Potential of virgin and calcined wheat bran biomass for the removal of chromium(VI) ion from a synthetic aqueous solution

Journal Pre-proof Potential of virgin and calcined wheat bran biomass for the removal of chromium(VI) ion from a synthetic aqueous solution Fumihiko Ogata (Visualization) (Writing - original draft) (Writing review and editing) (Project administration), Noriaki Nagai (Investigation), Ryo Itami (Investigation) (Visualization), Takehiro Nakamura (Investigation), Naohito Kawasaki (Conceptualization) (Supervision)

PII:

S2213-3437(20)30058-0

DOI:

https://doi.org/10.1016/j.jece.2020.103710

Reference:

JECE 103710

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

21 August 2019

Revised Date:

16 January 2020

Accepted Date:

20 January 2020

Please cite this article as: Ogata F, Nagai N, Itami R, Nakamura T, Kawasaki N, Potential of virgin and calcined wheat bran biomass for the removal of chromium(VI) ion from a synthetic aqueous solution, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103710

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Journal of Environmental Chemical Engineering

Potential of virgin and calcined wheat bran biomass for the removal of chromium(VI) ion from

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a synthetic aqueous solution

Fumihiko Ogataa, Noriaki Nagaia, Ryo Itamia, Takehiro Nakamuraa, and Naohito Kawasakia,b,*

Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502,

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a

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[email protected] and [email protected]

Antiaging Center, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

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b

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Japan

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Corresponding author: Naohito Kawasaki, Ph. D. Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502,

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Japan

TEL: +81-6-4307-4012, e-mail: [email protected]

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Graphical abstract

WB500

O

WB1000

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WB

O

Cr

Cr

30 25 20 15 10 5

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0

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Adsorption

Amount adsorbed (mg/g)

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WB WB

WB500 WB500

WB1000 WB1000

Amount of chromium(VI) ion adsorbed

Before adsorption

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After adsorption

Highlights

Amount of chromium(VI) ion adsorbed was related to the physicochemical properties.



Chromium(VI) ion adsorbed onto WB1000 was desorbed using sodium hydroxide.



WB1000 could be useful for removal and recovery of chromium(VI) ions.

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

Abstract

Virgin wheat bran (WB) and calcined wheat bran were prepared at 500 or 1000°C (WB500 or WB1000, respectively), and their physicochemical properties (such as morphological,

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thermal, specific surface area, point of zero charge pH, and surface functional groups) were investigated to assess their chromium(VI) ion adsorption capability. The amount of chromium(VI) ion adsorbed was in the order WB < WB500 < WB1000. These results showed that the chromium(VI) ion adsorption was related to the WB surface characteristics. Additionally,

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the amount of chromium (Cr) and oxygen (O) on the WB1000 surface increased after adsorption, which indicated chromium(VI) ions were adsorbed onto the WB1000 surface. Adsorption

isotherm and adsorption kinetic data fit the Freundlich (0.879–0.991) and pseudo-second-order

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models (0.979–0.997), respectively. The optimal pH condition for the removal of chromium(VI)

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ion from aqueous solution was approximately 2. Finally, WB1000 could be useful for repetition

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of chromium(VI) ion adsorption/desorption using sodium hydroxide at 1000 mmol/L (at least five times). It was shown that WB1000 has the potential for adsorption and recovery of

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chromium(VI) ion from aqueous solution using sodium hydroxide.

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Keywords: wheat bran, chromium(VI) ion, adsorption, recovery

1. Introduction

The International Agency for Research on Cancer (IARC) lists chromium(VI) compounds in Group 1 (carcinogenic to humans). Chromium(VI) is found in discharged

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industrial effluents such as electroplating, leather tanning, textile, paint, and pigment manufacturing wastewaters [1-3]. Therefore, chromium(VI) is regulated to be below 0.05 mg/L in surface water and drinking water by both the United States Environmental Protection Agency and European Union [3]. In addition, in contrast to chromium(III), the toxicity of chromium(VI)

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is 500–1000 times higher to a living cell [4]. Thus, the removal of chromium(VI) has become a greater concern during recent years. Chromium(VI) forms, which exist as oxyanions (CrO42-, Cr2O72-, HCrO4-, and H2CrO4) in aqueous solution, depend on the pH of the medium and are

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toxic even at low concentrations [5].

