial concentration of 100 mg/L, bed height of 50 cm and at room temperature

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(RT)is shown in Figure 3. It is observed that for the flow rates from 1 to 10 mL/min., no perchlorate was obtained in the effluent upto 1500 mL and for a breakthrough concentration (Ce/C0= 0.01 where Ce is the concentration of perchlorate in the effluent and C0 is the initial concentration of perchlorate) of 1mg/L was obtained at about 2500 mL. But when the flow rate was increased to 15 mL/min., the

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treatable volume was reduced to 2000 mL for the same breakthrough concentration of 1mg/L.

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As flow rate increased, the breakthrough curves become steeper and reached the breakthrough

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quickly. This is because of the residence time of the adsorbate in the column is not long enough for adsorption equilibrium to be reached at high flow rate. This means that the contact time between the

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adsorbate and the adsorbent is minimized, leading to early breakthrough. Increasing flow rate gave rise to a shorter time for saturation. This type of observation were also noted for the adsorption

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studies carried out using activated carbon prepared from oil palm fibre studied for the removal of Pb [Nwabanne et. al. 2012] and activated carbon prepared from Euphorbia antiquorum L. for the

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removal of basic Red 29 [Sivakumar et. al. 2009]. The breakthrough occurred faster at higher flow rate than at lower flow rate. The faster breakthrough curve exhibited was attributed to faster movement of the adsorption zone along the bed, thus reducing the contact time between the influent

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and the adsorbent [Sekhula et. al. 2012]. 3.2.2 Effect of bed height on breakthrough curves Breakthrough curves for the adsorption of perchlorate at variable bed heights of 10cm, 30cm and

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50cm, with an initial concentration of 100 mg/L, flow rate of 10 mL/min., at RT and breakthrough concentration of 1 mg/L are shown in Figure 4. The results indicate that the throughput volume of the aqueous solution increased with increase in bed height, due to the availability of more number of sorption sites. The equilibrium sorption capacity decreased with increase in bed height. This shows that at smaller bed height, the effluent adsorbate concentration ratio increased more rapidly than for a higher bed height. Further, the small bed height corresponds to less amount of adsorbent bed and is 8

saturated in less time [Nwabanne et. al. 2012].This was attributed to the fact that as the bed height increased from 10 to 50 cm, more contact time with the adsorbent resulted in higher removal of perchlorate. The slope of the breakthrough curves decreased with increase in bed height as a result of broadened influent movement zone [Sekhula et. al. 2012]. Higher perchlorate removal was observed for bed height of 50 cm because of the increase in the bed surface which provided more binding sites

3.2.3 Effect of initial perchlorate concentration on breakthrough curves

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for the adsorption of adsorbate onto the adsorbent.

The effect of initial adsorbate concentration at ambient temperature on the breakthrough curves at a bed height of 50 cm, flow rate of 10 mL/min and breakthrough concentration of 1 mg/L is shown in

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Figure 5. It is observed that as the initial ion concentration increased from 50 to 200 mg/L, the breakthrough time decreased. On increasing the initial ion concentration, the breakthrough curves became steeper and breakthrough volume decreased because of the lower mass-transfer flux from the

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bulk solution to the particle surface due to the weaker driving force. Similar observations found in

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the adsorption studies using activated carbon derived from oil palm fibre for the removal of Pb [Nwabanne et. al. 2012]. At higher concentrations, the availability of the perchlorate ion for the

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adsorption sites is more which leads to higher uptake of perchlorate even though the breakthrough

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time is shorter than that of lower concentrations. As the concentration of the adsorbate is increased the breakthrough curve shifts toward the origin. This behavior may be explained by the fact that the

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binding sites became saturated more quickly in the system at higher initial concentrations [Lavinia et. al. 2015].The saturation time and the volume of solution processed at the saturation point decreased with the increase in initial concentration.

According to the obtained results, the

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perchlorate adsorption capacity of GAC-HCl at breakthrough and saturation is greater at higher initial concentrations. This increasing trend is due to the fact that at higher initial concentration of

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perchlorate, the increased concentration gradient between the surface of the sorbent and the solution results in an improvement of the driving force for mass transfer initially which drastically reduces causing a reduction in the treatable volume. These results agree with those of other published studies

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concerning fixed bed column study on the removal of chromium by using hemp fibers [Lavinia et. al. 2015].

