Value-added utilization of maize cobs waste as an environmental friendly solution for the innovative treatment of carbofuran

Accepted Manuscript Title: Value-added utilization of maize cobs waste as an environmental friendly solution for the innovative treatment of carbofuran Author: K.Y. Foo PII: DOI: Reference:

S0957-5820(16)00025-2 http://dx.doi.org/doi:10.1016/j.psep.2016.01.020 PSEP 691

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

7-11-2015 7-1-2016 31-1-2016

Please cite this article as: Foo, K.Y.,Value-added utilization of maize cobs waste as an environmental friendly solution for the innovative treatment of carbofuran, Process Safety and Environment Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.01.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research Highlights

· Highlight the renewable use of maize cobs waste · Potential for the innovative treatment of carbofuran · Great monolayer adsorption capacity of 149.15 mg/g

Ac ce p

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M

an

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· Outline the adsorption isotherms, kinetics and thermodynamics

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· High BET and Langmuir surface area of 132.55 and 202.58 m²/g

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Value-added utilization of maize cobs waste as an environmental friendly solution for the innovative treatment of carbofuran

ip t

K.Y. Foo*

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River Engineering and Urban Drainage Research Centre (REDAC),

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Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia.

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* Corresponding author. Tel: +6045945874

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Fax: +6045941036

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te

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E-mail address: [email protected]

43 Page 2 of 50

Abstract

A new route for the conversion of maize cobs waste, a natural low-cost lignocellulosic

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biomass abundantly available from the food processing industries into an eco-friendly biosorbent (CC) via chemical treatment has been presented. The effectiveness for the

cr

adsorptive removal of a highly hazardous carbamate derivative pesticide, carbofuran

us

from the aqueous solution was attempted. The operational parameters including the effects of modification agents and chemical impregnation ratio on the adsorption

an

capability were investigated. The porosity, functionality and surface chemistry of CC were featured by means of low temperature nitrogen adsorption, elemental analysis,

M

scanning electron microscopy, Fourier transform infrared spectroscopy, evaluation of

d

surface acidity/basicity and zeta potential measurement. The effects of adsorbent dosage,

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initial concentration, contact time and solution pH on the adsorption performance were evaluated in a batch mode study. Equilibrium data were simulated by non-linear fittings

Ac ce p

using the Freundlich, Langmuir, and Temkin isotherm models. Kinetic modeling was fitted to the pseudo-first-order and pseudo-second-order equations. Langmuir isotherm model provided a better correlation to the experimental data, with a maximum monolayer adsorption capacity of 149.15 mg/g, while the adsorption kinetic was best fitted to the pseudo-second-order kinetic model. The results illustrated the potential of maize cobs waste derived biosorbent for the on-site remediation of pesticide contaminated wastewater.

Keywords: Adsorbent; Adsorption; Carbofuran; Maize cobs waste; Pesticide

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1.0

Introduction

To date, the abusive use and indiscriminate release of parental pesticides and their

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metabolites has prevailed to be a detrimental ecological concern, due to the great

tendency to exist indefinitely, streaming, and pseudo-persistent properties over the food

cr

chain (Ferreira et al., 2015). In the last two decades, these anthropogenic products have

us

raised scientific and public concerns regarding their low biodegradability and detrimental implications on the biological equilibrium of natural media and human health (Lladó et

an

al., 2015). It is well established that these emerging pollutants, notably in the form of herbicides, insecticides, fungicides, algaecides, antimicrobials, avicides, miticides,

M

molluscicides, nematicides, rodenticides, bactericides, defoliants, piscicides and

d

virucides, constitute the important pathway of a wide array of toxic substances released

te

into the environmental matrices (Sahithya et al., 2016). The continuous presence of these constituents, even at sub-therapeutic concentrations, indicates an external exposure to the

Ac ce p

public health, flora, fauna, and represents a sharp threat to the natural ecosystems (Ammari et al., 2015).

On a worldwide basis, intoxications attributed to pesticides have been estimated

to be as high as 3 million cases of acute and severe poisoning annually, with 220,000 of reported deaths/year (Yadav et al., 2015). Among all, carbofuran (2,3-dihydro-2,2dimethylbenzofuran-7-yl-N-methyl carbamate), a broad spectrum systemic acaricide, insecticide and nematicide included in the general group of the carbamate derivative pesticides, is widely used against a wide range of soil dwelling and foliar feeding insects, including corn root-worm, wireworms, boll weevils, mosquitoes, alfalfa weevil, stem

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borers, aphids, and white grubs (Agrawal and Sharma, 2010). Carbofuran is characterized by its high solubility and mobility, with the half-life of approximately 30-117 days. According to the United States Environmental Protection Agency (USEPA), the

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maximum acceptable carbofuran concentration in the potable drinking water is recorded at 0.09 mg/L, while the maximum concentration admitted by the World Health

cr

Organization (WHO) is 3 µg/L (Gupta et al., 2006).

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Prolonged exposure to carbofuran is vulnerable to a broad variety of reproductive failure and developmental toxicity presage as neurotoxicity, teratogenicity, mutagenicity

an

and carcinogenicity. Additionally, scientific evidence has documented that carbofuran could induce the disruption of estrous cycle in female hemiovariectomized albino mice,

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significant decrease in ovarian weight with the concomitant decrease of compensatory

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ovarian hypertrophy, reduction in epididymal sperm count; and exert neurotoxic effect by

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impairing mitochondrial functions leading to the oxidative stress. Due to its effect on the reproductive system, carbofuran has been listed as one of the endocrine disrupting

Ac ce p

chemicals (Gbadegesin et al., 2014). This highlights the necessity of priority attention to identify a dedicated pretreatment method of the leaching run-off from agricultural or deposited carbofuran.

