Evaluation of Arsenic(V) biosorption to charred orange peel in

aqueous environments

Muhammad Abida, Nabeel Khan Niazi a,b,*, Irshad Bibia,b, Ghulam Murtazaa, Abida Farooqic,

Yong Sik Okd, Anitha Kunhikrishnane, Fawad Alif, Shafaqat Alig and Avanthi Deshani

Igalavithanad

aInstitute of Soil and Environmental Sciences, University of Agriculture Faisalabad, Faisalabad

38040, PakistanbSouthern Cross GeoScience, Southern Cross University, Lismore 2480, NSW, Australia

cEnvironmental Geochemistry Laboratory, Department of Environmental Sciences, Quaid-i-

Azam University, Islamabad, PakistandKorea Biochar Research Center & Department of Biological Environment, Kangwon National

University, Chuncheon 200-701, KoreaeChemical Safety Division, Department of Agro-Food Safety, National Academy of Agricultural

Science, Wanju-gun, Jeollabuk-do, Republic of Korea

fDepartment of Plant Breeding and Genetics, University of Agriculture Faisalabad, Faisalabad

38040, PakistangDepartment of Environmental Sciences, Government College University, Faisalabad 38000

Pakistan

* CORRESPONDING AUTHOR FOOT NOTE:

Email: nabeelkniazi@gmail.com; nabeel.niazi@uaf.edu.pk

T: (+92) 41 920 1089

Permanent address: Institute of Soil and Environmental Sciences, University of Agriculture

Faisalabad, Faisalabad 38040, Pakistan

1

ABSTRACT

Biosorption efficiency of charred orange peel (COP) and natural orange peel (NOP) was

examined for the immobilization of arsenic (As) under environmentally relevant aqueous

environments in batch sorption experiments. The NOP was transformed to COP by pretreatment

with sulfuric acid. Sorption experiments were performed to determine influence of pH (3–10),

initial As concentration (5–250 mg L–1) and biosorbent dose (1–20 g L–1) on As sorption at a

20±2 °C. Arsenic sorption was found to be maximum at pH 6.5, with COP possessing higher As

removal (98 %) than NOP (68 %) at 4 g L -1 optimum biosorbent dose. Sorption isotherm data

exhibited the highest As sorption (60.9 mg g–1) for COP versus NOP (32.7 mg g–1). Langmuir

model provided the best fit to describe As sorption on COP (R2 =0.99) and NOP (R2 =0.95).

Fourier transform infrared spectra revealed that –OH, –COOH, –CH2 and –N-H surface

functional groups were involved in As biosorption. The SEM-EDX analyses unraveled that

meso- to micro-porous structure of COP biomass sequestered significantly higher As (0.07 wt.

%) than that of NOP (0.03 wt. %). Desorption of As from COB was observed to lower (10 %)

compared to NOP (26 %) up to third cycle. This novel pretreatment method could serve as a

simple and efficient means to produce the low-cost and unique ‘charred’ biomaterials from the

widely available biowaste, with enhanced As biosorption properties.

Keywords: Arsenic Remediation; Bioremoval; Drinking water; Wastewater; Biosorbents;

Contamination

2

Introduction

The contamination of surface water and groundwater reservoirs with elevated concentration of

arsenic (As) has emerged as a worldwide environmental and health issue given to the toxic and

carcinogenic nature of As (Smedley and Kinniburgh, 2002; Niazi, Bishop, and Singh, 2011).

Both geogenic processes as well as anthropogenic activities such as mining and smelting, coal

combustion, and leather tanning operations have led to the contamination of water resources with

As (Mahimairaja et al., 2005). In aquatic environments, As mainly exists in two forms, arsenate

(As(V)) and arsenite (As(III)), depending upon the pH and redox conditions. Arsenite prevails in

anoxic water environments, while As(V) mainly exists in oxidized conditions (Masscheleyn,

Delaune, and Patrick, 1991). The safe limit of As in drinking water is 0.01 mg L -1 according to

the United States Environmental Protection Agency (US EPA) and World Health Organization

(WHO) (Organization, 2006; Roychowdhury, 2010). Considering the strict environmental

protection regulations and public health concerns, there is a need to explore novel approaches to

remove As from aqueous environments (e.g. As-contaminated groundwater or wastewater).