Early appearance of breakthrough point with increasing concentration in solution is due to the arrival of high mass of solute per unit area on the adsorbent surface when the fresh feed solution appeared over the bed from the reservoir, and the primary adsorption zone (PAZ) mobilized fast across the bed. Late arrival of breakthrough point for the feed solution with lower concentration of the

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adsorbate was due to the appearance of less solute mass per unit area from the reservoir on the bed surface, indicating slow mobilization of PAZ across the bed [Sushanta et. al. 2014]. 3.3 Column kinetic study 3.3.1 Thomas Model Thomas model has been used to study the packed bed adsorption kinetics for 100% breakthrough

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concentration. The Thomas model is suitable for adsorption processes where external and internal diffusions will not be the limiting step. The column data were fitted to the Thomas model to determine the Thomas rate constant (KT) and the maximum adsorption capacity (qo). The kinetic rate

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(KT) and the adsorption capacity of the bed (qo) were determined from the plot of In [(Co/Ce)-1] against volume in mL as shown in Figures 6, 7 and 8.

The values of KT, qo and R2 were obtained using linear regression analysis according to equation (1) and are given in Table 2. Thomas rate constant, KT is dependent on flow rate, initial ion

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concentration and bed height. The KT value increased with increase in flow rate and bed height but

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decreased with an increase in initial ion concentration. The maximum adsorption capacity, qo

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increased with increase in flow rate and initial ion concentration but decreased with increase in bed height. This can be justified by taking into account of the fact that the gradient concentration is the

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driving force of the sorption process. Thus, a higher driving force due to an increased concentration of perchlorate results in an improved performance of the column. The R2 values range from 0.97 to

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0.99 which shows that Thomas model fits very well with the experimental breakthrough data. This model predicts monolayer sorption.

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Thomas rate constant, KT, decreases as the initial concentration increases. At the same time, the maximum sorption capacity determined by the Thomas model, qo increases as the initial metal

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concentration increases. These trends are in good agreement with recent literature data reporting on the sorption of Cr (III) ions from aqueous solution by hemp fibers in a fixed bed column [Lavinia et. al. 2015].

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3.3.2 Yoon - Nelson model The kinetic rate (KYN) and the 50% breakthrough time (τ) were determined from the plot of ln [Ce/( C0 - Ce) against time t in min. as shown in Figures 9, 10 and 11. The Yoon and Nelson model parameters KYN, τ and qo for various operating conditions are calculated based on the linear regression equation (2) and equation (3). The values of KYN, τ and qo are listed in Table 3.It is clear from Table 4 that the values of the Yoon–Nelson rate constant, KYN, increase as the initial 10

concentration increases. This may be explained by the fact that the increase in initial concentration of the perchlorate ion increases the competition between the sorbate species for the sorption sites, which ultimately results in a higher rate of retention. It is known that KYN and τ are inversely related, so that, as expected, the time required for 50% breakthrough (τ) decreases with increasing the influent concentration of perchlorate ion. The GACHCl column sorption capacity, calculated based on the results of the Yoon– Nelson model, with the increase in bed height, the τ and KYN values increased while KYN increased and τ decreased for

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increased flow rate. The maximum adsorption capacity, qo increased with increase in initial ion concentration but decreased with increase in flow rate and bed height. The R2 values range from 0.97

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to 0.99. 3.3.3 Adams Bohart model

The Adam- Bohart adsorption model was applied to experimental data for the breakthrough curve and a linear relationship was found for breakthrough time. For all breakthrough curves, respective

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values of NAB and KAB were calculated and presented in Table 5. The kinetic rate (KAB) and the

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saturation concentration (NAB) were determined from the plot of ln(Ce/C0) against time t in min and

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indicated by Figures 12, 13 and 14.

From Table 5, it can be seen that the values of KAB increased as the flow rate and the bed height

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increased and decreased with increase in initial ClO4- concentration. The value of NAB increased with increase in inlet concentration whereas it decreased with increase in bed height and flow rate.

