Saudi Arabia S Eastern Province Shallow Aquifers

REMOVAL OF ARSENICAND MERCURYFROM

WATER BY MODIFIED AND NON MODIFIED MULTI-WALLED CARBON NANOTUBES

استخلاص الزرنيخ و الزئبق من الماء باستخدام انابيب الكربون النانوية المتعددة الاسطح المعدلة و الغير معدلة

Abdallah Abu-AlKibash* (PI)

Mazen M. Khaled* (CoI)

Muataz Ali** (CoI)

*Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia.

* Department of Chemical Engineering, Department of Chemistry, King Fahd University of Petroleum and Mineral, Dhahran 31261, Saudi Arabia

; *

ABSTRACT

Due to low precipitation and limited fresh surface and groundwater resources, water is considered as a scarce commodity in the Kingdom of Saudi Arabia. These limited water resources need to be kept free of contamination in order for it to be used more efficiently. A number of classical water treatment methods are used for the removal of organic and inorganic contaminants. However, in recent years, nanotechnology has emerged as one of the latest editions of technological advancements in water treatment since its boost in the early 1990s. In this study, Multi-Walled Carbon Nanotubes (MWCNTs) will be used to remove Arsenic (As)andMercury (Hg) from water in a batch mode. Several parameters will be investigated to determine the optimum treatment conditions. These parameters include: pH, initial concentrations of As and Hg, dosage of the adsorbent (CNT), mixing rateand contact time. In addition, adsorption isotherms will be developed and studiedfor the various treatment conditions. A linear regression Model of adsorption isotherms will be used in the analysis. The physical structure and morphology of the exhausted CNTs used in the treatment experimentswill be also studied.Optimum treatment conditions will be then determined for the removal of As and Hg from water. The results of this studyare expected to contribute to the development of nanotechnology applications in water treatment processes in the Kingdom.

الملخص

نظرا لقلة الامطار و محدودية السطح المائي و قلة المياه الجوفية يعتبر الماء سلعة نادرة في المملكة. و يجب ان تبقى هذه المصادر المحدودة للمياه بعيدة عن أي نوع من انواع التلوث و ذلك ليتم استغلالها بكفاءة. يوجد العديد من الطرق التقليدية التي تستخدم في استخلاص الملوثات العضوية و غير العضوية من الماء الا انه في السنوات القليلة الماضية تم استخدام تكنولوجيا النانو في معالجة المياه. سيتم في هذه الدراسة استخدام انابيب الكربون النانوية متعددة الاسطح في ازالة كل من الزرنيخ و الزئبق من الماء و سيتم في هذه الدراسة التوصل الى العوامل المثلى التي تشمل درجة الحموضة, تراكيز الاولية لكل من الزئبق و الزرنيخ. و كمية المادة الممتصة بالاضافة الى معدل الخلط و زمن التماس و سيتم بيان منحنيات الادمصاص الحرارية لكل العوامل المؤثرة.كما سوف يتم استخدام نموذج احصاني خطي في عملية التحليل و ستتم دراسة التركيب و الشكل العام لانابيب الكربون النانوية التي سوف تستخدم في هذه الدراسة. يهدف البحث الى التوصل الى تحديد ظروف المعالجة المثلى لازالة كل من الزرنيخ و الزئبق من الماء. و ستساهم نتائج هذا البحث في تطوير تكنولوجيا النانو و استخداماتها في معالجة المياه في المملكة.

Table of Contents

List of figures AND TABLES

1. INTRODUCTION

Water is the new oil of the 21stCentury and is becoming more valuable due to the increased consumption and demand. Good quality water (i.e. water free of contaminants) is essential to human health and a critical feedstock in a variety of key industries including oil and gas, petrochemicals, mining, pharmaceuticals and food processing. The available supplies of water are decreasing due to (i) low precipitation, (ii) increased population growth, (iii) more stringent health based regulations, and (iv) competing demands from a variety of users e.g. industrial, agricultural and urban developments. Consequently water scientists and engineers are seeking alternative sources of water. These alternative sources include: seawater, stormwater, wastewater (e.g. treated sewage effluent), and industrial wastewater.