Comparatively, adsorption is a competitive treatment process for the removal of a

broad range of organic and inorganic pollutants, mainly ascribed to the ease of operation, simplicity of design, low sensitivity to the variation of temperature or toxic substances, and high adsorption capability (Abbas and Trari, 2015). Despite its prolific use in adsorption processes, the biggest barrier of its application by the industries is the high cost of adsorbents and difficulties associated with regeneration (Anirudhan and

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Ramachandran, 2015). Structurally, agricultural by-products, which composed of lignin, hemi-cellulose and cellulose as the major polymeric constituents, contain a variety of polarities to bind molecules via physical attractive forces, ion exchange and

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physiochemical interactions. Maize corncob, a highly voluminous, costless agricultural waste of corn milling process, is characterized with the bulk density of 0.32 g/cm3, and

cr

different functionalities, alcohols, aldehydes, ketones, acids, phenolic hydroxides, and

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ethers, making it an abundant source of potential scavenger of water pollutants. Additionally, dissection of maize fruit illustrated that the dry weight of grain represents

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only 38% of the entire fruit (Šæiban et al., 2008).

In the formal practice, some quantity of these residues is used as boiler fuel,

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where major portion is discarded by open burning. Although maize corncob has been

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reputed to be a valuable source of glucose, xylose, furfural, ethanol via catalytic

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hydrolysis, the reaction is accompanied by the co-generation of polysaccharide, arabinose, galactose, and mannose, which are difficult to be isolated from the biomass

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hydrolyzates to influence the downstream process (Ding et al., 2013). For this purpose, the present work was undertaken towards upgrading of the available maize corncob biomass from the food processing plants into a low-cost, active biosorbent by chemical modifications for the innovative treatment of a carbamate derivative pesticide, carbofuran. The significant influences of modification agents and chemical impregnation ratio on the adsorption capacity were investigated systematically. Structural, functional and surface chemistry of the prepared adsorbent was evaluated. Moreover, the adsorption equilibrium, isotherms, kinetics and thermodynamics were elucidated.

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Materials and Methods

2.1

Adsorbate

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2.0

Carbofuran, a carbamate derivative pesticide difficult to be degraded in natural

cr

environment was selected as the model adsorbate in this work. The standard stock

solution was prepared by dissolving accurately weighted carbofuran in double distilled

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water to the concentration of 300 mg/L. Working solutions of desired concentrations

Preparation of functionalized biosorbent

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2.2

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were prepared by successive dilution.

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Corn cob (CC), a by-product collected from the food manufacturing industries

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was selected as the raw precursor in this study. The collected sample was washed

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exhaustively with deionized water to remove adhering impurities from the surface. The raw precursors were manually chosen, cleaned, air-dried, crushed and screened to the desired mesh size of 125 to 250 µm. The modification process was performed by mixing 100 g of dried precursor with different modification agents at room temperature and 200 rpm for 24 hours, with the chemical impregnation ratio defined as:

IR 

w AG wCC

(1)

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where wAG and wCC is the dry weight of modification agent (g) and CC (g). The modified biosorbents were rinsed and washed repeatedly with hot and cool distilled water until the filtrate reached to the neutral pH. All modification agents [phosphoric acid (85%),

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sulphuric acid (95%), nitric acid (70%), sodium hydroxide (99%), and sodium carbonate

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Batch adsorption studies

an

2.3

cr

(99.995%)] used throughout the study were analytical-grade chemicals.

The batch adsorption experiments were conducted in a series of 250 mL

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Erlenmeyer flasks containing 200 mL of carbofuran solutions and prefixed amount of biosorbent. The flasks were capped and agitated in an isothermal water bath shaker at 30

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°C with an agitation speed of 120 rpm until the equilibrium was reached. The influence

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of biosorbent dosage on the adsorptive uptake of carbofuran was performed by varying

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the CC dosage from 0.05 to 0.80 g, while keeping the other parameters constant. The effect of pH was examined by regulating the pH from 2 to 12, with an CC dosage of 0.30 g/200 mL and adsorption temperature of 30 °C. The pH was measured using a pH meter. The carbofuran concentrations in the supernatant were withdrawn at

predetermined period and determined using a double beam UV–Vis spectrophotometer (UV-1601 Shimadzu, Japan) at 273 nm. All samples were filtered prior to analysis to minimize interference of the CC fines with the analysis. Each experiment was duplicated under identical conditions. The adsorptive uptake of carbofuran at time t, qt (mg/g) and equilibrium, qe (mg/g), was calculated by:

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qt 

W

(2)

( C  C e )V

(3)

W

ip t

qe 

(C  C t )V

cr

where C0, Ct and Ce (mg/L) are the liquid-phase concentrations of carbofuran at initial,

us

time t (min) and equilibrium, respectively. V is the volume of the solution (L), and W is

( C  C e ) C

X 100 %

(4)

Characterization of CC

te

2.4

d

M

R

an

the mass of adsorbent used (g). The pollutant removal (R %) was determined by:

Ac ce p

The pore structural analysis was characterized by nitrogen adsorption at 77 K with

an accelerated surface area and porosimetry system (Micromeritics ASAP 2020). Prior to the measurement, the sample was degassed under vacuum for 2 h at 573 K. The sample was then transferred to the analysis system where it was cooled in liquid nitrogen. The specific surface area (SBET) was calculated by the Brunauer-Emmett-Teller (BET) equation; the total pore volume (VT) was evaluated by converting the adsorption volume of nitrogen at relative pressure 0.95 to equivalent liquid volume of the adsorbate, while the micropore volume, micropore surface area and external surface area were obtained using the t-plot method.