In comparison to conventional remediation techniques (Garelick et al., 2005), in recent years

biosorption has seen significant advancement to remove heavy metal(loid)s such as As from

contaminated water due to its low-cost, eco-friendly nature, easy availability of the biosorbent

materials and no nutrient requirement (Mohan et al., 2007; Pehlivan et al., 2013). Agricultural

and food-industry biowastes are one of the best and commonly available sources for producing

3

low-cost biosorbent materials (Arief et al., 2008). Food processing biowastes, such as orange

peel has little economic value and can create disposal problems. Orange (Citrus reticulata L.)

peel waste is accessible in large quantities in different parts of the world, particularly in Pakistan,

which is the sixth largest orange producer in the world (2.1 m tons/year). The orange peel residue

primarily consists of cellulose, lignin, hemi-celluloses, carboxyl (–COOH) and hydroxyl (–OH)

surface functional groups, which can play a pivotal role in the removal of metal(loid) ions from

the water (Santos, Ntwampe, and Doughari, 2013).

Previous research, although partly, has transformed our scientific understanding on the use of

treated and natural orange peel (NOP) biomass for the immobilization of heavy metals (e.g. Pb,

Cd, Ni) from their aqueous solutions (Ghimire et al., 2002; Lu et al., 2009; Feng et al., 2011;

Lugo et al., 2012). For example, grafted copolymerization-modified orange peel led to a higher

removal of Ni, Cd and Pb cations from aqueous solutions (162, 293 and 476 mg g–1, respectively)

than the NOP (Feng et al., 2011). Except few studies and under a limited set of conditions,

previous research has been directed to evaluate effect of some pre-treatment methods on As

biosorption in As-contaminated aqueous environments. In a recent study, Ca(OH)2 treated NOP

was used for the remediation of As-contaminated water which showed the maximum As sorption

capacity of 43.7 mg g–1 at pH 7 (Peng et al., 2013). Ghimire et al. (2002) found that the orange

processing residue (pulp) treatment with ferric iron (FeIII) led to the maximum As sorption

capacity of 70 mg g–1 for As(V) at pH 3 and 68 mg g–1 for As(III) at pH 10. Khaskheli at al.

(2011) observed that up to 85 % of As(V) was removed by the NOP biomass from water

containing 0.01–50 mg L–1 As, at pH 7.

4

Although yet to be explored for As oxyanions, sulfuric acid (H2SO4) pretreatment has been used

to produce charred xanthated sugarcane bagasse (CXSB) material for the simultaneous removal

of cationic heavy metal species (Cd, Pb, Ni, Zn, Cu) from their aqueous solutions (Homagai,

Ghimire, and Inoue, 2010). To our scientific ability, examining the influence of H2SO4

pretreatment on orange peel to remove oxyanionic metal(loid) species, such as As(V) from

contaminated water represents a crucially important, although previously unexplored, aspect in

this area of research. Through this study, we unraveled a simple means of pretreatment of NOP

using H2SO4 for producing charred orange peel (COP), thereby enhancing its stability and As

sorption properties. This pretreatment method can help open the biopolymer ring present in the

NOP biomass, thus providing high number of pore spaces/active sites and stability to the

biosorbent material (Morrison and Boyd, 1994). Therefore, the objectives of this study were to

(1) investigate the biosorption efficiency of COP and NOP for the immobilization of As(V)

under environmentally relevant aqueous environments in batch sorption experiments, and (2)

delineate the influence of pH, initial As concentration and biosorbent dose on As sorption

capacity of the COP and NOP. Overall As biosorption was elucidated through isotherm modeling

of experimental data coupled with Fourier transform infrared (FTIR) spectroscopy and scanning

electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX)

techniques.