Many industries generate huge amounts of wastewater. Water recovery/recycle/reuse has proven to be effective and successful in creating a new and reliable water supply, while not compromising public health.[1]Removal of contaminants and reuse of the treated water would provide significant reductions in cost, time, and labor to industry and result in improved environmental stewardship. Most of the remediation technologies available today, while effective, very often are costly and time-consumingmethods. Advances in nanoscale science and engineering suggest that many of the current problems involving water quality could be resolved or greatly ameliorated using nanosorbents, nanocatalysts, bioactive nanoparticles, nanostructured catalytic membranes and nanoparticle enhanced filtration among other products and processes resulting from the development of nanotechnology.[2]

Nanotechnology is one of the latest editions of technological advancements since its boost in the early 1990s. Nanotechnology refers broadly to using materials and structures with nanoscale dimensions, usually ranging from 1 to 100 nanometers (nm). Nanometer-sized particles have been developed to improve the mechanical properties of materials, initiate photographic film development, and serve as vital catalysts in the petrochemical industry. Nanotechnology could substantially enhance environmental quality and sustainability through pollution prevention, treatment, and remediation.

Environmental scientists and engineers are already working with nanoscale structures and many products are increasingly reaching the market. Nanotechnology is not just about the size of very small materials, more importantly it is about the ability to work, observe, manipulate, and build at the molecular level. This results in materials and systems that often exhibit novel and significantly changed physical, chemical, and biological properties due to their size and structure. These new properties include improved catalysis, tunable wavelength sensing ability, and increased mechanical strength.[3]In the last decade there has seen a significant progress in every application of nanotechnology including nanoparticles, nanotubes, nanofibers, nanolayers nanodevices and nanostructured biological materials. [4]

Among these applications of nanotechnology, it is identified that Carbon Nanotubes (CNTs) emerged with very promising applications[4] since their boost by the report made by Iijima in 1991. [5][6] CNTs, a new member of the carbon family, have attracted special attentions to many researchers and engineers because they possess unique morphologies and have showed excellent properties and great potentials such as composite reinforcement, nanodevice component, gas adsorption material and catalyst support phases. [7]

Moreover, CNTs are also good anion and cation adsorption materials for water and wastewater treatment, as they exhibit exceptionally large specific surface area. [7]In addition to the remarkable mechanical properties, their hollow and layered nanosized structures make them a good candidate as adsorbers. [8]

1. LITERATURE REVIEW

Carbon nanotubes (CNTs) have recently drawn the attention of scientists and researchers for their exceptional mechanical and electronic properties, large specific surface area, high thermal stability and diverse applications.CNTs are tubular sheets of graphite which are divided into single-walled carbon nanotubes (SWCNTs) and multi-walledcarbon nanotubes (MWCNTs) basedon the carbon atom layers in the wall of the nanotubes(Figure 1).

Their hollow and layered nanosized structures make them a good candidate as adsorbers. Long and Yang (2001) reported that significantly higher dioxin removal efficiency is found with CNTs than with activated carbon. [9] Li et al. found that CNTs have high lead[7], fluoride [10], cadmium (II)[11] and chromium (II)[12]adsorption capacities and can be used as an adsorbent for the removal of the aforementioned metals from water. The high adsorption capacity of CNTs to Zinc (II) was confirmed by Lu and Chiu (2006) [13].Kandah and Meunier (2007) have evaluated nickel ions removal by CNTs from water [14]. Furthermore, other researchers have examined CNTs’ adsorption capacities to different organic pollutants such as 1,2-dichlorobenzene[15], trihalomethanes[16], o-xylene and p-xylene[17], polycyclic aromatic hydrocarbons (PAH) [18] and natural organic matter [19].However, the studies on the adsorption of heavy metals with CNTs and the effect of pH, metal ions concentration, dosage of the adsorbent i.e. CNT, mixing rate and time contacton the adsorption efficiency are still limited in the literature.

1. STATEMENT OF THE PROBLEM

Industrial processes such as paper manufacturing, textile processing, food and beverageproduction, metal processing, and petrochemical refining produce huge volumes of wastewater which is a valuable resource once properly treated. Removal of contaminants and reuse ofthe treated water would provide significant reductionsin cost, time, and labor to industry and resultin improved environmental stewardship. Arsenic (As) and Mercury (Hg) are among those contaminants that must be removed from water,due to their high toxicity and tendency to accumulate in tissues of living organisms.