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However, chromium is a relatively rare metal with several applications in different

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technological fields (e.g., stainless-steel fields). Chromium is stockpiled in Japan because it is imported and therefore subject to global supply shortages. In 2011, the global chromium

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production was 27,387 million tons, with greater than 40% of chromium produced in South

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Africa [6]. Japan is one of the main consumers of rare metals in the world, however, the reserves of rare metals, including chromium, are not sufficient. Therefore, it is very important to

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establish a chromium recycling technology [7]. During recent years, applications of biotechnologies in the removal (or recovery) and

control of metal pollution have attracted considerable attention because of their easy operation and low cost. Biosorption, as a relatively cost-effective metal removal technology, has been

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used for purification of wastewater or recovery of valuable materials including rare metals from aqueous solution via certain biomolecules (or types of biomass) [8-12]. Previously, some valuable material or rare metals which are recovered using biosprtion tequnie [13-17]. Additionally, these studies elucidated the adsorption/recovery mechanism.

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Briefly, the surface physicochemical properties on the WB were a very important factor and calcination treatment affected the WB characteristics and its adsorption/recovery capacity for heavy metals and rare metals from aqueous solution. However, there are no reports regarding

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the chromium(VI) ion adsorption capacity using virgin or calcined WB in aqueous solution.

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Therefore, if waste biomass WB could be explored, its value and applicability could

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significantly increase.

The objective of this study was to evaluate the adsorption isotherms, adsorption kinetics,

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and effect of contact time on the adsorption process and desorption of chromium(VI) ion from

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an aqueous solution using WB.

2. Materials and methods 2.1. Materials WB was purchased from Nisshin Seifun Group Inc. A chromium standard solution

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(K2Cr2O4 in 0.1 mol/L HNO3), sodium hydroxide, and hydrochloric acid solution were purchased from Wako Pure Chemical Industries, Ltd. Calcined WB was prepared by treating virgin WB in a muffle furnace at temperatures of 500 and 1000°C for 2 h. The WB morphology was measured using SU1510. Thermogravimetric-differential

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thermal analysis (TG-DTA) was performed using TG8210 (Measurement condition; atmosphere: 150–200 mL/min, heating time: 10°C/min, sampling time: 1 sec). The specific

surface area and surface functional groups were also determined using a NOVA4200e specific

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surface analyzer based on nitrogen adsorption isotherms (out gas and bath gas is 300 K and 77

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K) and Boehm’s titration method as previously reported [13]. The point of zero charge pH

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(pHpzc) of the samples was measured using the method previously reported by Faria et al. [18]. Additionally, elemental analysis was performed using electron probe microanalysis JXA-8530F.

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The pH in solution was measured using a digital pH meter.

2.2. Adsorption capability of chromium(VI) ion

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Each adsorbent (0.05 g) was added to a chromium(VI) ion solution at 0.1–50 mg/L (50

mL). The suspension was shaken at 100 rpm for 24 h at 25°C using a water bath shaker (MM-10) and then filtered through a 0.45-µm membrane filter. Subsequently, the filtrate was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES;

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iCAP-7600, Thermo Fisher Scientific Inc., Japan). The amount of chromium(VI) ion adsorbed was calculated by the following Eq. (1): qa = (C0 − Ce) V/W

(1)

where qa is the amount adsorbed (mg/g), C0 is the initial concentration (mg/L), Ce is the

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equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of the adsorbent (g).

Subsequently, the Langmuir equation (Eq. (2)) and the Freundlich equation (Eq. (3))

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were used for fitting the adsorption isotherm data and can be described, respectively, as follows: (2)

(3)

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log qe = log k + (1/n) log Ce

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Ce/qe = 1/Wsa + Ce/Ws

where Ce is the equilibrium concentration (mg/L); qe is the amount adsorbed at the equilibrium

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(mg/g); and Ws and a are the Langmuir constants relating to the monolayer adsorption capacity

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and energy of sorption, respectively. The Freundlich constants k and n are related to the

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adsorption of the adsorbent and intensity of adsorption, respectively [19].

2.3. Effect of contact time and pH on chromium(VI) ion adsorption For the kinetic studies, each adsorbent (0.05 g) was added to a chromium(VI) ion solution at 50 mg/L (50 mL). The suspension was shaken at 100 rpm for 0.5, 1, 2, 3, 6, 17, and

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24 h at 25°C. Subsequently, kinetic data were investigated applying the pseudo-first-order (Eq. (4)) and pseudo-second-order models (Eq. (5)), respectively, as follows: ln (qe – qt)/qe = - k1t

(4)

t/qt = 1/(k2qe2) + t/qe

(5)

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where qe and qt correspond to the amount of chromium(VI) ion adsorbed at the equilibrium and at time t (mg/g), respectively; k1 is the pseudo-first-order rate constant (1/h); and k2 is the pseudo-second-order rate constant (g/mg/h).