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The surface morphologies were imaged using a scanning electron microscope (Zeiss SUPRA 35TM VP), while elemental analysis was performed using Perkin Elmer 2400 Series II CHNS/O Elemental Analyzer for the rapid measurement of carbon,

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hydrogen, nitrogen and sulphur content. The surface acidity was estimated by mixing

0.20 g of CC with 25 cm3 of 0.05 M NaOH solution in a closed flask, and agitated for 48

cr

h at room temperature. The suspension was decanted, and the remaining NaOH was

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titrated with 0.05 M HCl. The surface basicity was measured by titration with 0.05 M NaOH after incubation 0.20 g of CC with 0.05 M HCl. The reliable prediction of pHpzc

an

was conducted by adjusting pH of 50 cm3 0.01 M NaCl solution to a value between 2 and 12. 0.15 g of CC was added and the final pH was measured after 48 h under agitation.

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The pHpzc is the point where pHinitial –pHfinal = 0.

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Chemical characterization of surface functional groups was detected using the

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pressed potassium bromide (KBr) pellets containing 5% of CC sample by Fourier transform infrared spectrometer (FTIR-2000, Perkin Elmer) in the scanning range of

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4000–400 cm-1. Prior to analysis, the sample was ground to a fine powder and dried at 105 °C for 24 h. About 5 mg of CC sample was pelletized into a thin pellet using a manual hydraulic press at 10 tones. The pellet was placed in a sample holder inside the analysis chamber.

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Results and discussions

3.1

Effect of preparation parameters

3.1.1

Effect of modification agents

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3.0

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Modification agent plays a decisive role in affecting the adsorption performance

of the functionalized adsorbents (Yari et al., 2015). The effect of modification agents on

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the adsorption uptake of carbofuran, was performed at the modification ratio of 1:1, as

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depicted in Figure 1. It is evident that phosphoric acid (H3PO4) treatment illustrated a dramatic increase in adsorption capacity (99.16 mg/g) as compared to the original

M

biosorbent (11.55 mg/g). During the modification, phosphoric acid would react as an acid catalyst to promote bond cleavage, hydrolysis, dehydration and condensation,

d

accompanied by cross-linking reactions that serve to improve the interior structure of the

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biosorbent matrix. In addition, H3PO4 molecules could be converted into phosphorus-

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containing constituents such as polyphosphoric acid (Hn+2PnO3n+1) and H2O (Marín et al., 2006) that accelerate the removal of volatile components, resulted to the increased porosity [Eq. (5)-Eq. (6)].

n H 3 PO4  H n 2 PnO3n1  (n  1) H 2O

 C  OH  H 3 PO4  C  O  PO(OH ) 2  H 2O

(5)

(6)

Sulphuric acid modified CC showed the adsorptive uptake for carbofuran of 70.23 mg/g. This modification is related to the formation of stable C–O complexes, which 52 Page 11 of 50

accounts for the formation of internal porosities (Guo et al., 2005). The incorporation of sulphuric acid into the interior biosorbent matrix would promote the introduction of

(7)

cr

2 H 2 SO4  C  2SO2  CO2  2 H 2O

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oxygen functional groups following the reaction (Izquierdo et al., 2001):

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The pore enlargement by nitric acid (HNO3) treatment is associated with the oxidative modification with the formation of oxygen complexes over the biosorbent surface.

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During the reaction, the functionalized adsorbents which consist of condensed aromatic

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rings structures are susceptible to oxidation according to the equation (Chingombe et al.,

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te

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2005):

 5HNO3 

COOH COOH

 5HNO2  H 2 O (8)

Concurrently, alkaline hydroxide modified CC exhibited a great enhancement of adsorptive uptake for carbofuran of 125.01 mg/g. The treatment involves the redox reduction and carbon oxidation to generate porosity (Piňero et al., 2005).

6 NaOH  2C  2 Na  3H 2  2 Na2 CO3

(9)

2 Na  CO2  Na 2 O  CO

(10)

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Conversely, sodium carbonate (Na2CO3) modification illustrated the highest adsorptive uptake of 141.99 mg/g. It was presumed that Na2CO3 treatment involved the reduction of

(12) (13)

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Na2 CO3  Na2 O  CO2 Na 2 O  C  2 Na  CO

(11)

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Na2 CO3  2C  2 Na  3CO

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Na2CO3 to form Na, Na2O, CO and CO2 according to the reactions (McKee, 1983):

The sodium compound formed during the modification would diffuse into the internal

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structure of biosorbent surface widening the existing pores. During the reactions, the evolution of CO, CO2 and H2 constituents, and additional reactions between the active

d

intermediates with the biosorbent surface are possible [Eq. (11)-Eq. (13)]. This

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observation is in agreement with the higher reactivity of Na2CO3. Thus, in the present

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study, Na2CO3 was selected as the modification agent due to the maximum adsorption capacity, lower environmental load in its life cycle, less corrosive ability and cheaper chemical cost.

