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NATURAL ADSORBENTS FOR CLEANING WATER FROM
ARSENIC
Akhalbedashvili L.
Janashvili N.
Kvatashidze R.
Todradze G.
Loria N.
Jalaghania S.
Al.Tvalchrelidze Caucasian Institute of Mineral Resources, TSU, Tbilisi, Georgia;
ABSTRACT
The problem of contamination of soil, surface and ground water with Arsenic compounds are particularly acute for a lot of countries.One of the best solutions to the problem of cleaning natural waters, is to use the sorption methods. In submitted work studies, we first compared the adsorption of As(III) in cationic (As3+) and in anionic (AsO33-) forms from water solutions on different adsorbents, such as diatomite, activated carbon and especially natural zeolites clinoptilolite and mordenite from deposits of Georgia, in initial and modified forms.
The dependence of the exchange capacity of arsenic from the form of finding it in a solution, to the type of modification of the zeolite, pre-treatment temperature and the concentration of the model solution was established. It is shown that the two mechanisms work in the adsorption of cationic form - ion exchange and donor-acceptor; during the adsorption of the anionic form AsO33- pre-dominates the physical adsorption.
Keywords: Arsenic (III), adsorption, zeolites, diatomite
Introduction
Widespread environmental contamination with Arsenic leads to the intensive study of its behavior, in particular the migration in natural environments. Elevated concentrations of Arsenic in groundwater used for drinking water supply are causing chronic diseases in humans, conditioned by anthropogenic pollution of soil and rocks, and high natural levels of Arsenic in the water-bearing rocks.
Over 70 million people in Eastern India, Bangladesh, Vietnam, Taiwan, and Northern China have been victims of Arsenic poisoning [1-5]. The USEPA has classified Arsenic as a "Class A" carcinogen [6] and recently reduced the Maximum Contaminant Level (MCL) in drinking water from 50 ppb to 10 ppb. To meet those drinking water standards, small water utilities need low cost and effective Arsenic removal techniques. Artificial pollution with Arsenic is associated with the use of pesticides containing this element as
[Pb3(AsO4)2], atmospheric deposition, mining processes - which uses Arsenic (gallium arsenide).
More widespread forms of Arsenic in the environment are arsenate [AsO43-, As (V)] and arsenite ions [AsO33-, As (III)]. Arsenite is more mobile and more toxic form of Arsenic [7]. In general, toxicity increases in the sequence: organic arsenical<As (V) <As (III)<arsine (AsH3) [8]. Adsorption of Arsenic on a mineral surface is a very important process, which controls its biological accessibility in natural systems. The biological accessibility and toxicology of Arsenic depends as on its forms and from a great quantity of chemical, physical and biological factors - which include pH, mineralogy, oxidative-reduced potential, microbiological population, presence of the ligands of different nature [7].
There is an intensive research worldwide to improve established techniques and to develop novel treatment technologies for removing Arsenic from
drinking water. The major technologies include precipitation-coagulation, membrane separation, ion exchange, lime softening and adsorption on iron oxides or activated alumina. Among these techniques proposed in the literatures, techniques involving immobilization of Arsenic by adsorption have received much attention on natural geo-materials, including natural zeolites, di-atomite.
Natural zeolites such as Clinoptilolite are excellent sorbents for several metallic and radioactive cations. Modifying the zeolite structure can effectively enhance the adsorption capacities of these zeolites for removal of different elements, including Arsenic.
The use of prepared sorbents is due to their relatively high capacity, selectivity, cation-exchange properties of some of them, comparatively low cost and availability (in Georgia as a local material). Enhancement of sorption on natural materials, modified with differentions is well known [9-13]. For example, modifying the zeolite structure by treatment with metals such as copper or iron effectively enhances the sorption capacity for Arsenic.
The problem of contamination of soil, surface and ground waters with Arsenic compounds is particularly acute in Georgia, where the hard rock mining began in 1932. Recently about 100 thousand ton of arsenic-contained substances located at Uravi factory, caused very high level of contamination. So, the problem of cleaning the environment from Arsenic is very important for our country.
As arsenite is more mobile and more toxic form of Arsenic [7], the goal of submitting work was the study of some natural materials in adsorption of As (III) in
Chemical content of z<
anionic and cationic forms, and to investigate which ad-sorbentor its modification has the higher removal efficiency of arsenic (III).