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However, the decrease in the value of NAB with increase in flow rate is not very large and the decrease in NAB with increase in bed height is very significant. This suggests that the overall system

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kinetics may have been influence by external mass transfer, particularly in the initial part of adsorption in the column [Sekhula et. al. 2012]. The correlation coefficient, R2 values were between 0.88-0.95. The lower R2 values relative to the other models suggest that the Adam-Bohart model is

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not an appropriate predictor for the breakthrough curve. 3.4 Kinetics of perchlorate uptake The kinetics of the solute uptake on the GAC-HCl column by varying the various parameters like

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initial concentration, the flow rate and bed height was studied. Ordinary first and second order models were tested for the data obtained. For first order model Ce/C0 on logarithmic scale was plotted against time and for second order model 1/Ce -1/C0 was plotted against time. 3.4.1

Effect of initial concentration on perchlorate uptake

All the parameters (bed height = 50 cm, flow rate = 10 mL/min.) other than initial concentration were kept constant at RT. At low concentration of perchlorate, the adsorption followed first order kinetics as shown in Figures 15 and 16. This indicates that adsorption is dependent on the number of 11

sites available for adsorption rather than the solute concentration. As the initial concentration is increased from 50 mg/L to 200 mg/L, the adsorption tends to go from first order to second order kinetics. This means that adsorption depends on both, the solute concentration and the available adsorbent sites. From the Figures15 and 16, it is evident that at low concentration of perchlorate (50 mg/L), uptake kinetics followed first order model better, with R2 value of 0.947 but the second order kinetics was

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not satisfactory with R2 value of 0.656. As the concentration increased, the solute uptake kinetics fitted first order equation better but tends to move towards the second order. This is confirmed by the fact that the R2 value of first order decreased and the R2 value of second order increased. Effect of flow rate on perchlorate uptake

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3.4.2

Effect of flow rate was studied at RT by keeping all other parameters constant ((bed height = 50 cm, initial concentration of perchlorate= 100mg/L) and the results are shown inFigure17and 18. The flow

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rate was varied from 1 mL/min to 15 mL/min. At lower flow rates (1, 5, 10 ml/min.), the kinetics was first order with R2 value ranging from 0.960 – 0.944, but did not follow the second order

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kinetics as the R2 value was in the range 0.664 – 0.491.At 15 ml/min. flow rate, second order kinetics

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(R2=0.950) was observed rather than first order equation (R2=0.883). The increase in flow rate

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allows more solute ions to come in contact with adsorbent; hence it dependents both on the availability of adsorbent sites and the solute coming in contact with these sites. Effect of bed height on perchlorate uptake

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3.4.3

Effect of bed height on perchlorate uptake kinetics at RT is presented in Figure 19and 20. The flow

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rate and the initial perchlorate concentration were fixed as 10 ml/min and 100 mg/L respectively. Figure 19 depicts the effect of bed height of 10 cm. The solute uptake kinetics fitted first order

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equation better with R2 value of 0.948 as against second order with R2 value of 0.924, which is comparable. The adsorption depends not only on the solute concentration but also on the sites available for adsorption. At lower bed height, due to insufficient adsorbent sites, the first order kinetics is followed better than the second order kinetics. With increase in bed height from 10 to 50

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cm, the solute uptake kinetics followed first order model rather than second order model. The R2values for first order kinetics are 0.927 which is higher compared to R2value of 0.532 for second order kinetics. It indicated that at bed height of 50 cm, though more adsorbent was available, the rate depends on solute concentration, which was kept constant. 3.5 Regeneration of Spent GAC-HCl

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Regeneration studies were carried out by three methods viz., thermal, hot water extraction and chemical regeneration methods. Thermal regeneration of the spent adsorbent in air showed a weight loss of about 10% and an additional step of surface modification with HCl was also required. A slight improvement in the perchlorate adsorption capacity (26.3mg/g from 24.8mg/g) was observed which is attributed to the increased surface area from 1341m2/g for non-exhausted GAC-HCl to 1405m2/g for the regenerated GAC-HCl. In the case of thermal regeneration in inert atmosphere (Ar),

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a weight loss of  5% was observed and the surface area was comparable (1395m2/g) with that of regenerated GAC-HCl in air. Five regeneration cycles were carried out and the perchlorate adsorption capacity remains consistent but a consistent weight lose was also observed in each cycle.