3.1.2

Effect of modification ratio

Effect of chemical modification ratio (MR) on the adsorption uptake of carbofuran was evaluated using Na2CO3 at the MR range from 0.25 to 2.00. (Figure 2). Augmenting MR from 0.25 to 1.50 showed an enhancement of adsorption uptake from 54 Page 13 of 50

18.88 to 142.01 mg/g, and then it steadily decreased. The best adsorption uptake of carbofuran was obtained at the MR ratio of 1.50. It was presumed that by increasing the ratio of Na2CO3/CC, the modification process would play a key role in pore formation.

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The pore width was successively broadened and new micropores-mesopores were formed in the original pore walls, giving a sustaining increase in BET surface area and pore

cr

volume. Correspondingly, the adsorption uptake was further enhanced.

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When the MR was lower than 1.50, the surface active sites reacted partially lowering the adsorption uptake. Beyond the optimum value, the excess of Na2CO3 and

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metallic sodium left in the biosorbent surface could cause blocking of the pores leading to a dramatic decrease of accessible area. Additionally, excessive Na molecules deposited in

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the biosorbent pore wall might entail catalytic oxidation and vigorous decomposition,

Ac ce p

te

2003):

d

which destroys the adsorbent framework lowering the adsorption uptake (Rodenas et al.,

2 Na Oxidation   Na2O Hydration   NaOH

(14)

4 NaOH  C  4 Na  CO2  2 H 2O

(15)

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Adsorption equilibrium studies

3.2.1

Effect of adsorbent dosage

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3.2

Adsorbent dosage is a profound parameter affecting the adsorption process due to

cr

the reason that it predicts the cost of pollutant to be treated (Maheshwari et al., 2015).

The variation of adsorbent dosage on the adsorptive uptake and adsorptive removal of

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carbofuran onto CC is displayed in Figure 3. It is clearly revealed that the adsorptive

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removal of carbofuran increased by increasing the adsorbent dosage from 0.05 g/200 mL to 0.3 g/200 mL, with approximately a linear relationship. It is plausible to suggest that

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by increasing the adsorbent dosage, there would be a greater availability of surface area and exchangeable binding sites. The optimum adsorptive uptake and adsorptive removal

d

of carbofuran are denoted at 148.82 mg/g and 74.77 %, respectively.

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Further increment in adsorbent dosage beyond 0.3 g/200 mL illustrated a steadily

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decrease of adsorption uptake, consequence of the unsaturated adsorption sites during the adsorption process. The reduction in adsorption capacity had been explained by the overlapping or aggregation of adsorption sites as a result of overcrowding of adsorbent particles beyond 0.3 g/200 mL (Malakootian et al., 2016). The statement was supported by the screening effect at a higher adsorbent dosage on the dense outer layer of CC, which shielding the binding sites from the carbofuran molecules, lowering the pollutant removal per unit adsorbent (Onundi et al., 2010).

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3.2.2

Effect of initial concentrations and contact time

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Figure 4 presents the variation of carbofuran adsorption with respect to time at the concentrations 50-300 mg/L. It is clear from Figure 4 that the adsorption process

cr

increased sharply at the initial stage, indicating of the availability of readily accessible

sites. The adsorption was progressively slowed down with prolonging the contact time,

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until the equilibrium was finally attained. Possibly, at the beginning, the solute molecules

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adsorbed onto the exterior surface of biosorbent. When the adsorption of the exterior surface reached to the saturation, the molecules need to diffuse into the interior surface of

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biosorbent. The process was gradually slower as the equilibrium approached. At this point, the amount of carbofuran desorbing from biosorbent is in a state of dynamic

d

equilibrium with the amount of carbofuran being adsorbed. The amount of pollutants

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adsorbed at the equilibrium time reflects the maximum adsorption capacity of the

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biosorbent under those operating conditions (Fakhri and Behrouz, 2015). Initial concentration provides an essential driving force for alleviating the mass

transfer resistance between the aqueous phase and the solid medium (Fakhri, 2015). In the present work, the adsorption equilibrium, qe of carbofuran increased from 33.12 to 149.80 mg/g with an increase in initial concentration from 50 to 300 mg/L. The time profile of carbofuran uptake is a single, smooth and continuous curve leading to saturation, suggesting possible monolayer coverage of carbofuran onto the surface of CC. Besides, it can be deduced from the results that longer contact time was required for the carbofuran solution of higher concentrations to reach to the equilibrium.

