Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions

Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions

Author’s Accepted Manuscript Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqu...

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Author’s Accepted Manuscript Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions Chi-Chuan Kan, Mario Jose R. Sumalinog, Kim Katrina P. Rivera, Renato O. Arazo, Mark Daniel G. de Luna www.elsevier.com/locate/gsd

PII: DOI: Reference:

S2352-801X(16)30073-X http://dx.doi.org/10.1016/j.gsd.2017.07.004 GSD65

To appear in: Groundwater for Sustainable Development Received date: 21 December 2016 Accepted date: 26 July 2017 Cite this article as: Chi-Chuan Kan, Mario Jose R. Sumalinog, Kim Katrina P. Rivera, Renato O. Arazo and Mark Daniel G. de Luna, Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions, Groundwater for Sustainable Development, http://dx.doi.org/10.1016/j.gsd.2017.07.004 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 galley proof before it is published in its final citable 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.

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Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for

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hexavalent chromium removal from aqueous solutions

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Chi-Chuan Kana1, Mario Jose R. Sumalinog IIb2, Kim Katrina P. Riverac3, Renato O. Arazob,d4,

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Mark Daniel G. de Lunac,*

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a

Institute of Hot Spring Industry, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan b Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines c Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines d College of Engineering and Technology, University of Science and Technology of Southern Philippines, Claveria 9004, Philippines *

Corresponding author. Ph.D, Tel.: +632-981-8500 local 3114; Fax: +632-929-6640

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[email protected],

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[email protected]

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[email protected]

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[email protected]

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[email protected]

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[email protected]

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ABSTRACT:

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Groundwater treatment residuals are rich in nano-sized metal oxides which have the potential to remove various water contaminants. In this study, hexavalent chromium adsorbents

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Tel: +886-6-366-2006; Fax: +886-6-366-2668 Tel: +632-981-8500 local 3114 3 Tel: +632-981-8500 local 3114 4 Tel: +63-917-827-5540 2

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were synthesized from the residuals of a 30,000 m3 d-1 groundwater treatment plant in Taiwan.

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The effects of acid type and ratio, acid concentration, ultrasonication time, and heating duration

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on adsorption capacity were examined. Adsorbents synthesized using dual-acid solutions yielded

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lower adsorption capacities compared to those which used pure acid solutions. Adsorption

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capacity decreased with higher ultrasonication time and heating duration. Chromium removal is

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improved with higher ionic strength and lower pH of the solution. Results showed high

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correlation between experimental data and the Freundlich isotherm (R2 = 0.9607) and the

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pseudo-second order kinetic model (R2 = 0.9740) suggesting that the dominant mechanism in the

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adsorption process is chemisorption. The involvement of Fe and Mn species during chromium

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adsorption was confirmed by the results of energy dispersive X-ray spectroscopy and Fourier

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transform infrared (FTIR) spectroscopy analyses.

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Keywords:

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Groundwater treatment residuals; Ultrasound irradiation; Hexavalent chromium; Adsorption;

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Isotherm studies

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1. Introduction

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Groundwater is a valuable source of drinking water in places where the supply of surface

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water is inadequate (Kan et al., 2012). Significant concentrations of soluble iron and manganese

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abound in groundwater, but between the two species, iron occurs in greater amounts (El Araby et

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al., 2009). In most aquifers, the redox conditions usually promote the formation of divalent

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manganese whether as hydroxide, sulfate or carbonate (Dashtban Kenari and Barbeau, 2014; Kan

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et al., 2013). During groundwater treatment, these metal species (Fe and Mn) are converted into

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toxic solid residuals (Sotero-Santos et al., 2007). The global implementation of stringent

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environmental policies to address problems related to the management of heavy metal-laded

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solids makes recycling as the most viable option for groundwater treatment residuals.

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The extraction of useful metal oxides from groundwater treatment residuals is a promising

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research area. Acid treatment and ultrasound irradiation have been used to dissolve organic

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components in residuals and concentrate the reusable heavy metals (Yang et al., 2014). Oxides of

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iron and manganese in groundwater treatment residuals have the potential to remove various

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wastewater contaminants including heavy metals (Chiang et al., 2012; Nelson and Lion, 2003).

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The oxides of iron have OH functional groups capable of reacting with other metals while oxides

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or hydroxides of other metals such as Al and Mn can react with dissolved contaminants to form

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ion pairs through surface complexation (Han et al., 2007).