Materials and methods

Materials

Arsenic was used as As(V) (Na2HAsO4.7H2O, Fluka, > 98.5%) in all batch sorption experiments.

All solutions were prepared by using Milli Q water (Millipore Corp. resistivity, 18.2 MΏ cm).

5

The glassware and plasticware (polypropylene) used throughout the sorption experiments were

soaked in 3 % nitric acid (HNO3) solution followed by two consecutive washings with the

deionized water prior to use for various analytical purposes.

Preparation of COP

The natural orange peel (NOP) was collected from a local juice shop situated in Faisalabad city

(Pakistan). The NOP sample was washed thoroughly with deionised water in order to remove

dust and impurities, and sun-dried for 48 h followed by oven drying at 65 oC for 72 h. The oven

dried NOP was ground and sieved (˂ 250 µm) prior to perform batch sorption experiments.

For the preparation of COP, the finely ground NOP biomass was treated with concentrated

sulfuric acid (H2SO4) in a 1:2 solid: solution suspension (wt/vol) and shaken for 30 min at 80

rpm using a magnetic stirrer. The suspension was centrifuged at 3000 rpm followed by 8

washings with deionised water until the pH of the washing solution was almost neutral. It is

worth mentioning that H2SO4 was almost fully recovered (>99 %) after reaction with NOP

allowing us to reuse it for treating other several batches of the biowaste material. Thus, we do

not expect any potential hazard of this pretreatment method to the environment in the form of

residual acid waste. The residual (black color) charred material was dried at 65 oC for 48 h,

ground (<250 µm) and stored in a desiccator prior to use in the batch sorption experiments, and

for Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM)

analyses. This black color material was referred to as COP.

Arsenate analysis in water using molybdene blue method

6

Preparation of reagents for As(V) determination

Arsenate was analysed in the filtered water samples following the molybdene blue color method

as described by Lenoble and his coworker (2003), whereby As(V) was determined by the

formation of an antimonyl-arsenomolybdate complex (detailed procedure is given in the

Supporting Information (SI)).

Colorimetric determination of As(V) in water samples

Briefly, 0.4 mL of reagent A (see SI) and 0.2 mL of ascorbic acid solutions were successively

added to a 8 mL of the sample aliquot in a 10 mL plastic (polyethylene) tube; the remaining

volume was made up by deionised water. A blank was also prepared according to the same

procedure using the appropriate volume of deionised water. After that, 60 min were allowed as

reaction time for the complex formation and blue color development at room temperature (20±2

°C), prior to As(V) analysis using an UV-visible spectrophotometer (Evolution 100,

Thermofisher) at 870 nm wavelength (Lenoble et al., 2003).

Batch sorption experiments

Sorption experiments were carried out in 50 mL plastic centrifuge tubes using 0.01 M NaCl

solution as a background electrolyte for the NOP and COP. All the sorption experiments were

conducted at 20±2 °C and the equilibration (shaking) time was 120 min based on the previous

literature since maximum sorption occurs in the first 90-120 min (Mohan and Pittman, 2007;

Khaskheli et al., 2011). For sorption isotherm experiments, initial As concentrations ranged from

5–250 mg L–1 considering the low and high As levels which may be found in As-contaminated

water environments (Lorenzen, Van Deventer, and Landi, 1995; Mcafee et al., 2001; Sari and

7

Tuzen, 2009). The suspension pH was set at 6.5 using 0.1 M HCl or 0.1 M NaOH, and the a

biosorbent dose of 4 g L–1 was used in order to evaluate effect of initial As concentration on the

sorption capacity of COP and NOP.

