The Application of Rapid Small-Scale Column Tests in Iron-Based

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 11, 2014 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch019

Chapter 19

The Application of Rapid Small-Scale Column Tests in Iron-Based Packed Bed Arsenie Treatment Systems Mohammad Badruzzaman and Paul Westerhoff Civil and Environmental Engineering Department, Arizona State University, Tempe, A Z 85287-5306

The rapid small-scale column test (RSSCT) is applicable to simulate pilot-scale performance for arsenate adsorption onto porous iron-based adsorbents. Commercially available granular ferric hydroxide (GFH) and Bayoxide Sorb33 (E33) have been evaluated for arsenate removal in this study. The B E T surface area of G F H is 236 m /g and for E33 is 129 m /g. Both G F H and E33 possess identical pore size distribution. Proportional diffusivity (PD) and constant diffusivity (CD) based RSSCTs have been conducted with G F H and E33 and compared against corresponding pilot column performance. PD-RSSCTs simulated pilot column breakthrough curves reasonably well, but CD-RSSCTs failed to correspond pilot scale performance. The column arsenate adsorption density (q ) varies from 0.3 - 0.4 μg arsenate/mg dry G F H and 0.3-1.53 μg arsenate/mg dry E33 for G F H and E33 respectively depending upon water quality. 2

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column

268

© 2005 American Chemical Society

In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

269


Introduction Arsenic contamination of drinking water is a global problem. Arsenic is present in the aquatic environment in both organic (i.e. methylated) and inorganic forms caused by weathering and dissolution of arsenic bearing minerals, rocks and ores (/). Arsenic usually occurs as arsenate (As(V)) or arsenite (As(lII)) in surface and ground waters used as raw potable water supplies. Arsenate ( H A s 0 , H As0 ", H A s 0 ' , or As0 ") occurs in anionic form over the pH range of 5 to 12. Arsenite ( H A s 0 , H A s 0 " , and HAs0 ") occurs in a lower Eh waters. Arsenic is classified as a Class A human carcinogen because of carcinogenic (prone to cancer of the bladder, lungs, skin, kidney, liver, and prostate) and non-carcinogenic (harmful to neurological and cardiovascular systems) effects (2). The U S E P A lowered the maximum contaminant level ( M C L ) for arsenic in drinking water from 50 μg/L to 10 μg/L in 2002, which will be enforced in 2006. The W H O , European Union and several counties have also recently lowered the recommended or required arsenic limit to 10 μg/L. The reduced M C L has increased the research need for arsenic treatment processes in the potable water supply systems and industrial discharges. Arsenic treatment technologies include adsorption on metal hydroxides during coagulation or by adsorptive packed-beds, removal by ion exchange resins or membrane systems (3-/3). But coagulation, membranes and ion exchange technologies might not be the feasible choices because of pretreatment requirements, high volume of waste brine handling and water loss associated with sewer disposal of waste brine. On the other hand, metal oxides/hydroxides packed adsorption systems are advantageous as the spent media is non hazardous and can be disposed to any municipal landfill (14). Many forms of adsorbents have been emerged recently for arsenic removal, but iron-based minerals have a high affinity for arsenic (75-/7). Several porous/nonporous adsorbents such as granular ferric hydroxide (GFH), Bayoxide E33, zero valent iron, sulfur modified iron, activated alumina, iron modified activated alumina, magnesium impregnated activated alumina, T i 0 etc. have been investigated for arsenic removal efficiency and adsorption mechanisms either through conventional batch isotherms or kinetic studies and using advanced electrochemical and spectroscopic instruments (14, 18-20, 22). Isotherms yield useful information on comparison of the adsorbents and the magnitude of competitive effects, but as a static equilibrium test, the extension of isotherm data is limitedly applicable to estimate important operational parameters. Moreover, batch experiments do not simulate the hydrodynamic condition of full-scale system. So the application of packed bed column experiments generating the breakthrough curves or the operation life of the adsorbents is very critical. But pilot tests are costly and could delay implementation of full-scale treatment systems design and construction. So there 2