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Ordinarily, metal oxide-containing residuals are not readily usable as adsorbents for

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environmental contaminants because, in their powdered form, they are difficult to separate from

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the treated solution. Also, poor fluid movement in saturated portions limits the application of

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powdered metal oxide adsorbents in continuous column operations (Kundu and Gupta, 2006).

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However, metal species extracted from residuals can be coated onto sand media to facilitate

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solid-liquid separation in metal adsorption applications (Babel and Kurniawan, 2003).

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Hexavalent chromium is among the widely used heavy metals in the metal plating, tanning,

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and electroplating industries (Sepehr et al., 2014). Waste streams from electroplating units may

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contain up to 2,500 mg L-1 Cr(VI) (Dermentzis et al., 2011) and these chromium-containing

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effluents eventually find their way into recipient rivers, lakes, and other water bodies. Chromium

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exists in two oxidation states: hexavalent and trivalent (Hawley et al., 2004). Between the two,

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hexavalent chromium is considered more hazardous to public health due to its carcinogenic and

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mutagenic properties (Costa, 2003; Hamdan and El-Naas, 2014). At acidic solution pH, Cr(VI)

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exists in its anionic form HCrO4- (Gode and Pehlivan, 2005). Due to repulsive electrostatic

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interactions, Cr(VI) anions are poorly adsorbed by the negatively-charged soil particles in the

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environment, allowing these molecules to move freely in the aqueous environments (Silva et al.,

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2009). The removal of chromium from wastewater is achieved mainly through various chemical

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processes. Adsorption is the technology that is commonly applied for heavy metal removal due

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to its low cost and high removal efficiency (Jung et al., 2013).

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In this study, hexavalent chromium adsorbents were synthesized from groundwater

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treatment residuals by varying acid type and ratio, acid concentration, ultrasonication time, and

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heating duration. The well adopted Langmuir (Langmuir, 1918) and Freundlich (Freundlich,

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1906) isotherm equations and the pseudo-first order and pseudo-second order kinetic models

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were applied to experimental data to understand the mechanism of Cr(VI) removal. The results

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of the adsorption studies were validated by various adsorbent characterization techniques

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including surface morphology, elemental composition, pH at point-of-zero-charge (pHPZC) and

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relevant functional groups.

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2. Materials and Methods

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2.1 Chemicals, materials and simulated Cr(VI) wastewater

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All glassware were detergent-washed and then acid-washed for 8 h before use. Residuals

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were obtained from the Chang-Hua Water Treatment Plant in central Taiwan which processes

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groundwater (with iron and manganese concentrations ranging from 0.2 to 0.6 mg L-1) in a series

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of unit operations: aeration, chlorination by sodium hypochlorite and manganese green sand

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filtration (Kan et al., 2012). Groundwater treatment residuals were acid treated using analytical

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grade nitric acid (HNO3, 69%, Merck), sulfuric acid (H2SO4, 96%, Merck), and hydrochloric

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acid (HCl, 37%, Sigma-Aldrich). Synthetic Cr(VI) solutions were prepared by dissolving

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potassium dichromate (K2Cr2O7, 99%, Merck) with deionized water (18.2 MΩ resistivity)

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generated by Purelab deionizers.

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2.2 Adsorbent synthesis

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Silica sand (0.71–1.2 mm) was soaked in 10% HNO3 for 2 h to remove impurities, rinsed

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with deionized water until pH 7 was obtained, and then dried at 105 oC in a precision oven (DV

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453, Channel) before use. Silica sand was coated with groundwater treatment residuals based on

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the procedure reported elsewhere (Kan et al., 2013). The residuals (0.5–0.7 mm) were treated

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with 7 mL acid (HNO3, H2SO4 or HCl) in an Erlenmeyer flask for 30 min. The acid-treated

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residuals were then mixed with 7.0 g sand. A ratio of sand to residual of 10:1 was used. Sodium

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hydroxide (1.0 N, NaOH, 99%, Merck) was introduced to the slurry until its pH reached 7–8.

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The slurry was agitated in a reciprocal shaker (BT-350, Yidher) for 24 h at 100 rpm to ensure

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complete mixing. The slurry was then heated at 105 °C in a precision oven (DV 453, Channel)

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for predetermined durations (8, 24, 36, and 48 h). Groundwater treatment residuals that did not

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coat the sand particles were separated through screening. The adsorbents were stored in sealed

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plastic containers. For ultrasound-assisted acid treatment of groundwater treatment residuals,

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ultrasonic bath (8510, Branson) set at 40 kHz was used instead of a reciprocal shaker. The

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durations of ultrasound irradiation were 5, 30, 60, and 90 min.