Different methodologies, with varying level of success, have been employed to remove heavy metals from water and wastewater. Efficiently removing heavy metals from water depends on the metal itself and its concentration. Biological treatment (aerobic and anaerobic), coagulation, precipitation,membrane filtration, ion exchange, oxidationand in-situ immobilization adsorption (activated carbon)are common methods of removing Asand Hg from wastewater streams.Adsorption is one of these methods which areregarded as the mostpromising and widely used method among them.Numerous materials were used as adsorbents to removeheavy metal ions from water, such as metal oxide, active carbon, sepiolite, chitin, biosorbent, metal sulfide, resin, and so on. Thesearch for new and more effective materials to be usedas adsorbents is a continuous effort for many researchers.[20]

One of the emerging technologies is CNT which is becoming a valuable tool in industrial applications.In this study we investigate the efficiency of CNT in removing As and Hg from water during the treatment process.The significance of this study is to have a better understanding on the applications of CNT in preserving the environment and also to hold paramount the safety, health and welfare of the society. Furthermore, this study is also conducted to determine the impact of the different solution conditions on CNTs adsorption efficiency. Hopefully, it will lead the way of process adjustments and devices that achieve desired arrangements of nanotubes during large-scale production.

1. OBJECTIVES

The emergence of nanotechnology and also the increasing rate of environmental contamination have encouraged me to proceed with this study which is a combination between these two fields. The main objectives of this study are as follows:

• Design and fabricateCVD reactor for CNT Production
• Experimental Setup and Trial Production of CNTs to Optimize the Process
• To remove Arsenic (As)and Mercury (Hg) from water during the treatment process by using nonmodified multi-walled carbon nanotubes.
• To impregnate gold nanoparticles on the surface of the carbon nanotube.
• To remove the arsenic and mercury metals by using impregnated CNT.
• To study the effect of solution conditions such as pH, metal ions concentration, dosage of the adsorbent i.e. CNT, time contact and agitation speed during mixing on the adsorption efficiency.
• To evaluate the adsorption isotherms of the experimental data.
• To characterize the exhausted CNTs after the removal usage.

1. SCOPE OF WORK

In order to achieve the objectives of this study, MWCNTs produced by KFUPM will be used to remove Asand Hgfrom water in this study. One parts per million (ppm) of analytical grade solutions of Asand Hg will be prepared to evaluate the efficiency ofMWCNTs in removing As and Hg.Furthermore, this study will be conducted with different experimental conditions foreach sample.The influences of the dosage of the adsorbents i.e. CNT, time contact, pH, and the mixing rate will be investigated. Therefore, the experiment optimization for As and Hg removal from water can be obtained and studied for the usage of further experimental development. Adsorption isotherm models will be used to obtain the best fit for the experimental data. After the removal ofAs and Hg, by field emission scanning microscopy (FESEM) and/or AFM will be utilized to characterize the CNTs.Hopefully; the outcome of this study will assist in the development of wastewater treatment, environmental contamination reduction and enhance the development of nanotechnology and its applications.

1. MATERIALS AND METHODS

1. Obtainment of MWCNTs

Multi-walled carbon nanotubes (MWCNTs) will be produced and optimized using the equipment procured in this project. The CNTs will be kept dry in a glass bottle at room temperature of 25ºC.

1. Impregnation of CNT with Gold nanoparticles

CNTs and 1 wt% HAuCl4 solution will be mixedby sonication for 5 min to make nanotubes dispersed equably. Then the suspended solution will be diluted to 100 ml with doublydistilled water. The flask will be placed in the centerof a microwave oven and heated to boiling by microwaveradiation for 60 s. Afterward, 4 ml of sodium citrate (1 wt%) willbe added to the heated solution which is kept boilingfor 2–5 min by microwave radiation until the color of the solutiondid not change. The resulting suspension will be centrifuged and thesolid product will be dried at 1200C overnight in a vacuum oven.

1. Preparation of As and Hg Stock Solutions

The preparation of As and Hg stock solutions will be done to produce standardsolutions of 1 mg/l concentrations. The glassware to be utilized for the experiment will be rinsed with 2% nitric acid in order to remove all the impurities that may be present and to prevent further adsorption of the heavy metals to the walls of the glasswares.