After shaking for 120 min on an end-to-end shaker, the suspensions were centrifuged at 4000

rpm, the supernatant solution was filtered through a 0.45 µm filtration membrane and the

equilibrium pH was measured. Filtered solutions were stored in a refrigerator at 4 °C prior to

As(V) analysis using an UV-visible spectrophotometer as described above. Arsenic in the

samples was analysed in triplicate and the standard deviation values for all the analysed water

samples were < 2% of their mean value. For quality assurance and precision of the As analysis,

As-spiked water samples were also analysed with each batch of the samples on

spectrophotometer.

The percent (%) removal of As from the solution was calculated as follows (Eq. 1):

% As removal = (Co – Ce)/Co × 100 (1)

where, Co and Ce represent the initial and final (equilibrium) As concentrations (mg L–1),

respectively. The sorbed As (mg) per unit mass of biosorbent (g), qe, was calculated by using the

following equation 2:

qe = Co – Ce/m × V (2)

where qe is the sorption capacity determined at equilibrium (mg g-1), Co and Ce are initial and

equilibrium As concentrations (mg L-1), m is oven dried weight of biosorbent (g), and V is the

volume of the solution (L).

8

Effect of biosorbent dose (1–20 g L-1) and solution pH (3–10) on As sorption by the COP and

NOP was investigated at a constant initial As concentration of 200 mg L–1. Arsenic desorption

experiments were conducted by using 0.01M HCl solution as a desorbent up to 3 cycles. After

each sorption/desorption cycle, the sorbent was filtered and equilibrated with 25 mL of 0.01 M

HCl for 1.5 hour. The sorbent was separated by filtration, thoroughly washed with deionized

water and used for the subsequent sorption cycle. Arsenic in filtered water samples was analysed

using a spectrophotometer as mentioned earlier.

Sorption isotherm models

Two parameter-based sorption isotherm models were used to evaluate the sorption mechanism of

As on the surface of COP and NOP. Four isotherm models, including: Freundlich, Langmuir,

Temkin, and Dubinin–Radushkevich, were used to fit equilibrium experimental data of As

(Ahmad et al., 2013) (see Supporting Information (SI) for details on isotherm models).

SEM-EDX and FTIR spectroscopy analyses

To determine the surface morphology, microstructure and elemental composition of the COP and

NOP, scanning electron microscopy (SEM) combined with the energy dispersive X-ray

spectroscopy (EDX) was used (SU8000, Hitachi, Tokyo, Japan). For the biosorbent surface

functional group characterisation, Fourier transform infrared (FTIR) spectroscopy (Excalibur

3000MX; Bio-Rad, Hercules, CA, USA) was employed. The FTIR scans were collected in the

wavenumber range from 400–4000 cm–1 with 32 successive scans at a resolution of 4 cm–1. The

9

transmittance spectra were normalised for difference spectroscopy using Essential FTIR software

(v 2.00.045). Specific surface area (SSA) of the two biosorbents (COP and NOP) was measured

by Brunauer-Emmett-Teller (BET) method (Walton and Snurr, 2007). The SSA of NOP was

3.84±0.97 and for COP it was 16.65±2.12 m2 g-1.

Sorption isotherm modeling and statistical analysis

Four sorption isotherms were fit to the equilibrium batch sorption experiment data by using the

Sigma Plot version 10.0.

Results and discussion

Sorption experiments

Effect of biosorbent dose

Arsenic sorption to both NOP (8.7–31.8 mg g–1) and COP (29.4–66.5 mg g–1) decreased as a

function of increasing biosorbent dose (4–20 g L–1) (Figure 1) with the highest As removal

observed at 4 g L–1. Relatively, greater As biosorption was found for COP than the NOP. A

decrease in the As sorption capacity with increasing amount of biosorbent could possibly be

attributed to blockage of pore spaces and sorption sites and/or increase in the competition of

negatively charged ligands (e.g., –HO, –COO–) with As oxyanions, thereby desorbing As from

the biosorbent surface (Christian et al., 2008).