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2.3 Analytical methods

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Total chromium was analyzed using inductively coupled plasma – optical emission

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spectroscopy (ICP-OES 2000 DV, Optima) while Cr(VI) concentrations were measured using a

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UV-Vis spectrophotometer (DR 3900, Hach) with a pre-set wavelength and automatic calibration

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for Cr(VI) determination. US EPA Method 8023 (1,5 diphenyl carbohydrazide method) was used

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to directly measure Cr(VI) concentration against blank samples (H2O) set as zero concentration.

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Samples (10 mL) containing Cr(VI) were mixed with ChromaVer 3 reagent powder for 5 min

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before Cr(VI) measurements. The Eq. 1 was used to calculate the adsorption capacity, qad (μg g-

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1

).

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)

(1)

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The determination of the pHPZC of residual coated sand adsorbents was based on the

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method of Noh and Schwarz (Reymond and Kolenda, 1989). RCS dosages of 0.15, 0.3, 1.5, 3.0,

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6.0, and 9.0 g were mixed with deionized water in sealed 30 mL glass bottles for 24 h at 100 rpm

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and 25 °C. The pH of each sample was recorded after 24 h of mixing. Adsorbent morphology

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and chemical composition were analyzed by a scanning electron microscope (SEM, S-3400N,

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Hitachi) and energy dispersive X-ray (EDX), respectively. The functional groups involved in the

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adsorption were analyzed using Fourier transform infrared (FTIR) equipment (6700 Nicolet,

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Thermo Scientific).

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2.4 Batch adsorption experiments

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A known amount of adsorbents (0.2 g) was added to an Erlenmeyer flask containing a 30

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mL Cr(VI) solution at pH 2 and having an initial Cr(VI) concentration of 20 mg L-1. The

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resulting suspension was agitated at 100 rpm in a reciprocal water bath shaker (BT 350, YIH-

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DER) for 24 h at 25 °C. Each sample was pre-filtered by a 0.45 µm syringe filter and stored in

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air tight plastic bottles before Cr concentration determination.

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Kinetic studies were conducted following the procedure described for typical adsorption

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experiments. The adsorption capacities at predetermined contact times were fitted to pseudo-first

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and pseudo-second order kinetic models. Isotherm studies were carried out using a similar

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procedure but with varying initial Cr(VI) concentrations (2, 10, 12, 15, and 20 mg L-1) and fixed

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contact time of 24 h. Experimental data was fitted to Langmuir and Freundlich isotherm models.

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The effect of ionic strength was determined by dissolving known amounts of K2Cr2O7 in

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dilute electrolyte solutions of varying KNO3 (99%, Aldrich-Sigma) concentrations (0.0, 0.05 and

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0.1 M). The pH of the solution was adjusted using NaOH and HCl. A fixed amount of adsorbents

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was added into each electrolyte solution, and the adsorption capacity was measured after 24 h.

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3. Results and Discussion

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3.1 Effect of ultrasound-assisted acid treatment

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Figure 1 shows that adsorbents which had ultrasound-assisted acid pretreatment of

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residuals had higher adsorption capacities (i.e. 147.75 μg g-1 at 1% w/w acid) compared to those

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produced from residuals pretreated with acid alone (i.e. 61.50 μg g-1 at 1% w/w acid). Increasing

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the acid concentration for pretreatment improved adsorption capacities such that at 10% acid

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treatment, the adsorption capacity for adsorbents which had ultrasound-assisted acid

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pretreatment of residuals reached 870.45 mg Cr(VI) per kilogram adsorbent. Ultrasound

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enhances the reduction in particle size of organics, metal oxides, and other residual components.

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With ultrasound-assisted acid treatment, more metals specifically Fe and Mn ions were extracted

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and liberated from the residuals and coated onto the sand. Deng et al. (Deng et al., 2009)

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reported the synergistic effect of ultrasonication and acid treatment to liberate Cu, Zn, and Pb

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ions from flocs.

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In this study, the breakdown of organics and other residual components was enhanced by

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ultrasonication. Sompech et al. (Sompech et al., 2012) reported that prolonged duration of

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ultrasound irradiation increased the surface area of metal oxides (from 20 m2 g-1 to 37 m2 g-1).

