xác định hàm lượng asen trong nước ngầm bằng phương pháp lò graphite (bằng tiếng anh)

water with the arsenic concentration is higher than the World Health Organization standard of 10 mg / l ppb.. GF-AAS method is highly sensitive method that can determine the total arseni

Also I am thankful to all my classmates for being great

friends and helping me so much

March, 29 th , 2012 Doan Thi Bich Ngoc

Table of Contents

Abstract (English)

water with the arsenic concentration is higher than the World

Health Organization standard of 10 mg / l (ppb) Arsenic in groundwater is of natural origin and it is released from the sediments due to the

anoxic conditions or from the weathered and runoff ores

containing arsenic Arsenic is a poisonous compound that can cause some diseases such as lung cancer, skin cancer, bladder cancer and respiratory diseases For this reason, there are many studies to find out the useful

method to reduce arsenic concentrations in water

In this report, I used the method of atomic absorption

spectrometry with graphite furnace to determine the arsenic levels

in water and study the use of laterite material to absorb arsenic from

water GF-AAS method is highly sensitive method that can determine the total arsenic in water with high repeatability and detection limits can be achieved ppb levels Using treated laterite as the adsorbent, water

quality is greatly improved, and arsenic can be reduced by 90%, in addition with the low cost and wide applicability

Abstract (Vietnamese)

Ô nhiễm asen là một vấn đề vô cùng nhức nhối không chỉ ở Việt Nam

mà trên toàn thế giới Người ta ước tính khoảng 57 triệu người đang sử dụngnguồn nước mặt có hàm lượng asen cao hơn tiêu chuẩn của Tổ chức Y tế Thếgiới là 10 μg/l (ppb) Asen trong nước mặt có nguồn gốc tự nhiên và nó đượcgiải phóng ra từ trầm tích do các điều kiện thiếu ôxy hoặc từ các quặng chứaasen bị phong hóa và rửa trôi Asen là chất độc có thể gây ra một số loại bệnhnhư: ung thư phổi, ung thư da, ung thư bang quang và các bệnh về hô hấp Vì

lý do đó mà rất nhiều công trình nghiên cứu nhằm tìm ra các phương pháphữu ích để làm giảm nồng độ asen trong nước

Trong bài báo cáo này, tôi đã sử dụng phương pháp quang phổ hấp thụnguyên tử với lò graphite để xác định hàm lượng asen trong nước và nghiêncứu sử dụng đá ong để làm vật liệu hấp thụ asen có trong nước Phương phápGF-AAS là phương pháp có độ nhạy cao, có thể xác định được tổng hàmlượng asen có trong nước với độ lặp lại cao và giới hạn phát hiện có thể đạthàm lượng ppb Sử dụng đá ong có xử lý làm vật liệu hấp phụ, chất lượngnước được cải thiện rất nhiều, hàm lượng asen có thể giảm đến 90%, hơn nữagiá thành rẻ và khả năng ứng dụng rộng rãi

In history, arsenic in science, medicine and technology has beenovershadowed by its notoriety as a poison in homicides Arsenic is viewed asbeing synonymous with toxicity Dangerous arsenic concentrations in naturalwaters are now a worldwide problem High arsenic concentrations have beenreported recently from the USA, China, Chile, Bangladesh, Taiwan, Mexico,Argentina, Poland, Canada, Hungary, Japan and India Among them thelargest population at risk is in Bangladesh followed by West Bengal in India.Twenty one countries in different parts of the word, groundwater containsarsenic Many countries in the world are carrying on investigation on arsenic.Historically, colorimetric and gravimetric methods have been used fordetermination of arsenic In recent years, atomic absorption spectrometry(AAS) has become the method of choices However a commonly usedtechnique for the measurement of arsenic is the highly sensitive hydridegeneration atomic absorption spectrometric method After examine arsenic, ifarsenic is over legal limit in water, those countries must remove arsenic.There are traditionally technologies to remove arsenic from water (oxidation,precipitation/coagulation/membrane separation) with far less attention paid toadsorption The sorption capacities of both available and developed sorbentsused for arsenic remediation together with the traditional remediationmethods We have incorporated most of the valuable available literature onarsenic remediation by adsorption Existing purification methods for drinkingwater; wastewater; industrial effluents, and technological solutions for arsenichave been listed Arsenic sorption by commercially available carbons andother low-cost adsorbents are surveyed and critically reviewed and theiroption efficiencies are compared Some commercially available absorbentsare also surveyed An extensive table summarizes the sorption capacities ofvarious adsorbents Some low-cost adsorbents are superior including treatedslag, carbons developed from agricultural waste (char carbons and coconuthusk carbons), biosorbents (immobilized biomass, orange juice residue),goethite and some commercial adsorbents, which include resins, gels, silica,treated silica tested for arsenic removal come out to be superior Desorption