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For the effect of pH in solution, each adsorbent (0.05 g) was added to a chromium(VI)

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ion solution at 50 mg/L (50 mL and pH 2, 4, 6, 8, and 10). The pH in the sample solution was

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adjusted using sodium hydroxide or hydrochloric acid solution. The suspension was shaken at

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100 rpm for 24 h at 25°C. The amount adsorbed was calculated using Eq. (1).

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2.4. Chromium(VI) ion recovery using sodium hydroxide solution First WB1000 (0.3 g) was added to a chromium(VI) ion solution at 50 mg/L (300 mL).

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The suspension was shaken at 100 rpm for 24 h at 25°C and then filtered through a 0.45-µm membrane filter. Subsequently, the filtrate was analyzed using ICP-OES. The amount of chromium(VI) ion adsorbed was calculated using Eq. (1). After adsorption, WB 1000 was collected, dried, and then used for the desorption experiment. The collected WB1000 was added

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to a sodium hydroxide solution at 0.001, 0.01, 0.1, and 1.0 mmol/L (50 mL). The suspension was shaken at 100 rpm for 24 h at 25°C and then filtered through a 0.45-µm membrane filter. Subsequently, the filtrate was analyzed using ICP-OES. The amount of chromium(VI) ion desorbed was calculated via Eq. (6) as follows: (6)

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qd = CeV/W

where qd is the amount desorbed (mg/g), Ce is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of the adsorbent (g). Additionally, the repetition of

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adsorption/desorption of chromium(VI) ion using WB1000 was conducted five times under this

3. Results and discussion

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experimental condition.

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3.1. Characteristics of virgin and calcined WB

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The SEM images of WB, WB500, and WB1000 are shown in Fig. 1. We observed the roughness on the WB surface. Subsequently, the pores on the WB500 and WB1000 surfaces were

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confirmed by calcination treatment. These results suggest that the specific surface area of WB samples increased with increasing calcination temperature. Similar trends were observed in a previous study [13]. Figure 2 shows the TG-DTA curves of the virgin and calcined WB. An endothermic peak was observed at approximately 100°C which indicated dehydration occurred.

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In addition, the organic compounds and fatty acids were burned at approximately 300°C in the WB; therefore, there were small differences between the TGA and DTA curves for WB 500 and WB1000. The physicochemical properties of virgin and calcined WB are shown in Table 1. The

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specific surface area, pHpzc, and basic functional groups were in the order WB < WB500 < WB1000. However, the acidic functional groups showed the opposite trend. Calcination treatment

particularly affected the specific surface area; the value of WB1000 (203.1 m2/g) was

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approximately 35 times higher than that of WB (5.7 m2/g). In addition, the yield percentage of

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WB500 and WB1000 was approximately 27–29 % and 22 %, respectively [13]. These

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phenomena were similar to the TGA curve of WB in Fig. 2. Finally, carbonaceous materials,

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which have different physicochemical properties, were obtained from waste biomass WB.

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3.2. Amount of chromium(VI) ion adsorbed The amount of chromium(VI) ion adsorbed was in the order WB1000 (29.3 mg/g) >

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WB500 (7.6 mg/g) > WB (4.2 mg/g). In this study, the relationship between the amount adsorbed and the physicochemical properties was evaluated (Table 1). As a result, the correlation coefficient between the amount adsorbed and the specific surface area, pHpzc, acidic functional groups, or basic functional groups was 0.998 (positive), 0.906 (positive), 0.685 (negative), or

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0.988 (positive), respectively. These results indicate that the specific surface area, pH pzc, and basic functional groups are related to the adsorption capability of chromium(VI) ion. Therefore, the WB surface physicochemical properties are very important to understand the adsorption mechanism. Then, the elemental analysis on the WB1000 before and after adsorption of the

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chromium(VI) ion was conducted (Fig. 3). Chromium(VI) ions usually exist as oxyanions (Cr2O72-, HCrO4-, and CrO42-); therefore, the intensity of Cr(chromium) and O(oxygen) on the

WB1000 surface before and after adsorption was measured. The intensity of both elements on the

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WB1000 surface increased after adsorption as compared to the intensity before adsorption. On the

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other hand, the intensity of C(Carbon) on the WB1000 surface decreased after adsorption. These

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results indicate that the chromium(VI) ions were adsorbed onto the WB1000 surface. Additionally, a comparison of the chromium(VI) ion adsorption capacity of WBs to that

for

chromium(VI)

ion

adsorption

from

aqueous

solutions

(except

for

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potential

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of other reported adsorbents is shown in Table 2 [20-25]. As a result, WB1000 has a great

surfactant-modified serpentine and Eichhornia crassipes root biomass-derived activated carbon).