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Overall, the increase in residual coated sand adsorption capacity can be attributed to the

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exposure and liberation of more Fe and Mn oxides brought about by the prolonged ultrasound

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and acid treatment of groundwater treatment residuals.

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On the other hand, long exposures (more than 30 min) of residuals to ultrasonication-aided

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acid treatment decreased the adsorption capacity of the adsorbents. After applying 60 min of

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ultrasonication-aided acid treatment, the adsorption capacity decreased from 541.95 to 136.95 μg

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g-1 (Figure 2a). The adsorption capacity decreased further to 10.35 μg g-1 at 90 min of

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ultrasonication time. Prolonged ultrasound-assisted acid treatment (t >30 min) effectively

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reduced the particle size of various residual components (Pilli et al., 2011). As a result, the

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ensuing smaller organic molecules formed Fe-C and Mn-C bonds through the halogen-metal

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exchange, ligand exchange, insertion, haptotropic migration, transmetallation, oxidative

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addition, reductive elimination, and other reactions (Bauer and Knölker, 2008). This increase in

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Fe- and Mn-organic complexes hindered Fe and Mn oxide precipitation, thereby lowering the

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available Cr(VI) adsorption sites.

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Acid concentration during acid treatment also affects the performance of residual coated

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sand. At 30 min ultrasound-assisted acid treatment, increasing acid concentration from 0.1% to

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0.5% improved adsorption capacity by 519.60 μg g-1 (2,324.83%). However, at 60 min of

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treatment, the improvement caused by the increase in acid concentration was only 120.90 μg g-1

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(753.27%). At 90 min of treatment, adsorption capacity increased almost negligibly by 0.90 μg

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g-1 (9.52%). The effect of ultrasound irradiation on adsorption capacity is far greater than that of

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acid concentration.

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In Figure 2b, the highest adsorption capacity (641.2 μg g-1) was obtained when 0.5% w/w

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H2SO4 was used during ultrasound-assisted acid treatment of residuals. The value is close to the

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adsorption capacity of residual coated sand synthesized with residuals treated with 7% w/w

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HNO3 in the absence of ultrasound irradiation (646.50 μg g-1). Other adsorption capacities

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obtained were 340.7 μg g-1 (HCl) and 533.8 μg g-1 (HNO3). H2SO4-treated residuals produced

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adsorbents with the highest adsorption capacity due to the very high acid strength. The pKa

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values, which are inversely proportional to acid strength, of H2SO4, HCl, and HNO3 are -7, -5.2,

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and -1.4 respectively (Daffalla et al., 2012). Also, the dibasic state of H2SO4 caused it to release

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one H+ more than the other acids, making it the strongest among the three acids investigated.

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H2SO4 caused the highest organic floc decomposition, thereby exposing more Fe and Mn oxides

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from residuals.

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The best among the acid combinations was HCl-H2SO4 giving 28.3 and 13.6 μg g-1 for 1:1

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and 1:2 acid molar ratios, respectively. The extent of floc dissolution was lower when two acids

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are combined compared to a single acid treatment. The hydrogen ions produced by the stronger

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acid tend to suppress the dissociation of the weaker one, and both will tend to suppress the

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dissociation of water, thus reducing the source of H+ (Merrill and Logan, 2009) and reducing the

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acid strength.

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The highest adsorption capacity is achieved at 24 h heating duration with 541.95 μg g-1

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(Figure 2c). The heating process caused the decrease of the hydroxide component making the

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anhydrous Mn-O-Mn the dominant Mn species in the oxide (Chang et al., 2004). In a separate

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study of Laurent et al. (Laurent et al., 2011), it was reported that preheating of residuals before

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adsorption increased its affinity to metal ions. Thermal treatment can cause the formation of

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amorphous Fe/Mn oxide phases, which are noted to have higher surface areas.

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Increasing the heating duration from 24 to 48 h, however, decreased the adsorption

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capacity to 241.20 μg g-1. The heat treatment can bring about phase changes in Fe and Mn

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oxides. According to studies conducted by Benjamen et al. (Benjamin et al., 1996) and

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Schwetmann et al. (Bowles, 2000), prolonged thermal treatment converts amorphous Fe/Mn

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oxide phases into crystalline Fe/Mn oxide phases. Also, maghemite transforms into crystalline

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hematite during thermal treatment (Bora et al., 2012). The Mn oxide alpha-MnO2 crystal

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(surface area 170 m2 g-1) transforms into alpha-Mn2O3 and then to alpha-Mn3O4 (surface area 30

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m2 g-1) when thermally heated. Further heating would cause crystallization, removing oxygen

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and producing Mn2O3 and Mn3O4 (Mi et al., 2011).