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270 is a need to develop a mini-column test to shorten the duration of pilot-column tests. Rapid small-scale column tests (RSSCTs) have been applied over the last two decades for simulating pilot-scale performance of organic micropollutants and natural organic matter (NOM) removal by granular activated carbon (GAC) (23-29). The adsorption mechanisms of inorganic compounds (arsenic) onto metal oxides are different from organic compounds adsorption onto porous activated carbon, as inorganic ions form inner-sphere complexes whereas organic compounds are adsorbed due to hydrophobic (van der waals) interaction. However, in both cases intraparticle diffusion appears to be rate the limiting mass transport process (30, 37). Therefore, the authors hypothesize that RSSCTs developed for organic compounds and activated carbon would be applicable to simulate full/pilot plant performance of arsenic adsorption onto metal hydroxides such as G F H and E33 with fraction of water, time and cost required to conduct field testing. The rationale of this research is to extend the theory and experimental methodologies of RSSCTs. The specific objectives of this study are: a) physical characterization of G F H and E33, b) evaluation of different approaches of RSSCTs through mini-column experiments, c) comparison of RSSCTs to pilot column performance and to determine appropriate scaling approach, and d) comparative evaluation of the performance of G F H and E33. This paper concentrates on arsenate removal from groundwater of the southwestern parts of the Unites States, primarily Arizona.

Scaling Procedure The dispersive flow pore surface diffusion model (DFPSDM) has been considered as the closest mathematical formulation of the performance of fullscale adsorber, as it considers three important mass transfer mechanisms such as - film diffusion, pore and surface diffusion (36). Analytical solution of the D F P S D M was successful in a very limited basis for simulating the behavior of full-scale performance due to the variability of model parameters from the real systems. But the RSSCT, designed from the dimensionless parameters obtained from the D F P S D M , was successful for scaling down a full-scale adsorber to bench-scale for organics adsorption onto G A C . Assumptions: First, boundary conditions for the full-scale and small-scale process must occur at the same dimensionless coordinate values in the dimensionless differential equations. Second, dimensionless parameters in the dimensionless differential equations must be equal for the full-scale and smallscale process. Finally, no change in mechanism can occur when reducing the size of the process. Scaling Approach: The dimensionless groups, that are used in the D P S D M to describe the relative importance of different transport mechanisms, are -


271 surface solute distribution parameter (D ) or Capacity factor (C ), Peclet number (Pe), Stanton number (St), surface diffusion modulus (Ed ), and pore diffusion modulus (Ed ). Equating the dimensionless quantities for small and large column dimensions, the operational design parameters for the RSSCT such as empty bed contact time (EBCT), loading rate, etc have been developed. Regarding intraparticle diffusion, the adsorption of organic compounds by activated carbon demonstrated that surface diffusion typically dominates over pore diffusion (33, 34). The adsorption of metal cations by porous iron oxide has also been reported as surface diffusion dominating adsorption process (32). Therefore, the authors assumed that intraparticle transport of arsenate onto porous sorbents is also controlled by surface diffusion. Therefore, equating the surface diffusion modulus (Ed ) for small (SC) and large (LC) column the equation 1 can be generated: g

f

s


p

s

E

B

C

T

L C

D

D

E

* s,LC * g,S,LC

R

B

C

T

S C *

P

s , S C *

R

LC

D g

, S C

(1)

SC

As the pore solute distribution parameter (Dg) of large-column may be equal to the small- scale, the equation 1 can be re-written as equation 2: E B C T ,LC

R LC

E B C X sc

V^sc

D

s,SC

(2)

V^s,LC J

J

Considering that surface diffusivity of arsenate (D ) onto porous sorbents is dependent on the adsorbent particle size according to equation 3: s

D

s,sc

D

L,LC

R LC

(3) J

V^sc

From equation 2 and 3, the general equation for empty bed contact time (EBCT) and particle radius (R) can be expressed as the equation 4. E B C X sc EBCT

L C

f

V

R SC R

,2-X

(4)

L C ;

Two different approaches of the RSSCT scaling can be established, as constant (CD) diffusivity based RSSCT (CD-RSSCT) and proportional (PD) diffusivity based RSSCT (PD-RSSCT). The two approaches differ if D values are independent (for CD) or a linear function (for PD) of adsorbent radius. In s


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other words, for CD-RSSCT and PD-RSSCT design, the values for X are zero and one, respectively should be used in equation 4. Considering similar breakthrough spreading for small and large column, the Reynolds number of a small column would be equal to that of a large column along with other dimensionless parameters such as St and Pe. Consequently, following operational design equation for the RSSCT can be developed:

V V

R

F,SC _ ^ P . L C

V V

F

.