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At lower concentrations of 50-100 mg/L, the ratio of initial number of dye molecules to the available surface area is low, subsequently, the fractional adsorption become independent on the initial concentration. However, at higher concentration of

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150-300 mg/L, the availability of adsorption sites become fewer (saturation of the

sorption sites) and longer contact time of 12 hr were required for carbofuran solutions to

cr

reach to the equilibrium. The observation could be explained by the fact that during the

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adsorption process, the carbofuran molecules had to first encounter the boundary layer,

Effect of solution pH

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3.2.3

an

before they could diffuse into the biosorbent and adsorbed onto the binding surface.

d

Solution pH affects the adsorption process by regulating the surface charge,

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heterogeneity, chemical nature, and degree of ionization and specification of the

Ac ce p

adsorbents (Guyo et al., 2015). The adsorptive behavior of carbofuran over a broad pH range of 2–12 is shown in Figure 5. Decreasing solution pH serves to increase the adsorption uptake, with a significant enhancement as the pH decreased from 10 to 8. At low acidic pH, the surface of the biosorbent would be surrounded by the hydronium ions, which may enhance the sorbate interaction with binding sites of the biosorbent by greater attractive forces to accelerate the removal of carbofuran. It is also possible that the surface properties of the biosorbent have been altered as a result of the variation of solution pH. The possible binding sites in the agricultural bioresidue may be carbohydrates, amines, hydroxyl, carboxyl and fiber carbonaceous CxOH (Chopra and Pathak, 2010). These functional groups may be dissociated at different pH 58 Page 17 of 50

values as per their dissociation constants, and take part in the surface complications. In the basic medium, negative charges would appear on the surface, due to the dissociation of the carbonyl and hydroxyl functionalities. The result also suggested a weaker

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interaction between the biosorbent with the deprotonated carbofuran, than with its neutral molecular form. This phenomenon was in agreement with the research finding reported

cr

by Krishna and Philip (2008), possibly ascribed to the masking of surface functional

Adsorption isotherm

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3.3

an

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groups at the high solution pH.

Adsorption isotherm is an invaluable curve describing the phenomenon governing

d

the retention or mobility of a substance from the aqueous porous media or aquatic

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environments to a solid-phase (Ebrahimi et al., 2015). It is important for practical design

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and operation of the adsorption systems. Langmuir isotherm (Langmuir, 1916) is physically plausible isotherm, which was developed from a theoretical consideration based on three assumptions, namely: adsorption cannot proceed beyond monolayer coverage; the ability of a molecule to adsorb at a given site is independent of the occupation of the neighboring sites; and there is no net change of surface coverage at the equilibrium stage. The mathematical expression of Langmuir isotherm model is derived as:

qe 

Q0 K LCe 1  K L Ce

(16)

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where Q0 (mg/g) and KL (L/g) are Langmuir constants related to adsorption capacity and energy of adsorption, respectively. Freundlich isotherm (Freundlich, 1906) is the earliest known relationship

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describing the non-ideal and reversible adsorption. This empirical model can be applied to multilayer adsorption, with non-uniform distribution of adsorption heat and affinities

qe  K F Ce n

(17)

an

1

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cr

over the heterogeneous surface. It is defined as:

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where KF (mg/g).(L/mg)n and 1/n are the Freundlich adsorption constant and a measure of adsorption intensity. Temkin isotherm (Tempkin and Pyzhev, 1940) assumes the heat of

d

adsorption of all molecules in the layer would decrease linearly with surface coverage

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rather than logarithmically due to adsorbent-adsorbate interactions. Its derivation is

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characterized by a uniform distribution of binding energies, given by:

qe  B ln ACe 

(18)

where B = RT/b, with b (J/mol), A (L/g), R (8.314 J/mol K) and T (K) are the Temkin constant related to heat of sorption, equilibrium binding constant, gas constant and absolute temperature, respectively. Due to the inherent bias resulting from linearization, in this study, the alternative isotherm parameter sets were determined by non-linear regression analysis, based upon minimizing the sum squares errors using an optimization algorithm. This provides a 60 Page 19 of 50

mathematically rigorous method for determining isotherm parameters using the original form of isotherm equations (Foo and Hameed, 2010). The validity of the models was

n

 qP )2

n 1

us

i 1

exp

(19)

an

RMSD 

 (q

cr

tool measuring the predictive power of a model derived as:

ip t

further verified by root-mean-square deviation (RMSD), the commonly used statistical

where qexp (mg/g) and qp (mg/g) are the experimental and theoretical adsorption

M

capacities, respectively. The detailed parameters of these different forms of isotherm models are listed in Table 1. The equilibrium data was getting valid for Langmuir

d

isotherm model, with the highest R2 and lowest RMSD, as compared to the other models.

te

Conformation of the data into Langmuir isotherm model indicated homogeneous nature

Ac ce p

of carbofuran onto the binding sites of CC that are identical and energetically equivalent. The results also demonstrated no interaction and transmigration of pesticide molecules in the plane of the neighboring surface. A comparison of monolayer adsorption capacity of carbofuran onto various adsorbents is provided in Table 2. It can be concluded that the low cost biosorbent prepared in this work showed relatively high adsorption capacity of 149.15 mg/g as compared to some previous works as reported in the literature.

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3.4

Kinetic modeling

ip t

Adsorption kinetic provides an invaluable insight into the controlling mechanism of adsorption processes, and the feasibility for scale-up operations (Anitha et al., 2015).

cr

When adsorption is preceded by diffusion through a boundary, the kinetics in most

form of:

 k   1 t  2.303

an

 qe ln   qe  qt

us

systems follow the pseudo-first-order rate equation (Lagergren and Svenska, 1898) in the

(20)

M

where k1 (1/h) is the adsorption rate constant. Contrary to pseudo-first-order equation,

Ac ce p

te

adsorption represented by:

d

pseudo second-order equation (Ho, 1995) predicts the behavior over the whole range of

1 1   k 2t qe  qt  qe

(21)

where k2 (g/mg h) is the adsorption rate constant of pseudo second-order equation. The experimental data of carbofuran adsorption onto CC at different time intervals were simulated by pseudo-first-order and pseudo second-order kinetic models, using the plots ln (qe −qt) against t, and t/qt versus t, respectively. The applicability of the kinetic model was validated by the normalized standard deviation ∆q (%), as defined in Eq (22).