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The formation of 3D crystallites requires surface energy, elastic strain, and surface

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diffusion kinetics (Gong et al., 2005). Hence, the heterogeneous surface of the residual coated

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sand is under constant strain during heat treatment because of metal oxide phase transformations.

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The strain, if not homogeneously relieved during the crystal growth, will grow as grooves or

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pits. Also, the formation of Fe and Mn crystalline oxide phases require stronger bonds rendering

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Fe-O-Si bonds between the coating and the sand weaker. As a result, the residuals easily get

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detached from the sand surface.

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The larger flocs and cracks formed at 30 min ultrasonication resulted from the uneven

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distribution of metal oxides and larger organic molecules (Figure 3b). During heating, the

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growth of unevenly distributed metal oxides and organics led to high strain sites (Gong et al.,

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2005), thereby creating large cracks and furrows. At 90 min of ultrasonication treatment, the

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majority of residual particles were reduced in size, forming a uniform coating and fewer cracks

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(Figure 3c). Higher concentrations of Fe and Mn were detected when 30 min ultrasonication was

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applied, compared to the 90 min treatment. The smaller organic molecules released at 90 min

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ultrasound irradiation readily coated Fe and Mn oxides on the adsorbent surface resulted in

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lower Cr(VI) uptake.

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Figures 4a, and 4b show the changes in surface morphology of the adsorbent as heating

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duration was increased to 36 and 48 h, respectively. More exposed surfaces of the sand can be

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seen when 48 h heating period was applied compared to 36 and 24 h. The almost negligible

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Cr(VI) uptake of sand and its less rough surface caused lower adsorption capacity (Tansel and

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Nagarajan, 2004).

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3.2 Kinetic studies The linearized equation used for pseudo-first order kinetic model is given in Eq. 2, (2)

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here qe is pollutant concentration in solid at equilibrium (mg g-1), where qt is pollutant

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concentration in solid at given time (mg g-1), and k1 is the rate constant (L min-1). Eq. 3 shows

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the linearized equation for pseudo-second order kinetic model (3)

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where qe is pollutant concentration in solid at equilibrium (mg g-1), where qt is pollutant

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concentration in solid at given time (mg g-1), h = k2qe2, and k2 is the rate constant (g mg-1-min-1).

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The statistical difference between the model and experimental data, can be computed using the

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Chi-squared equation given by Eq. 4 (Boparai et al., 2011). ∑

(4)

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where qe,exp is the experimental equilibrium adsorption capacity (μg g-1) and qe,cal is the

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calculated equilibrium adsorption capacity from the model (μg g-1). A smaller

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as it shows how much the experimental and model data differs.

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value is desired

Equilibrium was achieved in 3 h and at this point, the adsorption capacity of residual

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coated sand is 530.10 μg g-1. Figure 5 shows the linearized plots of the pseudo-first order and

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pseudo-second order kinetic models. In Table 1, the computed R2 values for both pseudo-first

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order (R2 = 0.982) and pseudo-second order (R2 = 0.974) were high. However, the computed

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for the pseudo-second order model was lower compared to the pseudo-first order model,

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indicating fewer errors. Hence, Cr(VI) adsorption onto residual coated sand follows the pseudo-

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second order kinetic model. This model validates the chemisorption phenomenon such as ion

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exchange during the adsorption of Cr(VI) onto Fe and Mn oxides on the adsorbent surface.

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3.3 Isotherm studies The Langmuir isotherm assumes monolayer attachment of molecules onto a surface

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containing a finite number of active sites, which are uniform for adsorption (Duranoĝlu et al.,

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2012). The linearized Langmuir isotherm is shown in Eq. 5 (5)

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where b is the Langmuir adsorption constant (L mg-1). A plot of Ce/qe versus Ce gave a straight

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line with a slope 1/qe and an intercept of 1/qob.

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The Freundlich isotherm assumes heterogeneous surface energies in which the energy term

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in Langmuir varies as a function of the surface coverage (Bello et al., 2011). The linearized

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Freundlich isotherm is given by Eq. 6

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(6) 272

where qe is pollutant concentration in solid at equilibrium (mg g-1), Ce is pollutant concentration

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in solution at equilibrium (mg L-1), kf is a measure of adsorption capacity (mg g-1), and n is

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adsorption intensity.