~R

F,LC

^P.SC

Where, V c and V , L C are loading rates of the small and large columns respectively. Experience with organic adsorption onto G A C suggests that for PD-RSSCT, equation 5 yields a long column and consequently generates an excessive pressure drop (39). As film diffusion is not important, the flowrate of mini-column can be reduced by the ratio of the product of the Reynolds and Schmidt number (Sch) in equation 5 until the dispersion does not become important. The modified form of equation 5 can be expressed as equation 6: F S

F

F,SC

R

P,LC

^Regc-Sch

V ,LC

R

P.SC

Re .Sch

V

F

LC

However, to ensure the similar dispersion effect for large and small-scale columns, the product of Reynolds number and the Schmidt number was mentioned in the mechanical range of 200-200,000 (35). The authors have experimented PD-RSSCTs for different values of the product of Re and Sch (Re *Sch) and concluded that a value of 2000 can simulate full-scale performance reasonably well (30, 37). A l l PD-RSSCTs of this study have been conducted using the value of 2000 as Re c*Sch. sc

S

Materials and Methods Water Samples Natural groundwater (DOC< 0.5 mg/L) from three different well sites of Arizona, U S A was used in the column studies. W l contained ~ 50 μg/L of arsenic at ambient pH of 8.9 and associated water quality data are: alkalinity ~ 156 mg C a C 0 / L , TDS of ~ 420 mg/L, and silica 22 mg/L. W2 contained 10 μg/L of arsenic with ambient pH of 7.8, alkalinity - 215 mg C a C 0 / L , and silica 31 mg/L. W3 contained 16.5 μg/L of arsenic, ambient pH of 8.27 and associated 3

3


273 important water quality parameters are: TDS - 291 mg/L, alkalinity - 214 mg C a C 0 / L , and silica -52 mg/L, sulfate ~12mg/L, fluoride ~ 0.5mg/L, nitrate ~ L5mg/L. 3


Experiments BET Surface Area. B E T surface areas and pore size distributions of "virgin" G F H and E33 were determined from N isotherm data collected at 77°K (Autosorb-l-MP, Quantachrome Corporation, Boynton Beach, F L ) . Prior to analysis particles were outgassed overnight at 423°K. The density functional theory (DFT) was used to calculate micropore volumes, mesopore volumes and micropore size distributions from the N adsorption data (Vulcan kernel, PC software version 1.19, Quantachrome, Boynton Beach, FL). Standard methods were employed for the determination of moisture content ( A S T M D2216) and packed bed porosity ( A S T M D854-58) (31). RSSCT Experiments. Laboratory columns consisted of 1.1 cm (Ace Glass, Vineland, NJ) diameter glass columns approximately 30.5 cm in length with Teflon end caps. Teflon tubing (3.2 mm) was used. Piston pumps with stainless steel heads were used (Fluid Metering Inc. Syosset, N Y ) . Schematic of the set up is shown is Figure 1. Columns were backwashed to remove fine particles, by operating the column in upflow mode with distilled water until the effluent water ran clear. The typical backwashing flow rate for the RSSCT columns was in the ranges of 10-20mL/min and resulted in bed expansion of approximately 40%. Pilot Columns. P V C pilot columns containing as-received media were operated by consulting ( W l , W3) personnel and arsenic breakthrough data provided to the authors. Pilot columns were backwashed approximately every 3 to 4 weeks to prevent excessive pressure development. Temperatures of the pilot columns influent water were similar to raw water of the RSSCT column experiments. Arsenic Analysis. A l l samples collected for arsenic analysis were preserved using nitric acid (JT Baker ultra pure reagent grade Ultrex II Nitric, Phillipsburg, NJ). A Varian SpectrAA-400 atomic absorption spectrometer (Palo Alto, C A ) with a Varian Graphite Furnace Tube Atomizer, Zeeman background correction, and auto-sampler was used for analyzing the samples for dissolved arsenic. 2

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Results and Discussion Physical Characterization of G F H and E33 2

The B E T surface area of G F H media was 231 m /g for 100x140 mesh size and 240 m /g for 10x30 mesh size, on an average 236 m /g. On the other hand, 2

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Figure 1. Schematic of the experimental set-up for Rapid Small Scale Column Tests (RSSCTs).

2

the B E T surface area of E33 media was 128 m /g for 100x140 mesh size and 129 m /g for 10x30 mesh size, on an average 127 m /g. The B E T surface area of "as received" médias were 227m /g for G F H and 129 m /g for E33. The variation of surface area as a function of media size is negligible. The pore volume distributions for the 100x140 mesh sizes of G F H and E33 are presented in Figure 2. Two major pore widths were detected such as - the micropore (

The Application of Rapid Small-Scale Column Tests in Iron-Based

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