62 Page 21 of 50

 q  100

[( qexp  qcal ) / qexp ]2 n 1

(22)

ip t

where n is the number of data points, qexp (mg/g) and qcal (mg/g) are the experimental and calculated adsorption capacity, respectively. The corresponding correlation coefficients

cr

and kinetics constants are presented in Table 3. The experimental data showed good

us

compliance with the pseudo-second-order kinetic model, with the normalized standard deviation, Δq values which ranged between 5.55 and 16.20 %. Additionally, the

an

correlation coefficient values for the pseudo-second-order kinetic model were higher for all carbofuran concentrations.

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A better fit to the pseudo second-order kinetic model suggested that the adsorption rate is dependent more on the availability of the adsorption sites rather than

te

d

the concentration of the pesticides in the aqueous solution. This observation indicated that the adsorption of carbofuran onto CC is controlled by chemical adsorption involving

Ac ce p

valence forces through electrons sharing or exchange between the biosorbent and the adsorbate molecules.

3.5

Adsorption thermodynamic

Thermodynamic parameters including the Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) are among the most critical aspect in predicting the stability of an adsorption system (Ezechi et. al, 2015). The values of enthalpy change (ΔH), Gibbs free energy change (ΔG), and entropy change (ΔS) were computed following the equations: 63 Page 22 of 50

ln K d 

S H  R RT

(23)

G   RT ln K d

ip t

(24)

an

C Ae Ce

(25)

M

Kd 

us

temperature, and Kd is the distribution coefficient defined as:

cr

where R (8.314 J/mol K) and T (K) are the universal gas constant and absolute

where CAe (mg/L) is the amount adsorbed on solid at equilibrium, and Ce (mg/L) is the

te

d

equilibrium concentration. The values of ΔH and ΔS were determined from the slope and intercept of the van’t Hoff plot of ln Kd versus 1/T (van't Hoff, 1887). The calculated

Ac ce p

values are tabulated in Table 4.

The negative value of ΔH represents exothermic nature of the adsorption

interaction. This exothermic process was attributed to the weakening of adsorptive forces between the binding sites and the carbofuran species, and between the adjacent carbofuran molecules on the adsorbed phase. The effect of temperature can be explained on the basis of solubility. Higher temperature may enhance the solubility and desorption rate of carbofuran, hence these solute molecules exhibited a lower tendency to be adsorbed. Similar behavior was reported by the previous researchers, where increasing temperature dictated an increase in the mobility of the adsorbate molecules, which in turn reduced the adsorption uptake (Salman and Hameed, 2010a). 64 Page 23 of 50

The negative value of ΔS illustrated there is a decrease in the state of randomness at the solid-solution interface during the adsorption process, mainly associated with the binding force between the solute molecules with the adsorbent surface. It reflected low

ip t

significant change in the internal structures of biosorbent during the sorption process. The negative ΔG obtained for the adsorption of carbofuran onto CC indicated spontaneous

cr

nature and feasibility of the adsorption process. A special case to be noted here is the

us

value of ΔG was negative at 30 and 40 °C, and this ΔG value turned positive at higher operating temperature of 50 °C. This positive ΔG is related to the surface tension of the

an

prepared adsorbent minimizing their respective surface area. As a result, an extra surface free energy is required to extend the surface area by stretching or distorting their

M

respective surface. The results were consistent with the earlier findings, where the

Textural and surface characterization

Ac ce p

3.6

te

d

adsorption of carbofuran onto CC was exothermic.

The examination of the textural structure of CC was evaluated from the scanning

electron micrograph, with a magnification of 150 X, as depicted in Figure 6. The prepared biosorbent displays a well organized, uniform and circular porosity, with a series of irregular cavities distributed around the surface. These cavities were resulted from the evolution of volatile matter and evaporation of impregnated modification agent, leaving the space previously occupied by the reagents (Li et al., 2015). The reconstitution of the biosorbent matrix and substantial changes occasioned by the chemical modification

65 Page 24 of 50

played a substantial role to facilitate pore diffusion during the adsorption process (Mohapatra et al., 2009). The surface physical parameters obtained from the N2 adsorption isotherms were

ip t

summarized in Table 5. It was inferred that the modified biosorbent demonstrated the high BET surface area, Langmuir surface area and total pore volume of 132.55 m2/g,

cr

202.58 m2/g, and 0.075 m3/g, respectively. Meanwhile, the mesopores of CC accounts

us

approximately 53 % of the total pore volume, with a well developed porous structure. The deviation of the BET surface area of CC with the unmodified form (< 5 m2/g)

an

verified pore development and widening of the existing pores during the modification

Functional, elemental, and surface chemistry

te

d

3.7

M

stage.