The standard solutions will be prepared by pipetting 1ml of Asand Hg from the stock solutions of 1000mg/l into a 1L volumetric flask and mixed thoroughly. The calculations fordetermining the volume of As and Hg to be taken from the stock solutions are as follows:

V1M1 = V2M2(1)

Where:

V1 = Volume of standard solution (L)

V2 = Final desired volume (1L)

M1 = Concentration of the standard solution (1000mg/L)

M2 = Concentration of the stock solution that we need (1 mg/L)

Once the stock solutions are prepared, the pH of the solutions will be adjusted using 0.1M HCl and 0.1M NaOH and buffer will be used for maintaining the pH of the solutions according to the pH required i.e. 3, 5, 7, 8 and 9.

1. Batch Mode Adsorption Experiment

The standard solutions of fixed concentration of 1 mg/L will be prepared by dissolving the required amount ofAs and Hg stock solutions with distilled water. The desired weight of adsorbent, in this case, the CNTs (5 and 10mg) will be added to 50 mL Erlenmeyer flasks containing the fixed concentration (1.0 mg/L). Then, Erlenmeyer flasks arethen agitated at the desired speed (50,100,150 and 200 rpm) using a mechanical shaker. The pH will be adjusted using 0.1 M HCl and 0.1 M NaOH.

After the desired equilibrate time (10, 20, 40, 60 and 120 min) has passed, the adsorbent will be filtered on 0.45µm filter papers. The filtered solution will be analyzed for As usingthe purchased atomic absorption spectrophotometer with graphite furnace, where as the filtered Hg solutions will be analyzed bymercury analyzer system manufactured by P.S. Analytical Ltd available in the RI.

The effects of the dosage of adsorbent, the initial concentration of As and Hg, pH, contact time and the agitation speed will be studied. The amount of As and Hg adsorbed on CNTs will be determined by the difference between the initial concentration (Ci) and the equilibrium concentration (Cf).The percentage removal of As and Hg ions from the solution will be calculated using the following relationship:

(2)

The metal sorption capacity (qt) will be calculated by the following equation:

(3)

Where:

V = volume of the solution (L)

Ms = weight of adsorbent (g)

1. Experimental Design

Table 2 shows the experimental parameters and their variationswhich will be used in the batch mode adsorption experiment. The initial concentrations of As and Hg in this studywill be fixed at (1.0 mg/L).

Table 2: Experiment parameters and their variation

1. Adsorption isotherms models

Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among liquid and adsorbent, based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage, and possibility of interaction between the adsorbate species. The Langmuir model assumes that there is no interaction between the adsorbate molecules and the adsorption is localized in a monolayer. The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes froma liquid to a solid surface, and assumes that different sites with several adsorption energies are involved.In order to model the adsorption behavior and calculate the adsorption capacity for the adsorbent, the adsorption isotherms will be studied.The Langmuir adsorption isotherm is perhaps the best known of all isotherms describing adsorption and it is often expressed as:

(4)

Where;

Qe = the adsorption density at the equilibrium solute concentration Ce (mg of adsorbate per g of adsorbent)

Ce = the concentration of adsorbate in solution (mg/l)

Xm = the maximum adsorption capacity corresponding to complete monolayer coverage (mg of solute adsorbed per g of adsorbent)

K = the Langmuir constant related to energy of adsorption (l of adsorbent per mg of adsorbate)

The above equation can be rearranged to the following linear form:

(5)

The linear form can be used for linearization of experimental data by plotting Ce/Qe against Ce. The Langmuir constants Xm and K can be evaluated from the slope and intercept of linear equation.

In addition, we can describe adsorption with Langmuir if there is a good linear fit. If not then maybe some other model will work. Therefore, we can use Freundlich Isotherm.

(6)

Where;

Qe is the adsorption density (mg of adsorbate per g of adsorbent)

Ce is the concentration of adsorbate in solution (mg/l)

Kf and n are the empirical constants dependent on several environmental factors and n is greater than one.

This equation is conveniently used in the linear form by taking the logarithmic of both sides as:

(7)

A plot of lnCe against lnQe yielding a straight line indicates the confirmation of the Freundlich isotherm for adsorption. The constants can be determined from the slope and the intercept.

1. Kinetic Modeling

The study of sorption kinetics is applied to describe the adsorbate uptake rate and this rate evidently controls the residence time of adsorbate at solid liquid interface. In order to investigate the mechanism of sorption of Asand Hg by the CNTs, Pseudo second-order kinetic is chosen. It is used to describe the relationship between the adsorbent and time. The pseudo second order equation can be expressed by the following relationship:

(8)

Where:

qe = sorption capacity at equilibrium

qt = sorption capacity at time (mg/g)