of arsenic followed by regeneration of sorbents has been discussed Strongacids and bases seem to be the best desorbing agents to produce arsenicconcentrates Arsenic concentrate treatment and disposal obtained is brieflyaddressed This issue is very important but much less discussed.

33 and an atomic mass of 74.91

Arsenic is ubiquitous and ranks 20th in natural abundance, comprisingabout 0.00005% of the earth’s crust, 14th in the seawater, and 12th in thehuman body It’s concentration in most rocks ranges from 0.5 to 2.5 mg/kg,though higher concentrations are found in finer grained argillaceoussediments and phosphorites It is a silver-grey brittle crystalline solid, specificgravity 5.73, melting point 817 oC (at 28 atm), boiling point 613 oC and vaporpressure 1mm Hg at 372 oC

1.1.2 Chemical and physical properties of arsenic compounds

Arsenic is a metalloid widely distributed in the earth’s crust It can exist

in four valency states; –3, 0, +3, and +5 In strongly reducing environments,elemental arsenic and arsine (–3) can exist Under moderately reducingconditions, arsenite (+3) may be the dominant form, but arsenate (+5) isgenerally the stable oxidation state in oxygenated environments

Environmental forms include arsenious acids (H3AsO3, H3AsO3,

H3AsO32-), arsenic acids (H3AsO4, H3AsO4-, H3AsO42-), arsenites, arsenates,methyl arsenic acid, dimethylarsinic acid, arsine, etc Arsenic (III) is a hardacid and preferentially complexes with oxides and nitrogen Conversely,

Inorganic forms of arsenic most often exist in water supplies Arsenic isuniquely sensitive to mobilization (pH 6.5–8.5) and under both oxidizing andreducing conditions among heavy metalloids Two forms are common innatural waters: arsenite (AsO33-) and arsenate (AsO43-), referred to as arsenic(III) and arsenic (V) Pentavalent (+5) or arsenate species are AsO43-,HAsO42-, H2AsO4 -while trivalent (+3) arsenates include As(OH)3, As(OH)4-,AsO2OH2- and AsO33-

1.2 Arsenic and water

Arsenic can be found in seawater (2-4 ppb), and in rivers (0.5-2 ppb).Half of the arsenic present is bound to particles Freshwater and seas algaecontain about 1-250 ppm of arsenic, freshwater hydrophytes contain 2-1450ppm, marine mollusks contain 1-70 ppm, marine crustaceans 0.5-69 ppm, andfishes 0.2-320 ppm (all values are based on dry mass) In some marineorganisms, such as algae and shrimp, arsenic can be found in organiccompounds.[1]

The legal limit for arsenic in water applied by the World HealthOrganization (WHO) is 10 µg/L

1.2.1 Arsenic react with water

Elementary arsenic normally does not react with water in absence ofair It does not react with dry air, but when it comes in contact with moist air alayer is formed The layer has a bronze color, and later develops a blacksurface An example of an arsenic compounds that reacts strongly with water

is orpiment This is an amorphous arsenic compound Reaction mechanism:

As2S3 + 6 H2O → 2 H3AsO3 + 3 H2S

In natural water arsenic participates in oxidation and reduction reactions,coagulation and adsorption Adsorption of arsenic to fine particles in waterand precipitation with aluminum or iron hydroxides causes arsenic to entersediments

1.2.2 Arsenic present in water

Metallic arsenic is processed in lead or copper alloys, to increasehardness The extremely toxic arsenic gas ASH3 plays an important role inmicrochip production Copper arsenate (Cu3(AsO4)2.4H2O) is applied as apesticide in viticulture, but its use is currently prohibited in many countries.Paxite (CuAs ) is an insecticide and fungicide