Ac ce p

The representative FTIR spectra of CC is presented in Table 6. The broad band at 3406 cm-1 is assigned to the overlapping of O-H groups, attributed to the decrease in lignin content with respect to cellulose. The transmittance at 2922 cm-1 is representative of symmetric and asymmetric characterize stretching of –CH2 and –CH3, while the sharp peak at 1730 cm-1 is identical to the C=O stretching vibration. The presence of bonded C=C and N-H groups showed the signals at 1635 and 1512 cm-1. The O-H bending vibration in alcohol and ether, and symmetric C-O-H can be associated with the intensities at 1373 and 1252 cm-1 (Arief et al., 2008). The region between 1160 and 1045 cm-1 is ascribed to the asymmetric C-O-H and C-O stretch of dimmers, and intensive

66 Page 25 of 50

peak at 895 cm-1 is corresponded to the glycosidic C1-H group deforming with ring vibration and OH bending (Zheng et al., 2010). The surface chemistry is governed by the heteroatoms bonded to the edges of the

ip t

biosorbent surface (Ersan et al., 2015). The chemical compositions of CC are listed in Table 7. As suggested by the result, the modified biosorbent contains heteroatom

cr

mixtures of carbon, hydrogen, oxygen, nitrogen and sulfur. The analysis showed the

us

presence of C (48.77%), O (42.89%), H (7.32%), N (1.99%) and a small amount of S (0.03%), to rule out the detection of sulfur based functional groups. The surface acidity

an

and basicity is an important criterion interpreting the surface chemistry of the functionalized adsorbents (Sayğılı et al., 2015). CC exhibited a basic behavior, with the

M

surface basicity of 1.83 mmol/g and 1.15 mmol/g as surface acidity. It can be postulated

d

that the basicity nature of modified corn cob was derived primarily from the presence of

te

oxygen-free Lewis sites, carbonyls, pyrone and chromene type structures. However, the surface acidity was associated with the carboxylic, anhydrides, lactones and phenol

Ac ce p

containing groups (Shafeeyan et al., 2010). The surface chemistry was further justified by determination of zero point of charge (pHZPC), an index of the propensity of the surface charge as a function of pH. From the result, the pHZPC of the prepared CC was 8.62. This basic surface would result in more significant electrostatic attraction and chemical bonding between the carbofuran molecules and the basic functional groups, and thereby facilitating the adsorption process (Shattar et al., 2015).

67 Page 26 of 50

4.0

Conclusion

The potential conversion of maize cobs waste into an efficient biosorbent (CC)

ip t

has been highlighted. The prepared CC illustrated a well developed porous structure, with the BET surface area and total pore volume of 132.55 m2/g and 0.075 m3/g. The

cr

versatility for the innovative treatment of carbofuran has been demonstrated. Adsorption

us

equilibrium was satisfactory represented by the Langmuir isotherm model, with a monolayer adsorption capacity of 149.15 mg/g. Adsorption kinetic was favorably fitted to

an

the pseudo-second-order kinetic model, suggesting a chemisorption process.

d

M

Acknowledgement

te

The authors acknowledge the financial support provided by Universiti Sains Malaysia under the Research University (RUI) Grant Scheme (Project No.

Ac ce p

1001/PPSK/814272)

Reference

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ip t

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Chen, J.Q., Hu, Z.J., Ji, R., 2012. Removal of carbofuran from aqueous solution by orange peel. Desalin. Water Treat. 49, 106-114.

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Chopra, A.K., Pathak, C., 2010. Biosorption technology for removal of metallic pollutants-An overview. J. Appl. Nat. Sci. 2, 318-329. Ding, L., Zou, B., Liu, H., Li, Y., Wang, Z., Su, Y., Guo, Y., Wang, X., 2013. A new route for conversion of corncob to porous carbon by hydrolysis and activation. Chem. Eng. J. 225, 300-305.

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ip t

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75 Page 34 of 50

Figure captions

Figure 1: Effects of modification agents (modification ratio = 1:1; adsorbent dosage = 0.2

ip t

g/ 200mL; Co=300mg/L; T=30 °C) on the adsorptive uptake of carbofuran

Figure 2: Effects of chemical modification ratio (modification agent: Na2CO3; adsorbent

cr

dosage = 0.2 g/ 200mL; Co=300mg/L; T=30 °C) on the adsorptive uptake of

us

carbofuran

Figure 3: Variation of adsorbent dosage on the adsorptive uptake and adsorptive removal

an

of carbofuran onto CC at 30 °C

carbofuran onto CC at 30 °C

M

Figure 4: Effect of contact time and initial concentrations on the adsorptive uptake of

d

Figure 5: Effect of solution pH on the adsorptive uptake and adsorptive removal of

te

carbofuran onto CC at 30 °C

Ac ce p

Figure 6: SEM micrograph (150 X) of CC

76 Page 35 of 50

Tables

Table 1: Isotherm parameters for the adsorption of carbofuran onto CC

ip t

Table 2: Comparative evaluation of adsorption capacities onto various adsorbents

cr

Table 3: Kinetic models parameters for the adsorption of carbofuran onto CC at different initial concentrations

us

Table 4: Thermodynamic parameters for the adsorption of carbofuran onto CC

an

Table 5: Porosity structures of CC Table 6: FTIR spectra of CC

Ac ce p

te

d

M

Table 7: The chemical composition, surface acidity and basicity of CC

77 Page 36 of 50

Table 1

Freundlich

Constants n

KF

R2

RMSD

(mg/g).(L/mg)1/n 0.818

6.576

Qo (mg/g)

KL (L/mg)

R2

RMSD

149.15

0.883

0.998

0.639

A (L/g)

B

R2

RMSD

27.87

21.01

0.918

4.391

us

cr

72.726

Ac ce p

te

d

M

Temkin

5.41

an

Langmuir

ip t

Isotherms

78 Page 37 of 50

ip t cr Modification/

Adsorption

preparation method

capacity (mg/g)

Na2CO3

Banana stalks activated carbon

-

Date seed activated carbon

KOH

Commercial activated carbon

-

Commercial activated carbon

-

ce pt

(Filtersorb 300)

ed

Corn cob

M an

Adsorbent

us

Table 2

References

149

Present work

148

Salman and Hameed, (2010a)

137

Salman et al. (2011)

99

Fernandez-Perez et a. (2005)

96

Salman and Hameed, (2010b)

NaCl

84

Chen et al. (2012)

Blast furnace sludge activated

-

23

Gupta et al. (2006)

Blast furnace dust activated carbon

-

13

Gupta et al. (2006).