Materials and methods
To study the sorption properties, we prepared the initial and modified zeolites CL and MOR with higher ion-exchange capacity, for comparisons - were used synthetic zeolite NaY, diatomite, and activated charcoal brand "Silcarbon". All natural materials were taken from the deposits of Georgia.
The samples were studied in initial and modified forms, which were obtained by using different methods of treatment. The modification was carried out for increasing the adsorption capacity and activity of natural zeolites at 353K according to the method developed by us, i.e.by means of combination of acid, thermal and alkali treatment in different sequence and variation of treatment time and concentration of solutions. Acid modification was carried out by treatment with 0.25, 0.5, 1.0 and 2.0N hydrochloric acid in the same conditions, alkali treatment with 0.25 - 0.30 % KOH solutions with purpose to increase thermal and mechanical stability. Multiple ion-exchange of zeolites was carried out with transition metals' salt solutions in dynamic conditions after preliminary treatment.
The chemical composition of initial and modified samples was determined using conventional chemical analysis, AAS, flame spectrometry and flame photometry. The retention of structure was controlled by X-ray. The chemical content of some initial samples are given in table. Exchange degree of Cu and Fe were 4.56 and 6.32% accordingly.
Table
ites and diatomite, %._
Sample SiO2 Al2O3 Fe2O3 TiO2 SO3 MgO CaO K2O Na2O MnO P2O5
CL(GeO) 69.64 10.25 1.00 0.50 0.02 0.95 2.17 3.46 9.93 0.04 0.50
CL(Ar) 68.16 12.29 1.48 0.43 0.12 0.30 6.33 2.46 0.76 - -
CL(GeO)H 84.92 5.46 0.05 0.01 0.03 0.49 1.05 0.78 3.24 0.09 1.10
Diatomite 83.74 7.33 4.43 0.23 - 1.00 1.84 1.65 0.29 - -
In carrying out the experiments and analysis, we used chemicals, reagents and standard solutions of ultrapure qualification, production companies Sigma-Al-drich, Perkin Elmer. Arsenic in form of arsenic trioxide (As2O3) (99.9%) was used for preparing the adsorbents. The initial model solutions were prepared for trivalent Arsenic in cationic and anionic forms. Arsenic concentrations in the resulting solutions were 0.1 g/l, which is further diluted to working concentrations.
The experiments were performed in a static-circulation mode with variation of ratio solid: solution, contact time, concentration of arsenic ion in model solution (5, 10, 20, 25 and 50mg/l), temperature of pretreatment of samples. The analysis of Arsenic ions in initial model and waste solutions was carried out by AA Spectroscopy, on apparatus "Perkin Elmer' AAnalyst200 -used as the source of atomization flame air-acetylene [14]. Quantitative determination of Arsenic in the analyzed solutions was performed according to the obtained calibration graphs in mg/l. The exchange adsorption value, coefficients of adsorption and distribution were calculated on the base of received data.
Results and Discussion
According to fig. 1 the curves of dependence of exchange capacity (EC) from concentration till 20-25 mg/l have had practically parallel ways for arsenic anions AsO33- and arsenic cations (As3+), and then the saturation for AsO33-was reached. But the saturation of EC of arsenic cations (As3+) wasn't reached in the investigated area of concentrations. Thus, the dependence of the concentration of the solution EC is more expressed for the cationic form of As (III).
The dependence of the exchange capacity of both forms of arsenic from heat treatment temperature of adsorbent was studied on the example of the initial non-treated zeolite CL (fig.2). Both dependence have a maximum, at the same time value of EC (As3+) twice exceeds magnitude EC (AsO33-). The temperature of preliminary heat treatment doesn't influence on EC (As3+) until 573K, but adsorption of AsO33- reaches maximum value in narrow temperature interval of heat treatment 573-673K, and then sharply falls down.
It is known that heat treatment of natural zeolites upper 673-723K reduces to narrowing of channels and
decreasing of adsorption volume [15], but as the effec- than in three times less the size of AsO33- anion, there-tive ionic radius of As3+is 0.1037 nm [16] and more fore its adsorption parameters decreases more sharply.