1.3 Effect of arsenic

1.3.1 The environmental effects of arsenic in water

Arsenic is an essential compound for many animal species, because itplays a role in protein synthesis It is unclear whether arsenic is a dietarymineral for humans Arsenic toxicity is another important characteristic Theboundary concentration of arsenic is 2-46 ppm for freshwater algae.Plants absorb arsenic fairly easily, so that high-ranking concentrations may bepresent in food The concentrations of the dangerous inorganic arsenics thatare currently present in surface waters enhance the chances of alteration ofgenetic materials of fish

1.3.2 The health effects of arsenic in water

Arsenic related illness is usually caused by consumption ofcontaminated drinking water In the old days it was applied as a poison,because symptoms of arsenic poisoning resemble cholera symptoms, andtherefore the intentional factor was shaded

Arsenic in drinking water is an issue of global importance; therefore thelegal limit was decreased to 10μg /L This legal limit is not met in countriessuch as Vietnam and Bangladesh, where millions of people consume drinkingwater with an arsenic content of over 50μg /L This problem results in long-term chronic health effects, such as skin disease, skin cancer, and tumors inlungs, bladder, kidneys and liver

1.3.3 Arsenic contamination of water in the world

Arsenic in natural waters is a worldwide problem Arsenic pollution hasbeen reported recently in the USA, China, Bangladesh, Taiwan, Mexico,Argentina, Poland, Canada, Hungary, New Zealand, Japan, and India Thelargest population with known groundwater arsenic contamination is inBangladesh, followed by West Bengal in India Larger regions in the USA areaffected Vulnerable areas in Nepal, Pakistan, Thai-land, Laos, Cambodia,and Sumatra have barely or not been examined so far Many other countriesand districts in South East Asia, such as Vietnam, Cambodia, and China havegeological environments conducive to generation of high-arsenic

1.3.3.1 At Bangladesh and West Bengal

According to the World Health Organization, “In Bangladesh, WestBengal (India) and some other areas, most drinking-water used to be collectedfrom open dug wells and ponds with little or no arsenic, but withcontaminated water transmitting diseases such as diarrhea, dysentery, typhoid,cholera and hepatitis Programmes to provide ‘safe’ drinking-water over thepast 30 years have helped to control these diseases, but in some areas theyhave had the unexpected side-effect of exposing the population to anotherhealth problem—arsenic.” The acceptable level as defined by WHO formaximum concentrations of arsenic in safe drinking water is 0.01 mg/L TheBangladesh government's standard is at a slightly higher rate, at 0.05 mg/Lbeing considered safe WHO has defined the areas under threat: Seven of thenineteen districts of West Bengal have been reported to have ground waterarsenic concentrations above 0.05 mg/L The total population in these sevendistricts is over 34 million, with the number using arsenic-rich water is morethan 1 million (above 0.05 mg/L) That number increases to 1.3 million whenthe concentration is above 0.01 mg/L According to a British GeologicalSurvey study in 1998 on shallow tube-wells in 61 of the 64 districts inBangladesh, 46% of the samples were above 0.01 mg/L and 27% were above0.050 mg/L When combined with the estimated 1999 population, it wasestimated that the number of people exposed to arsenic concentrations above0.05 mg/L is 28-35 million and the number of those exposed to more than

0.01 mg/L is 46-57 million [2]

1.3.3.2 United States

There are many locations across the United States where thegroundwater contains naturally high concentrations of arsenic Cases ofgroundwater-caused acute arsenic toxicity, such as those found inBangladesh, are unknown in the United States where the concern has focused

on the role of arsenic as a carcinogen

Some locations in the United States, such as Fallon, Nevada, have longbeen known to have groundwater with relatively high arsenic concentrations(in excess of 0.08 mg/L) Even some surface waters, such as the Verde River

in Arizona, sometimes exceed 0.01 mg/L arsenic, especially during low-flowperiods when the river flow is dominated by groundwater discharge [3]