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In hot water extraction method, the spent GAC-HCl was heated at 70- 90°C with 100 mL DI water and at this temperature, the leached perchlorate content in the DI water was only 60% of the total perchlorate adsorbed in the spent GAC-HCl. Hence, the regeneration efficiency was lower. Chemical regeneration was done using (i) 100 mL of 2M sodium chloride and (ii) 100 mL of 2M HCl. Both the

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regenerating agents gave very good regenerating efficiency of >95% for 2M sodium chloride

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and >96% for 2M HCl for five cycles. Figure 21 shows the regeneration efficiency obtained for the

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five cycles using HCl as the regenerating agent.

The main drawback of using sodium chloride as the regenerating agent, is the involvement of an

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additional step of washing off the excess sodium ions in the material and further modification using HCl. But this additional step is eliminated if HCl is used as the regenerating agent. In both the cases

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the resultant solution can be mixed with the mother liquor used for the production of ammonium perchlorate. Hence, HCl is the most suitable regenerating agent for the treatment of spent GAC-HCl

4. Conclusions

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and make it reusable with less complication.

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Granular Activated Carbon (GAC) is found to be an effective and low-cost adsorbent for the removal of perchlorate from water. The present study shows that the perchlorate removal efficiency of virgin GAC can be increased by acid treatment. Results suggest that HCl modified GAC is a potential material for perchlorate removal from water. The process variables such as flow rate, bed

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height and initial perchlorate concentration have significant impact upon the adsorption of perchlorate in a fixed bed column. Within the studied flow rate range of 1 to 15 mL/min, an optimum breakthrough and exhaustion points were found at a flow rate of 10 mL/min for a breakthrough concentration of 1ppm. The fixed bed adsorption system was found to perform better with lower perchlorate inlet concentration, lower feed flow rate and higher GAC-HCl bed height. Comparing the values of R2 and breakthrough curves, both the Thomas and Yoon-Nelson models

13

can be used to describe the behaviour of the adsorption of perchlorate in a fixed-bed column. The qo values calculated by these models are also comparable. The experimental data fit well with the Thomas and Yoon-Nelson models, but the Adam-Bohart model predicted poor performance of fixed bed column. At low initial concentration of perchlorate, the adsorption followed first order kinetics and tends to move towards the second order at higher concentrations. The same trend was observed in the case of flow rate. While with increase in the bed height, the break through time was delayed and perchlorate uptake kinetics followed first order rather than second order. Regeneration study was

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carried out by thermal, hot water extraction and chemical methods. Based on the regeneration efficiency, yield of the regenerated material and ease of operation, chemical regeneration using HCl

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is most effective. This gives a closed loop approach there by all the three environmental media (air, water and soil) are protected. 5. Acknowledgement

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The authors acknowledge Director and Deputy Director (Propellants, Polymers, Chemicals and

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Materials Entity), Vikram Sarabhai Space Centre for the support.

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Figure Captions Fig.1 SEM images of virgin and treated GAC a) GAC and b) GAC HCl Fig.2 Variation of the adsorption capacity q0 with concentration of perchlorate (mg/L) Fig. 3Effect of flow rate on breakthrough curves for perchlorate Fig. 4Effect of bed height on breakthrough curves for perchlorate Fig. 5Effect of initial adsorbate concentration on breakthrough curves for perchlorate

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Fig. 6Thomas plot for the effect of flow rate on adsorption of perchlorate on GAC-HCl

Fig.7Thomas plot for the effect of bed height on adsorption of perchlorate on GAC-HCl

Fig. 8Thomas plot for effect of initial concentration on adsorption of perchlorate on GAC-HCl

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Fig. 9Yoon - Nelson plot for effect of flow rate on adsorption of perchlorate on GAC-HCl

Fig. 10Yoon - Nelson plot for effect of bed height on adsorption of perchlorate on GAC-HCl Fig. 11Yoon - Nelson plot for effect of initial conc. on adsorption