Fig. 1. Dependence of exchange capacity EC on initial concentration ofAsO33- (1) and As3+ (2)
Fig.2. Dependence of exchange capacity EC on calcinations temperature of CL(GeO) for AsO33- (1) and As3+ (2)
It is known that materials available for sorption of arsenic include activated alumina, iron [17], synthetic ion exchange resins, and fly ash [6]. So the adsorption activity of zeolites may be explained by the presence of three-coordinated aluminum in crystal structure and impurity iron in inter-zeolite cavities of original zeolite. The acid and alkali treatment of CL causes the decatenation and de-alumination of structure and as a result, decrease of the adsorption ability, but following the introduction of iron and copper by ion-exchange method caused an increase in the adsorption of both adsorbents (fig.3, 4).There with the sorption of arsenic cations occurs more actively on most adsorbents.
The low sorption capacity with respect to the ani-onic form AsO33- of CL(Ar) (Noemberyan deposit, Armenia) and the MOR (mordenite from Georgia deposit) could be explained by steric hindrance and high content of non-zeolite impurities (up 35%).But in this case it is difficult to explain the low adsorptive of synthetic zeolite NaY in regard to AsO33-, the size of cavities and channels of which is respectively 1.5 and 0.9 nm [18], and is available for arsenic cations (fig.4).On the sorbents such as a raw diatomite, and particularly the activated charcoal - the exchange capacity reaches values of 3.5 - 4.3 meq/g, indicating on the mechanism of physical adsorption on these macro-porous amorphous adsorbents.
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Fig.3. Comparative activity of studied samples in adsorption of AsO33- anions
Sample
Fig.4. Comparative activity of studied samples in adsorption of As3+cations
But it may be assumed that in the process of ad- by exchange the adsorbed ion with compensate cations sorption As3+cations on zeolites implemented two of the zeolite - Ca2+, Mg2+, K+, Na+ [19]. Ion exchange mechanisms: ion exchange and donor-acceptor. The of cations As3+ with modified zeolite can be represented ion-exchange mechanism suppose that sorption of As3+ by diagrams: ions from solutions by modified zeolites occurs by substitution of H+ ions in hydroxyl groups of zeolite, and
3(=SiO-)2R2+ + 2As3+ ^ 3(=SiO-^2 As3+ + 3R2+(R-Ca) 3(=SiO-)2R2+ + 2As3+ ^ 3(=SiO-^2 As3++ 3R2+(R- Mg) 3[(=SiO-)R+] + As3+ + ^ 3(=SiO-)2As3++ 3R+(R- K) 3[(=SiO-)R+] + As3+ ^ 3(=SiO-)2 As3+ + 3Ra+(R- Na)
3[(=SiO-)H+] + As3+ ^ 3(=SiO-)2As3++ 3H+ In addition to the ion-exchange interaction is also possible and the formation of strong bonds between the modified zeolite and adsorbed cations, which has the donor-acceptor nature. May be the free d-orbitals of
As3+ presented as acceptors. Donors can be groups with oxygen atoms, located on the surface of zeolite, and having an unshared electron pair. Such an adsorption mechanism may be represented by the following scheme [19]:
Similar mechanisms of adsorption are unacceptable to the anionic form of arsenic - AsO33- , as evidenced by the sorption parameters of NaY in regards to both forms of arsenic (compare Figures 3 and 4). Likely the adsorption centers in zeolites are the transition metal cations in exchangeable or in the impurity forms. Thus, the iron content is almost the same values in the natural CL (deposit Dzegvi, Georgia) which obtain the ion-exchange process.
So, the study of the dependence of adsorption value, coefficients of adsorption and distribution from modification type of initial - showed that the more active adsorbents are cation-modified CL, especially Cu-and Fe-contained samples. The introduction of copper and iron cations in the preliminary de-cationated zeolite twice increases adsorption value of AsO33-ions as compared to the initial zeolite, and 3-fold compared with de-cationated. It can be inferred that metal ions used for modification form a strong bond with CL and do not leach off the surface in aqueous phase. Acid treated CL showed the smallest adsorption ability in relation to AsO33- and As3+ ions. The increase of concentration from 0.5 mg/l till 50.0 mg/l caused growth of adsorption value, but the saturation wasn't reached in these limits.
It should be noted that CL from different Georgian deposits indicated different adsorption ability. This may depend on the different content of CL in and different chemical composition of zeolite.
Conclusions
The dependence of the exchange capacity of arsenic from the form of finding it in a solution, from the type of modification of the zeolite, from pre-treatment temperature and the concentration of the model solution was established. It is shown that the two mechanisms work in the adsorption of cationic form - ion exchange and donor-acceptor; during the adsorption of the anionic form AsO33- pre-dominates the physical adsorption.
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