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As shown in Table 2, the data fitted to Langmuir and Freundlich isotherms revealed that

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Cr(VI) adsorption by residual coated sand follows the Freundlich isotherm (R2 = 0.9605). This

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model validates the heterogeneity of the surface adsorption due to various Fe and Mn oxide

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phases. The computed value for 1/n (0.149) is within the acceptable range (0.1 to 0.5) for

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favorable adsorption (Bello et al., 2011). The Kf value of 464.5 μg g-1 was approximately close to

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the adsorption capacities obtained.

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3.4 Effect of ionic strength Heavy metal ion adsorption onto Fe oxide surfaces is a combination of ion exchange and

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inner-sphere complexation with OH groups where metal ions replace the OH. Since heavy metals

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exist as anions at acidic pH, the adsorption of these anions on metal oxides follows a ligand

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exchange reaction (Eq. 7) where the surface OH group of a metal cation (denoted as M) is

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exchanged by the anion ligand (denoted as L) (Su and Suarez, 1997). (7)

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These ion-exchange processes can occur either through the formation of outer or inner

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sphere complexes. According to Goldberg (Goldberg, 2005), the mechanism of ligand exchange

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with surface hydroxyl is defined by the specific adsorption of anions onto the mineral surfaces

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forming inner-sphere complexes. When this happens, increasing ionic strength increases

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adsorption. This result can be explained by the principle of mass action. Increased adsorption is

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caused by the high solution activity of the counter ion of the background electrolyte available to

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compensate surface charge generated specific ion adsorption (McBride, 1997).

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In Figure 6, increasing the ionic strength from 0.0 to 0.1 M KNO3 improved the adsorption

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capacities of residual coated sand (67.82 to 166.96 μg g-1). Goldberg and Johnson (Goldberg and

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Johnston, 2001) reported similar findings on the relationship of ionic strength and adsorption

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capacities. In the same study, it was postulated that there was a formation of inner-sphere surface

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complexes during the adsorption process. The phenomenon of inner sphere complexation by

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metal oxides during adsorption has also been discussed in separate studies by Shuman (Shuman,

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1977) and Al-Sewailem et al. (Al-Sewailem et al., 1999).

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The increased adsorption at higher ionic strength can be explained by the principle of mass

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action. At higher KNO3 concentration, the solution activity of the counter ion of the background

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electrolyte increases. It compensates for the surface charge generated by the specific ion

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adsorption (McBride, 1997). Therefore, Cr(VI) anions were adsorbed more due to the charges

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induced at higher KNO3 concentration. Since inner-sphere complexation was formed, the Cr(VI)

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anions were directly adsorbed on the surface of Fe and Mn oxides of the adsorbents. The NO3-

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ions did not compete with the adsorption onto the Fe and Mn oxides. This inner-sphere complex

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mechanism confirms the ligand exchange of OH groups from the Fe and Mn oxides which is a

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chemisorption process removing the Cr(VI) from the solution.

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3.5 Adsorbent characterization

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The FTIR spectra of residual coated sand before and after Cr(VI) adsorption are shown in

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Figure 7. Similar peaks were detected for synthetic oxide adsorbents containing Fe and Mn (Lǚ

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et al., 2013). The characteristic peaks at wavelength 460 cm-1 (Figure 7c) and 1070 cm-1 (Figure

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7b) indicates Mn-O stretching vibration (Malankar et al., 2010) and OH stretching vibration

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(Laurent et al., 2009). After Cr(VI) adsorption, a broadening of these peaks showed the

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involvement of Mn-O and OH in Cr(VI) removal. The peak near 1500 cm-1 indicates Fe oxides

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(Boujelben et al., 2009). The disappearance of this peak after Cr(VI) adsorption implies the

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involvement of Fe oxide - Cr(VI) interaction. Overall, the high Cr(VI) adsorption capacities can

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be attributed to Cr(VI) interactions with Fe and Mn oxides.

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The pHPZC (Figure 8) of RCS was also indicative of Fe and Mn oxides. The measured

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pHPZC of the residual coated sand was 6.7, which was close to the pHPZC (7.4) of synthetic Fe-

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Mn coated sand (Chang et al., 2012). The pHPZC was also close to the following pHPZC values of

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Fe and Mn containing minerals: hematite (Fe2O3) = 7 (Chang et al., 1983), maghemite (γ-

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Fe2O3) = 5.2 (Plaza et al., 2002), geothite (α -FeO(OH)) = 7.1 ± 0.4 (Kosmulski, 2009),

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hausmannite (Mn3O4) = 5.7 (Kosmulski, 2009), manganite (MnOOH) = 7.4 (Kosmulski, 2009).