Chest nut activated carbon

-

2

Memon et al. (2007)

carbon

Ac

Orange peel activated carbon

79 Page 38 of 50

Table 3

(mg/L)

(mg/g) k 1 (1/h)

Pseudo-second-order

R2

qe, calc (mg/g)

Δq

k2

qe, calc

(%)

(x 103)

(mg/g)

0.190

44.63

0.906

34.75

100

66.08

0.249

85.87

0.912

29.95

150

98.52

0.156 178.12 0.861

80.77

200

128.31 0.149 256.92 0.838

250

142.65 0.125 322.46 0.830

300

149.80 0.133 340.27 0.856

(%)

30.90

30.95

0.999

6.55

15.77

71.82

0.999

8.69

11.82

103.99

1.000

5.55

100.23

9.64

143.77

0.998

12.05

126.05

8.86

155.79

0.999

9.22

127.15

6.88

174.07

0.998

16.20

Ac ce p

te

d

an

33.12

Δq

M

50

us

(g/mg.h)

R2

cr

qe, exp

ip t

Pseudo-first-order

C0

80 Page 39 of 50

1

Table 4

2

303 K

313K

-2.322

-0.893

323K 0.377

us an M d te Ac ce p

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

-135.10

ΔG° (kJ/mol)

cr

-42.23

ΔS° (J/mol K)

ip t

ΔH° (kJ/mol)

Table 5

Properties

CC

BET surface area (m2/g)

132.55

Micropore surface area (m2/g)

66.11

External surface area (m2/g)

66.44

81 Page 40 of 50

0.035

Mesopore volume (cm3/g)

0.040

Average pore size (nm)

3.578

ip t

Micropore volume (cm3/g)

cr

0.075

us

Total pore volume (cm3/g)

te

Table 6

d

M

an

202.58

Ac ce p

24 25 26 27 28 29 30 31 32 33 34

Langmuir surface area (m2/g)

82 Page 41 of 50

Wavelength (cm-1)

Assignation

1

3406

Bonded –OH groups

2

2922

CH vibration of –CH2 andCH3 groups

3

1730

C=O stretching

4

1635

Bonded C=C

5

1512

NH bending vibration

6

1373

Phenolic OH vibration and bending of the O-

cr

ip t

IR peak

1252

Symmetric C-O-H

8

1160

Asymmetric C-O-H

9

1045

C-O stretch of dimmers

898

us

7

an

H group

C-O-C group of primary hydroxyl stretching

Glycosidic C1-H group deforming

M

10

with ring vibration and OH bending

te

d

35

37 38 39 40

Ac ce p

36

Table 7

Element

CC (Wt %)

Carbon

48.77

Oxygen

42.89

Hydrogen

7.32

Nitrogen

1.99

Sulphur

0.03

Acidity (mmol/g)

1.150 83 Page 42 of 50

Basicity (mmol/g)

1.825

Total content of surface oxides (mmol/g)

2.975

pHzpc

8.62

Ac ce p

te

d

M

an

us

cr

ip t

41 42 43 44 45 46 47 48 49 50 51

84 Page 43 of 50

ip t cr us an HNO3

H3PO4

Na2CO3

te

Figure 1

d

Modification agents

Ac ce p

51 52 53 54 55 56 57 58 59 60

M

H2SO4

85 Modification agents

Page 44 of 50

ip t cr us an M d Figure 2

te

Modification ratio

Ac ce p

61 62 63 64

86 Page 45 of 50

ip t cr us an M te

d

Figure 3

Ac ce p

65 66 67 68 69 70 71 72 73 74

87 Page 46 of 50

ip t cr us an

M

Time (h)

te

d

Figure 4

Ac ce p

75 76 77 78 79 80 81 82 83

88 Page 47 of 50

ip t cr us an M te

d

Figure 5

Ac ce p

84 85 86 87 88 89 90

89 Page 48 of 50

ip t cr us an M d te

Figure 6

Ac ce p

91 92 93 94 95 96 97

90 Page 49 of 50

97

Value-added utilization of maize cobs waste as an environmental friendly solution

98

for the innovative treatment of carbofuran

99 K.Y. Foo *

101 102

River Engineering and Urban Drainage Research Centre (REDAC),

103

Engineering Campus, Universiti Sains Malaysia,

104

14300 Nibong Tebal, Penang, Malaysia.

105

* Corresponding author.

106

Tel: +6045945874

107

Fax: +6045941036

108

E-mail address: [email protected]

cr

us

an

M

d

Graphical abstract

Ac ce p

110 111

te

109

ip t

100

Maize cobs biosorbent

112

91 Page 50 of 50