1.3.4 Arsenic contamination of water in Vietnam

Arsenic contamination of water has become a crucial water qualityproblem in many part of the world The contamination of groundwater byarsenic in Bangladesh is the largest poisoning of a population in history AtVietnam the Vietnamese capital of Hanoi is situated at the upper end of the 11

000 km2 Red River Delta of northern Vietnam, which is inhabited by 11million people and is one of the most populous areas in the world Togetherwith the Mekong Delta, the Red River Delta (Bac Bo Plain) has become one

of the most productive agricultural regions of Southeast Asia The ruralpopulation is growing rapidly and has, in the last 5-7 yr, moved away fromusing surface water or water from shallow dug wells as sources for drinkingwater in favor of groundwater pumped from individual private (family based)tube wells Groundwater exploitation in the city of Hanoi began 90 yr ago.Today, eight major well fields supply water to city treatment facilities, whichprocess 500 000m3 of water per day [4-6]

The results of the measuring campaign of September 1999 in the ruraldistricts The results from the investigated family-based tube wells reveal that50% of the samples exceed the Vietnamese guideline value of 50 µg arsenicper liter with an average concentration of all the samples amounting to 159 µg/l Peak values of 3000 µg arsenic per liter, south of Hanoi The situation in adistrict (peak value of arsenic in water) is particularly alarming: with anaverage value of 432 µg /l, 90% of the analyzed samples revealedconcentrations of 51– 3000 µg /l

Chapter 2: Analytical arsenic

Historically, colorimetric and gravimetric methods have been used forthe determination of arsenic However, these methods are either semi-quantitative or lack sensitivity In recent years, atomic absorptionspectrometry (AAS) has become the method of choice, as it offers thepossibility of selectivity and sensitivity in the detection of a wide range ofmetals and non-metals including arsenic Popular methods for generatingatoms for AAS are flame and electrothermally heated graphite furnaces.However, a commonly used technique for the measurement of arsenic is thehighly sensitive hydride generation atomic absorption spectrometric method(HGAAS) However, although it is suitable for total arsenic determinationafter appropriate digestion the technique is only routinely used to speciate alimited number of compounds – arsenite, arsenate, MMA, DMA,trimethylarsine oxide (TMAO)

2.1 Sample preparation and treatment

2.1.1 Sampling and collection

Care must be taken to avoid contamination and prevent speciationchanges during sample collection and storage Plastic containers should beacid washed and traces of oxidizing and reducing agents avoided to preservethe oxidation state of arsenic compounds

2.1.2 Oxidative digestion

Acid digestion and dry ashing are the two basic methods which havebeen widely employed for oxidative digestion of samples before analysis Inmore recent years, microwave-assisted digestion has been used For analysis

of biological soft tissues by ICP techniques, a simple partial digestion in aclosed vessel at low temperature and pressure is often sufficient for thesample preparation and pretreatment step

2.1.3 Extraction

For speciation of arsenic, solvent extraction is often required beforeanalysis For example, arsenite and arsenate in soil can be speciated after ahydrochloric acid and chloroform extraction procedure Water has been used

for the extraction of soluble arsenic compounds from soil with the aid ofultrasonic treatment

2.1.4 Supercritical fluid extraction

There are very few publications on the use of supercritical fluidextraction (SFE) for the determination of arsenic Wenclawiak & Krah (1995)reported a procedure for the measurement of arsenic species using SFEfollowed by GC or SFC detection

2.2 Macro-measurement

Most procedures for the separation and determination of arsenic arebased on distillation and hydrogen sulfide precipitation methods Beard &Lyerly (1961) reported a gravimetric method for the measurement of arsenicfollowing extraction of arsenic as AsCl3 by benzene in strong hydrochloricacid The recovery was close to 100% when 20 mg was spiked into anaqueous solution

2.3 Colorimetric methods

George et al (1973) carried out a collaborative study for a colorimetricmeasurement of arsenic in poultry and swine tissues using silverdiethyldithiocarbamate (AgDDTC) as the complexion agent The sensitivitywas 0.1 mg/kg in tissues Dhar et al (1997) reported a detection limit of 0.04mg/liter with 95% confidence limit using AgDDTC in chloroform withhexamethylenetetramine