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The measured pHPZC indicates that at solution pH below 6.7, the residual coated sand surface

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becomes positively charged (Lǚ et al., 2013). This result agrees with optimum pH (<6.7)

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determined by Chang et al. (Chang et al., 2012), allowing the protonated residual coated sand to

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attract more HCrO4- anions. A similar observation was reported by Lǚ et al. (Lǚ et al., 2013),

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wherein more anions were adsorbed by a similar adsorbent at pH lower than pHPZC.

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4. Conclusion In this study, adsorbents, synthesized from groundwater treatment residuals by ultrasound-

336

assisted acid treatment, were used to remove Cr(VI) from aqueous solutions. Ultrasound

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irradiation of residuals until 30 min improved the adsorption capacity of the ensuing residual

16

338

coated sand. Surface morphology and EDX elemental concentrations show that lesser Fe and Mn

339

oxides were present on the RCS due to prolonged ultrasonication-aided acid treatment.

340

Acid treatment with 0.5% w/w H2SO4 yielded the highest RCS adsorption capacity. The

341

exceptionally high acid strength of H2SO4 rendered it capable of extracting the greatest amount

342

of Fe and Mn for oxide precipitation. DWT residuals treated by mixed acid solutions yielded

343

lower RCS adsorption capacities due to reduced acid strength.

344

RCS adsorption capacity is improved with heating duration until 24 h. Further increase in

345

heating duration decreased adsorption capacity of RCS due to the change in Fe and Mn oxide

346

phases, causing the loss of adhered Fe and Mn oxides on the Si sand. Surface morphology and

347

EDX elemental concentration indicating smoother surface and lesser Fe and Mn concentrations

348

on the RCS also confirmed the effect of prolonged heating duration.

349

The Cr(VI) adsorption onto RCS follows the Freundlich isotherm (R2 = 0.9605). This

350

finding is consistent with the non-uniform surface of RCS, arising from the different Fe and Mn

351

oxide phases and various compounds. The kinetic data fits the pseudo-second order model

352

indicating chemisorption, which confirms the ion exchange between Cr ligands and Fe-Mn

353

oxides. The Cr(VI) removal increased with solution ionic strength, indicating inner-sphere

354

complexation of Cr(VI) with the edge OH-groups. The measured pHPZC of the RCS adsorbent

355

was 6.7, suggesting favorable adsorption at low pH systems. Based on FTIR spectra peaks, Fe

356

and Mn oxides comprised the functional groups primarily responsible for Cr(VI) adsorption on

357

RCS.

358 359

Acknowledgements

17

360

The authors would like to thank the Ministry of Science and Technology, Taiwan (Contract No.

361

MOST-102-2221-E-041-005) and the Department of Science and Technology, Philippines for

362

providing financial support for this research undertaking.

363

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516 517

24

1000 Without ultrasonication

qt (μg g-1)

800

With ultrasonication

600

400

200

0 1

518 519 520 521

5

7

10

HNO3 concentration (% w/w) Figure 1. Adsorption capacity of residual coated sand at varying amounts of acid.

25 700

(a)

600

0.1% w/w 0.5% w/w

qt (μg g-1)

500 400 300 200 100 0 30

60

90

Ultrasonication time (min)

700

(b)

600

0.1 % w/w 0.5 % w/w

qt (μg g-1)

500 400 300 200 100 0 HCl

HNO3

H2SO4

HCl-H2SO4 HNO3HCl-H2SO4 (1:1) H2SO4 (1:1) (1:2)

Acid type

700 600

(c) 0.1% w/w

qt (μg g-1)

500

0.5% w/w

400 300 200 100 0 24

36

Heating duration (h) 522

48

26

523

Figure 2. Adsorption capacities at varying acid concentration and varying (a) ultrasonication

524

time, (b) acid type and (c) heating duration.