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Moreover, the correlation coefficient (r2) between amount of chromium(VI) ion adsorbed and the specific surface area was evaluated in this study. The value of r2 (0.033) was very low. Therefore, the specific surface area did not affect the adsorption capability of chromium(VI) from aqueous phase in Table 2.

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3.3. Adsorption isotherms of the chromium(VI) ion Adsorption isotherms of the chromium(VI) ion onto WB, WB500, and WB1000 are shown in Fig. 4. The amount adsorbed was in the order WB < WB500 < WB1000. Particularly, the amount

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of chromium(VI) ion adsorbed onto WB1000 significantly increased at an equilibrium concentration of approximately 20 mg/L.

The correlation coefficients of the Freundlich model (0.879–0.991) were higher than

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those of the Langmuir model (0.632–0.998). Additionally, the value of Ws for WB, WB500, and

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WB1000 was -0.2, 0.2, and -0.5, respectively, which indicated the obtained adsorption isotherm

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data in this study are not fitted to the Langmuir model. Therefore, the adsorption isotherm data best fitted the Freundlich model, implying that chromium(VI) ion adsorption on the WBs was

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governed by monolayer and heterogeneous sorption [26]. In addition, chromium(VI) ions were

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easily adsorbed onto the WB surface when 1/n was within the range 0.1–0.5 but not when 1/n >2 [27]. The values of 1/n (0.8–1.8) obtained in this study indicate that the chromium(VI) ion

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was easily adsorbed under this experimental condition. 3.4. Effects of contact time and pH in solution on chromium(VI) ion adsorption Adsorption kinetic studies are shown in Fig. 5(a). The amount of chromium(VI) ion adsorbed using WB, WB500, and WB1000 significantly increased within 3, 3, and 6 h under this

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experimental condition, respectively. Thereafter, the equilibrium adsorption using WB was attained at approximately 24 h. The correlation coefficient of the pseudo-second-order model (0.984–0.997) was higher and the value of qe,exp (4.15, 7.56, and 29.3 mg/g for WB, WB500, and WB1000, respectively) is

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nearer the value of qe,cal (4.48, 8.59, and 31.84 mg/g for WB, WB500, and WB1000, respectively) in the pseudo-second-order model compared to that in the pseudo-first-order model. These results show that the chromium(VI) ion adsorption process using WBs was mostly controlled by

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chemisorption behavior [20,28,29]. However, the contribution ratios of physical and chemical

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adsorptions are quite different between each WB materials. This study could not elucidate the

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contributions of physical and chemical adsorption for removal of chromium(VI) ions from aqueous phase using WB materials. Therefore, further studies are needed to elucidate the

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adsorption mechanism.

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In addition, the effect of pH in solution on chromium(VI) ion adsorption is shown in Fig. 5(b). As a result, the amount adsorbed under an acidic condition was higher than that under a

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neutral or basic condition. The chromium(VI) ion can exist in several stable anions such as CrO42-, HCrO42-, Cr2O72-, and HCr2O7- and the relative abundance of a particular complex depends on the chromium(VI) ion concentration and pH in the solution [23]. Table 1 shows the values of pHpzc of the WBs which are in the order WB (6.19) < WB500 (8.10) < WB1000 (9.77).

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The WB adsorbent surface may be positively or negatively charged depending on the pH in the solution, which governs the acid–basic balances of the WB adsorbent surface conditions [30]. At a lower pH value (pH < pHpzc), the positive charge on the surface of WBs increases and thus more strongly attracts the negatively charged chromium(VI) ion. However, at a higher pH value

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(pH > pHpzc), the WB surfaces are negatively charged. Then, a repulsion exists between the WB surfaces and chromium(VI) ions and there is competition with hydroxyl ions and chromium(VI) ions under a basic condition, leading to a decrease in the adsorption capability of chromium(VI)

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ion [31]. These observations confirm that chromium(VI) ion adsorption onto the WBs resulted

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in chemisorption phenomena, involving valence forces through the sharing or exchanging of

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electrons between the surface of the WBs and chromium(VI) ions [30].