Sorption isotherm experiment

Sorption isotherm experiments were performed to examine influence of the initial As

concentration (5–250 mg L–1) on As sorption by the two biosorbents at a constant pH 7 and

10

biosorbent dose of 4 g L-1. Arsenic sorption was found to be significantly higher for the COP

compared to the NOP (1.41–60.94 and 1.29–32.7 mg g–1, respectively) (Figure 2a,b).

The shape of isotherm obtained from the isotherm experiments indicated that for NOP As

sorption became constant after a certain point (39 mg g–1; at 150 mg L-1 initial As), while COP

showed an increasing, although curvy, As sorption trend with increasing As concentrations

(Figure 2a,b), under investigated experimental conditions. It is evident from sorption isotherm

data that the COP possessed higher As removal efficiency than NOP, thus COP can be used to

immobilize As in aqueous environments.

The sulfuric acid treatment of NOP can increase the number of surface sites and surface area by

opening the biopolymer rings of the biosorbent material, as described elsewhere (Homagai et al.,

2010). Significantly higher SSA of COP (16.65 m2 g–1) compared to NOP (4.84 m2 g–1) is in

agreement with our earlier argument. The mechanism of solute transfer to the biosorbent

includes diffusion through fluid film around sorbent particle and diffusion through surface pores

to internal sorption sites (Ranjan, Talat, and Hasan, 2009). As COP constituted a larger number

of pore spaces and available reactive sites (based on SSA results) than the NOP, thus more As

was immobilized by COP than that of NOP (Bibi et al., 2014).

Effect of pH

Both COP and NOP demonstrated the highest As removal from aqueous solutions at pH 6.5

followed by at pH 7.2 and 8. At pH 6.5, the COP removed up to 98 % of aqueous As that was 1.5

times higher than that removed by the NOP (68 %) (Figure 3). Our results showed that the COP

can be used as a potential biosorbent for (>98 %) removal of As from water having a natural pH

11

range of 6.5–8, in order to reduce the As concentration below the WHO safe limit of 0.01 mg L–

1.

The pH can affect the protonation of the functional groups on the biosorbent biomass as well as

As speciation and aqueous chemistry thus pH plays a crucial role in the sorption of As (Das, Das,

and Guha, 2007). Our results are in agreement with previous work on the effect of pH on

sorption of As using different kinds of sorbents (Hansen, Ribeiro, and Mateus, 2006; Baig, Kazi,

and Elci, 2012; Bibi et al., 2014). Sorption data from pH experiments showed that the maximum

% sorption of As was observed at pH 6.5 (98 % for COP and 68 % for NOP) followed by pH 7.2

and 8. Between pH 4 and 7, As(V) predominantly exists as negatively charged HAsO42–, H2AsO4

–

species. The low As sorption at pH 3–5 could be associated with the introduction of additional

protons in the solution using acid solution, thereby leading to competition for carbonyl sites and

reduction of As sorption at low pH (Feng et al., 2011). However, in alkaline pH range a decline

in As sorption on the COP and NOP surface could possibly be due to competition between

negatively charged hydroxyl ions at higher pH and As-oxyanions for the available sorption sites

as described elsewhere (Rahaman, Basu, and Islam, 2008; Boddu et al., 2008).

Sorption isotherm modeling

In the present study, four isotherms models: Freundlich, Langmuir, Temkin and Dubinin–

Radushkevich were applied to the equilibrium experimental data in order to evaluate its

credibility (Table 1; Figure S1a–d, SI).