(a)

Element Na K Mg K Si K PK Ca K Cr K Mn K Fe K

Weight % 4.67 12.35 55.52 24.92 2.39 0.04 0.05 0.06

Atomic % 5.72 14.29 55.61 22.63 1.68 0.02 0.03 0.03

(c)

(b)

Element Na K Mg K Al K Si K PK KK Ca K Ti K Mn K Fe K

Weight % 6.45 4.79 3.94 4.06 4.07 6.17 7.36 11.82 23.27 28.08

Atomic % 11.62 8.17 6.05 5.99 5.44 6.53 7.61 10.22 17.55 20.83

Element Al K Si K KK Ti K Mn K Fe K

Weight % 14.79 77.66 2.59 4.83 0.06 0.07

525 526

Figure 3. SEM images and EDX atomic composition of (a) uncoated sand; residual coated sand

527

at heating duration = 24 h and ultrasonication time of (b) 30 min, and (c) 90 min.

528 529 530 531 (a)

Element Na K

Weight % 8.96

Atomic % 11.46

Element Na K

Weight % 40.22

Atomic % 46.40

Atomic % 15.74 79.40 1.90 2.90 0.03 0.04

27 Mg K Si K Ca K Ti K Mn K Fe K

10.57 54.92 22.19 3.17 0.09 0.11

12.77 57.46 16.27 1.94 0.05 0.06

Si K KK Ti K Cr K Mn K Fe K

49.18 10.40 0.01 0.05 0.06 0.08

46.45 7.05 0.01 0.03 0.03 0.04

532 533

Figure 4. SEM images and EDX atomic composition of residual coated sand at ultrasonication

534

time = 30 min and heating duration of (a) 36 h, and (b) 48 h.

535 536 537 2.73375

(a) log (qe-qt)

2.73370 2.73365 2.73360 2.73355 0

50

100

150

200

t (min) 400

t/qt

300 200 100

(b) 0 0

50

100

150

200

t (min) 538 539 540 541

Figure 5. Linearized plots of (a) pseudo-first order and (b) pseudo-second order kinetics.

28

200

No ions added

0.05 M KNO3

0.10 M KNO3

qt (μg g-1)

160 120 80 40 0 3

542 543 544

5

pH Figure 6. Adsorption capacity at varying initial solution pH and ionic strength.

29

70

70

70

60

60

50

50

50

40

30

40

30

Absorbance

60

Absorbance

before Cr(VI) adsorption

(c)

40

30

after Cr(VI) adsorption

2,000

1,800

1,600

20

20

20

10

10

10

0 1,400

Wavenumber (cm-1)

0

0 1,400

1,200

1,000

800

Wavenumber (cm-1)

800

600

400

200

Wavenumber (cm-1)

545

Figure 7. FTIR spectra of residual coated sand indicating (a) FeOH, (b) OH, and (c) MnOH

546

functional groups.

547 548

Absorbance

(b)

(a)

30

8

pH

6 4 2 0 0

5

551

Table 1. Kinetic constants of residual coated sand adsorbents.

555

Value 540.75 1.00 x 10-6 0.982 0.3694

528.80 8.78 x 10-2 0.974 0.3212

Table 2. Isotherm constants of residual coated sand adsorbents. Parameter Langmuir qo (μg g-1) b (L mg-1) R2 Freundlich Kf (μg g-1) 1/n R2

554

20

Figure 8. Point of zero charge approximation for residual coated sand.

Parameter pseudo-first order qe (μg g-1) k1 (L min-1) R2 χ2 pseudo-second order qe (μg g-1) K1 (g mg-1-min1 ) R2 χ2 552 553

15

Solids content (wt %)

549 550

10

Value 1.631 x 103 1.732 x 10-1 0.6704 4.645 x 102 1.490 x 10-1 0.9605

31

556 557 558

Highlights

559



Groundwater treatment residuals underwent ultrasound irradiation and acid treatment

560



Cr(VI) adsorbents were synthesized by coating pretreated residuals on sand

561



Cr(VI) removal improved with higher solution ionic strength and lower solution pH

562



Freundlich isotherm and pseudo second order kinetics define the adsorption process

563 564 565 566 567 568 569 570 571 572 573 574 575 576 577

Statement of Novelty  This work takes advantage of the residuals, rich with nano-sized metal oxides, from ground water treatment plant in the synthesis of adsorbent.  The adsorbent is uniquely synthesized through ultrasound-assisted irradiation and acid treatment.  The synthesized adsorbent was tested for the removal of hexavalent chromium from aqueous solution.  The result showed that the synthesized adsorbent ably removed the hexavalent chromium which gives huge implication both in the ground water treatment operation and the removal of heavy metals from wastewater like chromium produced in various industries such as metal plating, tanning, electroplating.