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3.5. Chromium(VI) ion recovery from WB1000

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To evaluate the WB field application and repetition for adsorption/desorption, we investigated WB regeneration and re-adsorption using a sodium hydroxide solution at different

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concentrations (Fig. 6). Our previous studies elucidated that sodium hydroxide solution could be useful for desorption treatment [13,15]. Therefore, sodium hydroxide solution was selected for desorption in this study. The amount of chromium(VI) ion released from WB after adsorption depended on the sodium hydroxide concentration (the desorption percentage using 0.001, 0.01,

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0.1, and 1.0 mmol/L was 5, 12, 36, and 44%, respectively). These results suggest that the bonds between the adsorption sites and chromium(VI) ion can be broken and that the chromium(VI) ions adsorbed onto WB1000 can be desorbed by the sodium hydroxide solution under a high pH condition (The schematic of adsorption/desorption of chromium(VI) ions using WB1000 is shown

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in Fig. 7). In addition, abovementioned in “3.4. Effect of pH in solution on chromium(VI) ion adsorption”, the solution pH is one of the important factors for removal of chromium(VI) ions from aqueous solution. These results indicated that amount of chromium(VI) ions desorbed

observed

in

previous

section.

Therefore,

we

evaluated

the

repetition

of

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also

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increased with increasing the concentration of sodium hydroxide solution. Similar trends were

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adsorption/desorption of chromium(VI) ion using a sodium hydroxide solution at 1000 mmol/L. WB1000 could be re-used for adsorption/desorption of chromium(VI) ion at least five times under

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this experimental condition. The recovery percentage of chromium(VI) ion from WB1000 was 49,

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45, 30, 46, and 48% for the 1st, 2nd, 3rd, 4th, and 5th use, respectively. WB1000 is useful for the repetition of adsorption/desorption of chromium(VI) ion using a sodium hydroxide solution.

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Thus, WB1000 has great potential for wastewater purification and rare metal (Cr) recovery. 4. Conclusions

In this study, we evaluated the potential of waste biomass WB for adsorption/desorption capability of chromium(VI) ion from an aqueous solution system. Calcination treatment affected

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the WB physicochemical properties, particularly the specific surface area which significantly rose with increasing calcination treatments at 1000°C (WB1000). The amount of chromium(VI) ion adsorbed onto WB1000 was higher than that onto WB or WB500. These results indicate that the physicochemical properties, including the specific surface area, are related to the adsorption

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capability of chromium(VI) ion. The adsorption isotherm data fit to the Freundlich model (0.879–0.991) and the adsorption kinetics follows the pseudo-second-order model (0.984–0.997). The optimal pH condition for chromium(VI) ion removal onto WBs was an

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acidic condition, which indicates that the adsorption capacity involves the electrostatic

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interaction between the WB surface charge and the ion form of chromium(VI) (oxyanions).

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Additionally, chromium(VI) ion adsorbed onto WB1000 was desorbed using a sodium hydroxide solution at 1000 mmol/L. WB1000 could be useful for repetition for adsorption/desorption of

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chromium(VI) ion from aqueous solution (at least five times). Collectivity, this study found that

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waste biomass WB is efficient for chromium(VI) ion adsorption and recovery from an aqueous

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solution.

CRediT author statement Fumihiko Ogata: Visualization, Writing- Original draft preparation, Writing - review & editing, Project administration. Noriaki Nagai: Investigation Ryo Itami: Investigation, Visualization 16

Takehiro Nakamura: Investigation. Naohito Kawasaki: Conceptualization, Supervision. Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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[27] I. Abe, K. Hayashi, M. Kitagawa, Adsorption of non-ionic surfactants, Yukagaku 25(1976) 145–150.

[28] N. Ghasemi, P. Tamri, A. Khademi, N.S. Nezhad, S.R.W. Alwi, Linearized equations of

-p

pseudo second-order kinetic for the adsorption of Pb(II) on pistacia atlantica shells, IERI

re

Procedia 5 (2013) 232-237.

methylene

blue

dye

by

lP

[29] C. Zhou, Q. Wu, T. Lei, I.I. Negulescu, Adsorption kinetic and equilibrium studies for partially

hydrolyzed

polyacrylamide/cellulose

nanocrystal

na

nanocomposite hydrogels, Chem. Eng. J. 251 (2014) 17-24.

ur

[30] M. Gueye, Y. Richardson, F.T. Kafack, J. Blin, High efficiency activated carbons from African biomass residues for the removal of chromium(VI) from wastewater, J. Environ. Chem.