Freundlich isotherm constants qf and 1/n were calculated from intercept (qf) and slope (1/n) of

the linear plot of lnqe versus lnCe, respectively (Table 1; Figure S12a, SI). This model describes a

12

distribution of monolayer sorption on heterogeneous energetic active sites, accompanied by

interactions with sorbed ions (As in this study) (Crini et al., 2007). Coefficient of variation (R2)

was higher for COP (0.91) than that of NOP (0.83) (Table 1). It was worth noting that the q f

value was significantly greater for COP (34.07 mg g-1) compared to NOP (1.54 mg g-1),

indicating its highest capacity for As sorption. Freundlich parameter 1/n indicated the extent of

favorability of the biosorption processes under the ambient aqueous conditions. The 1/n is a

measure of sorption intensity or sorbent surface heterogeneity that reflects deviance from

linearity of sorption as follows: if the value of 1/n = 1, the sorption is linear; for 1/n < 1, the

sorption process is chemical; if 1/n > 1, the sorption is a favorable physical process and sorption

is cooperative (Feng et al., 2011; Zhang et al., 2013). The 1/n values were 0.507 and 0.901 for

COP and NOP, respectively, (Table 1) indicating that As biosorption was a favorable chemical

process in the batch experimental systems (Figure S1, SI).

Langmuir sorption isotherm parameter, qL and R2, values for the COP (98.03 mg g–1 and 0.99,

respectively) were found to be higher than the NOP (86.95 mg g–1 and 0.95) (Table 1; Figure

S1b, SI). Separation factors (RL) were determined from the Langmuir isotherm which represents

favorability for As sorption (Figure 4). Typically, if the RL > 1, sorption is an unfavorable

process; if RL = 1, sorption is linear; if 0 < RL< 1, sorption is favorable, and RL = 0, the sorption

is irreversible (Juang, Wu, and Tseng, 1997). The RL values were < 0.13 (with RL < 0.005 at

higher As concentrations) for COP and < 0.65 for NOP, providing the mounting evidence that As

sorption was favorable, and particularly for COP sorption was nearly irreversible process at high

As concentrations (Figure 4).

13

Modeling data indicated that the Langmuir model was able to explain As sorption better than

Freundlich model for both biosorbents based on the higher R2 values as mentioned earlier,

suggesting that the monolayer sorption mechanism of As is favored on the biosorbent surfaces

that is controlled by chemisorption (Lugo et al., 2012).

Temkin and Dubinin–Radushkevich models demonstrated better performance to fit the

equilibrium experimental data for the COP (R2 = 0.96 and 0.97, respectively) compared to the

NOP (R2 = 0.91 and 0.65) (Table 1; Figure S1c,d, SI). In the case of Temkin model, heat of

sorption, b, is an important parameter to differentiate between the sorption efficiency of the

biosorbents in the Temkin isotherm (Foo and Hameed, 2010). Relatively, lower values of b were

obtained for the COP (225) compared to the NOP (321), demonstrating that a linear decrease in

the heat of sorption resulted in high coverage of As on the surface layer of COP (Foo and

Hameed, 2010).

For Dubinin–Radushkevich model, bonding energy (E) values for the COP and NOP were higher

(Table 1) compared to the typical bonding energy range of 8–16 kJ g -1(Ahmad et al., 2013) . The

E values < 8 kJ g–1 reflects that the sorption process is physical in nature and driven by the pore

filling mechanism, and E value from 8 to 16 kJ g–1 represents that the sorption process involves

ion exchange and chemisorption is the favorable process (Memon et al., 2009). As mentioned

earlier, E values were significantly higher than 16 kJ g–1 which may suggest that this model was

unable to explain As biosorption accurately, although R2 value was reasonable higher for model

fits. Modeling data showed that Langmuir model was found to be the best followed by Temkin

model to delineate As biosorption on COP compared to NOP.