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Fig. 12 Adams Bohart plot for effect of flow rate on adsorption of perchlorate on GAC-HCl Fig. 13Adams Bohart plot for effect of bed height on adsorption of perchlorate on GAC-HCl

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Fig. 14Adams Bohart plot for effect of initial concentration on adsorption

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Fig.15a First order kinetics, initial concentration 50 mg/L

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Fig.15b Second order kinetics, initial concentration 50 mg/L Fig.16a First order kinetics, initial concentration 200 mg/L

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Fig.16b Second order kinetics, initial concentration 200 mg/L Fig.17a First order kinetics, flow rate1mL/min Fig.17b Second order kinetics, flow rate1 mL/min.

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Fig.18a First order kinetics, initial concentration 15 mL/min. Fig.18b Second order kinetics, initial concentration 15 mL/min.

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Fig.19a First order kinetics, bed height 10 cm Fig.19b Second order kinetics, bed height 10 cm Fig.20a First order kinetics, bed height 50 cm Fig.20b Second order kinetics, bed height 50 cm

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Fig.21 Regeneration efficiency using HCl as the regenerating agent.

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Figures

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Table caption Table 1 Characterisation of Granular activated carbon (GAC) GAC

GAC-HCl

Moisture (%)

ASTM-D-2867

3.0

0.23

Apparent Density (g/cc)

ASTM-D-2854

0.522

0.49

Iodine No. (mg/g)

ASTM-D-4607

1156

1332

Surface Area (m2/g)

ASTM-(BET-N2)

1159

1341

pH

ASTM-D-3838

10.2

2.3

Ash (%)

ASTM-D-2866

2.8

0.80

Ball Pan Hardness (%)

ASTM-D-3802

98.3

98.5

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Test Method

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Table 2 Calculated column kinetic parameters by Thomas Model for perchlorate adsorption on GAC-HCl under various operating conditions Flow rate Bed Height [ClO4-]initial KT qo (mg/g) R2 (mL/min.) (cm) (mg/L) (mL/min/mg) 1 50 100 0.04 22.1 0.993 5 50 100 0.2 21.5 0.992 10 50 100 0.4 21.5 0.995 15 50 100 0.45 22.6 0.977 10 10 100 0.3 62.6 0.980 10 30 100 0.3 32.8 0.965 10 50 100 0.4 21.7 0.984 10 50 50 0.6 20.1 0.983 10 50 100 0.3 25.0 0.989 10 50 200 0.2 33.4 0.970

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Model for perchlorate R2

22.7 21.5 21.1 19.8 56.9 25.9 21.8 20.1 22.7 25.1

0.991 0.994 0.996 0.979 0.980 0.965 0.984 0.983 0.989 0.970

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qo (mg/g)

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Table 3Calculated column kinetic parameters by Yoon-Nelson adsorption on GAC-HCl under various operating conditions Flow rate Bed Height [ClO4-]initial KYN τ (min.) (mg/L) (mL/min.) (cm) (mL/min/mg) 100 1 50 0.004 3860 100 5 50 0.02 733.5 100 10 50 0.041 359.3 100 15 50 0.052 224.8 100 10 10 0.033 199.2 100 10 30 0.038 259.0 100 10 50 0.040 370.0 50 10 50 0.030 683.3 100 10 50 0.033 386.7 200 10 50 0.043 213.6

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Table 4Calculated column kinetic parameters by Adams Bohart Model for ClO4- adsorption on GAC-HCl under various operating conditions Flow rate Bed Height [ClO4-]initial KAB NAB R2 (mL/min.) (cm) (mg/L) (x 10-4L/mg (mg/L) min) 1 50 100 0.3 8486 0.936 5 50 100 1.5 8306 0.939 10 50 100 2.9 8421 0.933 15 50 100 3.4 8267 0.955 10 10 100 2.0 27462 0.882 10 30 100 2.5 10668 0.943 10 50 100 2.6 8565 0.934 10 50 50 3.8 7971 0.936 10 50 100 1.9 9739 0.934 10 50 200 1.35 11261 0.939

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