2.4 Methods for total inorganic arsenic

Methods for the analysis of inorganic arsenic based on its conversion toarsenic trichloride or arsenic tribromide by treatment with 6 mol/literhydrochloric acid or hydrobromic acid have been described The arsenictrihalide is separated from the remaining organic arsenic either by distillation(Maher, 1983) or by solvent extraction (Brooke & Evans, 1981) The methodshave been applied routinely to the measurement of inorganic arsenic in avariety of foodstuffs, including those of marine origin where any inorganicarsenic is a small percentage of the total arsenic present

2.5 Atomic spectrometry

Common flame atomic absorption spectrometric methods are flameAAS (FAAS), electrothermal AAS (ETAAS) and hydride generation AAS(HGAAS) FAAS is relatively less sensitive for the determination of arsenicthan ETAAS and HGAAS Its detection limit is usually in the range of sub-milligram quantities per liter, and therefore it has limited application,especially for biological samples

HGAAS is probably the most widely used method for thedetermination of arsenic in various matrices Most of the reported errors in thedetermination of arsenic by HGAAS with NaBH4 can be attributed tovariation in the production of the hydride and its transport into the atomizer.The reaction and atomization of arsine have been reviewed and discussed byWelz et al (1990) The addition of a solution of l-cysteine to a sample beforehydride generation eliminates interference by a number of transition metals inthe generation of arsine from arsenite and arsenate, and improves responses ofarsine generated from MMA and DMA in the presence of arsenite andarsenate

Atomic fluorescence spectrometry (AFS) has recently been used for thedetection of arsenic hydrides in the ultraviolet spectral region because of thesmall background emission produced by the relatively cool hydrogendiffusion flame The use of cold vapor or hydride generation, together withintense light sources, allows very low detection limits to be achieved Forexample, arsenic species in seawater have been measured using hydridegeneration and cold trapping, coupled with AFS detection at 193.7 nm Theyfound detection limits of 2.3, 0.9, 2.4 and 3.7 ng/liter for arsenite, arsenate,MMA and DMA respectively (in a 5 ml sample), with a precision of 3.5%

2.6 ICP methodologies

ICP-MS is more susceptible to isobaric interferences arising from theplasma For example, hydrochloric acid and perchloric acid are not desirablefor sample preparation, because the chloride ions generated in the plasmacombine with the argon gas to form argon chloride (ArCl) This has the samemass as arsenic (75) which could lead to error if not corrected Therefore,whenever possible, only nitric acid should be used in sample preparation

2.7 Voltammetry

Voltammetric stripping methods are mostly based on the chemicalreduction of As (V) to As (III) before the deposition step, because it has beengenerally assumed that As (V) is electrochemically inactive Mercury andgold (or gold-plated) electrodes are most commonly used for thedetermination of arsenic

2.8 X-ray spectroscopy

Particle-induced X-ray emission spectrometry (PIXES) is an analyticaltechnique that entails the bombardment of a sample (target) with chargedparticles, resulting in the emission of characteristic X-rays of the elementspresent PIXES is a multi-elemental technique with a detection limit ofapproximately 0.1 μg As/g It has the advantage of using small samples (1 mg

or less) and being a non-destructive technique

Chapter 3: Arsenic remediation

There are several methods available for removal of arsenic from water

in large conventional treatment plants The most commonly used technologiesinclude oxidation, co-precipitation and adsorption onto coagulated flocs, limetreatment, adsorption onto sorptive media, ion exchange resin and membranetechniques A detailed review of arsenic removal technologies is presented bySorg and Logsdon (1978) Jackel (1994) has documented several advances inarsenic removal technologies In view of the lowering the drinking waterstandards by USEPA, a review of arsenic removal technologies was made toconsider the economic factors involved in implementing lower drinking waterstandards for arsenic Many of the arsenic removal technologies have beendiscussed in details in AWWA reference book A comprehensive review oflow-cost, well-water treatment technologies for arsenic removal with the list

of companies and organizations involved in arsenic removal technologies hasbeen compiled by Murcott (2000) with contact detail.[8]

[9-10]