14

Surface characterization and morphology of biosorbents

FTIR spectroscopy

The FTIR spectra of the As-loaded and -unloaded COP and NOP were obtained to determine the

stretching and bending vibrations of the functional groups responsible for As sorption (Figure

S2a–d; SI). The broad and intense spectral peaks at 3405 and 3420 cm–1of COP and NOP,

respectively, were associated with the –OH stretching vibrations of cellulose, pectin, adsorbed

water, and lignin components (Figure S2a,b, SI). The spectral bands at 2930 and 2891 cm –1 could

possibly be attributed to the C–H stretching vibrations of alkyl functional groups such as methyl,

methylene and methoxy groups of the COP and NOP, respectively (Figure S2a,b, SI). The

spectral peaks at 1711 for COP and at 1742 cm–1 for NOP were due to the stretching vibrations

of C–O bonds of the non-ionic carboxyl groups (i.e., –COOH, –COOCH3) and can be linked to

carboxylic acids and/or their ester groups (Feng et al., 2011; Lasheen, Ammar, and Ibrahim,

2012; Niazi, Singh, and Budiman, 2014). The symmetric and/or asymmetrical –COO– vibration

bands could be associated with ionic carboxylate groups at 1406, 1623 and 1422, 1624 cm –1, of

the COP and NOP, respectively (Figure S2a,b, SI) (Mohan et al., 2007). The peak observed at

801 cm–1 is an indication of presence of amide (–N-H) group from proteins (Figure S3, SI).

The FTIR spectra of As-sorbed COP and NOP revealed a shift in the position of spectra peaks

(see Figure S2c,d). For the COP, the spectral bands at 3405, 1711, 1406, 1623 cm–1 (Figure S2a)

shifted to 3395, 1715, 1616, 1367 cm–1, respectively (Figure S2c, SI). In the case of NOP, the

bands at 3420, 2891, 1742, 1634, 1422 cm–1 (Figure S2b, SI) were shifted to 3430, 2899, 1735,

1618, 1421, respectively (Figure S2d). These shifts in spectral band positions could possibly be

associated with the surface sorption of As at the biosorbent surface through ion exchange process

15

of As oxyanions with the –OH and –COO– functional groups (Feng et al., 2011; Agrafioti,

Kalderis, and Diamadopoulos, 2014). A slight shift in –N-H band in COP spectra is seen toward

800 cm–1, which indicates an association of As with –N-H group from protein molecules present

in biomass. The shift in spectral peak (wavenumbers) was observed for both biosorbents,

however, for shift in the peak position for COP was greater compared to the NOP suggesting

high As sorption on the surface of COP.

Scanning electron microscopy (SEM)

Surface micromorphology of the COP and NOP was investigated before and after As sorption by

SEM (Figure 5a,b and Figure 6a,b). It is evident from the SEM micrographs that COP has a

larger surface area due to pore spaces and patchy surfaces compared to the NOP, which is also in

agreement with the SSA results obtained by BET method for both biosorbents. As such, a large

number of surface sites were available for sorption of As on COP (Figure 5a vs. Figure 6a). After

As sorption, the SEM micrographs in the Figures 5b and 6b confirmed that the available pore

spaces were well-covered and filled due to the sorption and aggregation of As, thereby

developing layer-like depositions on the porous, rough and irregular surfaces. However, it is

worth noting that the surface sorption and pore-filling mechanism of As was more pronounced

for COP (Figure 5b) compared to the NOP (Figure 6b), thus strongly supporting our data

obtained from the As sorption isotherm experiments showing high As sorption capacity by COP.

Elemental composition of the As-loaded and -unloaded COP and NOP was determined using the

SEM-EDX analysis (Figure S3a,b, SI). The SEM-EDX analysis provided evidence that COP

showed higher wt. % distribution of As (0.07) than the NOP (0.03 wt. %), confirming that COP

possessed the highest As sorption efficiency in this study (Figure S3, SI). Additionally, it is

16

supporting our (Langmuir) modeling findings where we observed the greatest R2 value and As

sorption capacity for COP. Our data indicated the presence of Al in the COP and NOP which can

make complexes with As(V) present in the aqueous solutions and bind As on the surface of the

biosorbent materials in form of Al-arsenate complexes (Khaskheli et al., 2011). We also

observed calcium (Ca) ions in COP and NOP, hence As association with Ca as precipitates may

also be a reason to remove As from the aqueous solution. As a result, As from water was

depleted and immobilized on the biosorbent surface by forming these precipitates (Agrafiotia,

Kalderisb, and Diamadopoulos, 2014).