Jo

Eng. 2 (2014) 273-281.

[31] Q.Q. Zhong, Q.Y. yue, B.Y. Gao, Q. Li, X. Xu, A novel amphoteric adsorbent derived from biomass materials: Synthesis and adsorption for Cu(II)/Cr(VI) in single and binary systems, Chem. Eng. J. 229 (2013) 90-98.

21

ro of

WB

WB500

re

-p

×500

WB1000

na

lP

×1000

Jo

ur

Fig. 1. SEM images of virgin and calcined WB.

22

ro of lP

re

TGA (%)

na

WB1000

Jo

ur

Fig. 2. TG-DTA curves of virgin and calcined WB.

23

DTA (µV)

-p

WB500

O

N

Cr

-p

C

ro of

Before adsorption

C

O

Cr

na

lP

N

re

After adsorption

High concentration

Low concentration

Jo

ur

Fig. 3. Elemental analysis of WB1000 surface before and after adsorption.

24

ro of

25

●WB ●WB500 ●WB1000

-p

15

10

re

Amount adsorbed (mg/g)

20

0 0

lP

5

10

20

30

40

50

na

Equilibrium concentration (mg/L)

Jo

ur

Fig. 4. Adsorption isotherms of chromium(VI) ion onto WBs.

25

(a)

(b) 25

25 20 15 10 5 0

0

5

10

15

20

20

15

10

5

0

25

■WB ■WB500 ■WB1000

2

Contact time (h)

ro of

●WB ●WB500 ●WB1000

Amount adsorbed (mg/g)

Amount adsorbed (mg/g)

30

4

6

8

10

re

-p

Initial pH

lP

Fig. 5. Effect of contact time(a) or pH(b) on the adsorption of chromium(VI) ion onto WBs.

Jo

ur

na

Conditions (a) and (b): Initial concentration of chromium(VI) ion is 50 mg/L.

26

18 ■Adsorption □Desorption

16 14 12

Amount desorbed

10 8 6 4 2 0

30

■Adsorption □Desorption

25 20 15 10 5 0 1

1.0

0.1

0.01 0.001

ro of

Amount adsorbed

Amount adsorbed or desorbed (mg/g)

Amount adsorbed or desorbed (mg/g)

20

2

3

4

5

Cycle (times)

re

-p

Concentration of sodium hydroxide (mmol/L)

Jo

ur

na

lP

Fig. 6. Amount of chromium(VI) ion adsorbed or desorbed using WB1000.

27

Calcination at 1000 ℃

WB

WB1000

Adsorption

WB1000 could be recycled

ro of

and

Cr

Desorption

Cr

Adsorption site (Adsorbent surface)

-p

Chromium(VI) ions could be recovered

Adsorption site (Pores)

Jo

ur

na

lP

re

Fig. 7. The schematic of ad/desorption of chromium(VI) ions using WB1000.

28

Table 1 Physicochemical properties of virgin and calcined WB.

Specific surface area (m2/g)

pHpzc

Acidic functional groups (mmol/g)

Basic functional groups (mmol/g)

WB

5.7

6.19

0.43

0.06

WB500

18.0

8.10

0.20

WB1000

203.1

9.77

0.17

Jo

ur

na

lP

re

-p

ro of

Samples

29

0.10

0.20

Table 2 Comparison of chromium(VI) ion adsorption capacity of WBs with other reported adsorbents.

Adsorption capability (mg/g)

Specific surface area (m2/g)

pH

Temperature (℃ )

Initial concentration (mg/L)

Contact time (h)

Adsorbent (g/L)

References

Aminefunctionalized modified rice straw

15.82

5.52

2.0

45

200

1.0

10

20

Eichhornia crassipes root biomass-derived activated carbon

36.34

109.23

4.5

25±2

100

Raw rice bran

0.07

1050–1200

5.0

-

-

Activated carbon derived from coconut shell

21.75

1565

2.0

25

100

Groundnut shell

3.79

89.74

8.0

41.5

25

Surfactantmodified serpentine

53.6

-

-

25

140

WB

4.2

5.7

Approximately 2.0

25 25

18.0

WB1000

29.3

203.1

Approximately 2.0

7

21

-

-

22

48

2

23

2

2

24

2

0.2

25

50

24

1

This study

50

24

1

This study

24

1

This study

-p

7.6

0.5

re

WB500

Approximately 2.0

Jo

ur

na

lP

25

30

ro of

Samples

50