3.1 Co-precipitation and adsorption processes

Water treatment with coagulants such as aluminum alum,

Al2(SO4)3.18H2O, ferric chloride , FeCl3 and ferric sulfate Fe2(SO4)3.7H2O areeffective in removing arsenic from water Ferric salts have been found to bemore effective in removing arsenic than alum on a weight basis and effectiveover a wider range of pH In both cases pentavalent arsenic can be moreeffectively removed than trivalent arsenic In the coagulation-flocculationprocess aluminum sulfate, or ferric chloride, or ferric sulfate is added anddissolved in water under efficient stirring for one to few minutes Aluminium

or ferric hydroxide micro-flocs are formed rapidly The water is then gentlystirred for few minutes for agglomeration of micro-flocs into larger easilysettable flocs During this flocculation process all kinds of microparticles andnegatively charged ions are attached to the flocs by electrostatic attachment.Arsenic is also adsorbed onto coagualted flocs As trivalent arsenic occurs innon-ionized form, it is not subject to significant removal Oxidation of As(III)

to As(V) is thus required as a pretreatment for efficient removal This can beachieved by addition of bleaching powder (chlorine) or potassiumpermanganate as shown in Equations 2 and 3 The possible chemical

equations of alum coagulation are as follows: Alum dissolution:

Al2(SO4)3.18H2O = 2Al3+ + 3SO42- + 18H2O (1)

Aluminium precipitation(acidic):

2Al3+ + 6H2O = 2Al(OH)3 + 6H+ (2)

Co-precipitation (Non-stoichiometric, non-defined product):

H2AsO4- + Al(OH)3 = Al-As (complex) + Other Products (3)Arsenic adsorbed on aluminiun hydroxide focs as Al-As complex is removed

by sedimentation Filtration may be required to ensure complete removal ofall flocs Similar reactions take place in case of ferric chloride and ferricsulfate with the formation of Fe-As complex as end product which is removed

by the process of sedimentation and filtration The possible reactions ofarsenate with hydrous iron oxide are shown below where [≡FeOHo] representsoxide surface site

Fe(OH)3 (s) + H3AsO4→ FeAsO4.2H2O + H2O (4)

≡FeOHo + AsO43- + 3 H+ → ≡FeH2AsO4 + H2O (5)

≡FeOHo + AsO43- + 2 H+ → ≡FeHAsO4- + H2O (6)

The immobilization of arsenic by hydrous iron oxide, as shown in Eqs 4 to 6,requires oxidation of arsenic species into As(V) form for higher efficiency.Arsenic removal is dependent on pH In alum coagulation, the removal ismost effective in the pH range 7.2-7.5 and in iron coagulation, efficientremoval is achieved in a wider pH range usually between 6.0 and 8.5 [11]

3.2 Arsenic removal by adsorption onto Fe hydroxide and oxidation by hypochlorite

The use of naturally occurring iron precipitates in ground water inBangladesh is a promising method of removing arsenic by adsorption It hasbeen found that hand tubewell water in 65% of the area in Bangladeshcontains iron in excess of 2 mg/L and in many acute iron problem areas, theconcentration of dissolved iron is higher than 15 mg/L Although no goodcorrelation between concentrations of iron and arsenic has been derived, ironand arsenic have been found to co-exist in ground water Most of the tubewellwater samples satisfying Bangladesh Drinking Water Standard for Iron ( 1mg/L) also satisfy the standard for Arsenic (50 mg/L) Only about 50% of thesamples having iron content 1 - 5 mg/L satisfy the standard for arsenic while75% of the samples having iron content > 5 mg/L are unsafe for having highconcentration of arsenic The iron precipitates [Fe(OH)3] formed by oxidation

of dissolved iron [Fe(OH)2] present in groundwater, as discussed above, havethe affinity for the adsorption of arsenic The Fe-As removal relationship withgood correlation in some operating IRPs has been plotted in Figure 4 Resultsshow that most IRPs can lower arsenic content of tubewell water to half toone-fifth of the original concentrations The efficiency of these communitytype Fe-As removal plant can be increased by increasing the contact timebetween arsenic species and iron flocs Community participation in operationand maintenance in the local level is absolutely essential for effective use ofthese plants.[12]

1 Reagents and apparatus

- Twice distilled water

- Standard solution As 1000ppm for AAS, Merck

1.2 Apparatus

1 Device Name: Atomic Absorption Spectrophotometer (AAS)

2 Marking: AA - 6800