Desorption of As from biosorbent

Sorption experiments were conducted using both biosorbents (COP and NOP) with As(V) at an

initial concentration of 150 mg L-1. After sorption, it was observed that the sorbed As could be

efficiently desorbed by mixing with 0.01 M HCl for 1.5 hour. The desorbed sorbent was then

washed with deionised water and the As sorption cycle repeated three times. The results (Figure

7) indicate that after 3 cycles the sorption of As(V) was reduced by 10 and 26 % for COP and

NOP, respectively. These observations demonstrate that the recyclability and reuse of the COP

was more than that of NOP, thus COB can be used to remove As from water up to many cycles.

Table 2 shows the comparison of As sorption capacity of various biosorbents in aqueous solution

in comparison with the present study. It appears that COP used in this study showed a

significantly higher As sorption capacity, thus possessed a remarkable potential for the removal

of As from water.

Significance and implications

17

Our results revealed that COP showed more As sorption capacity than the NOP and removed 99–

100 % of total As at pH 6.8 and at 4 g L-1 biosorbent dose. This indicates that 4 g of dried COP

and NOP was sufficient to purify the 1 L of As-contaminated drinking water/wastewater, below

the WHO safe limit (0.01 mg L–1). This novel biosorbent (COP), as evident from our data, is a

cheap and can provide an efficient solution for the remediation of As-contaminated water.

Transformation of biowastes to charred materials by adopting this innovative method could offer

a quick and efficient way to produce charred biosorbents for testing their potential to remove

anionic species, like As, chromate, phosphate from aqueous environments.

After use, As-loaded biosorbents can be converted into ash at low temperature (< 100 °C) to

avoid As loss by volatilization and for safe disposal. Although desorption of As has been studied,

further research is warranted to develop recovery methods for As after desorbing it from As-

loaded biosorbent and potentially utilize As in relevant industries (e.g., semiconductor

manufacturing).

While yet to be explored, further work should be directed to (i) investigate the simultaneous

biosorption mechanisms of As(III) and As(V) by COP in natural groundwater or wastewater

samples using X-ray absorption and X-ray photoelectron spectroscopy techniques; (ii) examine

the influence of competing anions such as phosphate, chromate, sulfate and humic acids on the

As sorption capacity of COP in batch as well as column systems.

Conclusions

This study shows that at 4 g L–1 biosorbent dose maximum As removal (> 98–100 %) was

achieved by COP. The results indicated that COP immobilized up to 99 % of As from As-

18

contaminated water containing 150 mg L–1 of As. The highest As removal was observed at pH

6.8 (99 %) followed by pH 7.2 and 8, thereby providing the best and highly suitable biosorbent

material to remediate the As-contaminated water in a natural pH range. The R2 values showed

that Langmuir isotherm explained 99% of variation between the equilibrium As concentration

and As sorption capacity which was higher than that described by the Freundlich and Temkin

isotherms. Recyclability experiments revealed that As immobilization capacity of COB

decreased to 10 % up to third cycle, which was more than that of NOP (26 %). The SEM-EDX

analyses provided evidence that porous surface of COP resulted in higher As removal from water

than NOP. Our data reveal that the novel method used in this study for producing COP as

biosorbent can offer a cheap and eco-friendly solution in order to treat As-contaminated water

below the WHO safe limit (10 μg L–1).

Acknowledgements

The authors are thankful to the Grand Challenges Canada – Stars in Global Health (Round 5,

Grant No. 0433-01) for the financial assistance. The SEM-EDX and FTIR instrumental analyses

were supported by the Basic Science Research Program through the National Research

Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology

(2012R1A1B3001409).

Supporting Information

19

The 3 figures, isotherm modeling details are available in the Supporting Information on the

Internet at the journal web site.

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