Harmful Algae 19 (2012) 61–67

pH effects on the adsorption of saxitoxin by powdered activated carbon

Honglan Shi a,b, Jie Ding c, Terry Timmons d, Craig Adams e,*a Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409, USAb Environmental Research Center, Missouri University of Science and Technology, Rolla, MO 65409, USAc Department of Civil, Architectural & Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USAd Missouri Department of Natural Resources, Jefferson City, MO, USAe Department of Civil and Environmental Engineering, Utah State University, Logan, Utah 84322, USA

A R T I C L E I N F O

Article history:

Received 9 June 2011

Received in revised form 29 May 2012

Accepted 30 May 2012

Available online 21 June 2012

Keywords:

Adsorption

Algal toxin

Cyanobacterial toxin

pH

Powered activated carbon

Saxitoxin

A B S T R A C T

Increasing occurrence of cyanotoxins in surface waters worldwide pose significant problems, including

those for drinking water utilities. In this study, the removal of saxitoxin (STX) from three different

powdered activated carbons (PACs) was studied. STX is one of the most toxic paralytic shellfish toxins

(PSTs), albeit not the most prevalent. The results showed that a wide range of non-electrostatic and

electrostatic interactions appeared to play a role in the sorption of STX on PAC, depending on the solution

pH, NOM concentration, and other factors. A bituminous coal-based PAC, that was studied in greatest

detail, showed a trend of increasing sorption capacity for STX with increasing pH. NOM appeared to

significantly inhibit adsorption when the pH was nearly neutral (e.g. 7.05), yet it had less effect at higher

pH levels of 8.2 and 10.7.

� 2012 Elsevier B.V. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Harmful Algae

jo u rn al h om epag e: ww w.els evier .c o m/lo cat e/ha l

1. Introduction

In the last few decades, a variety of cyanotoxins (also known asalgal toxins or cyanobacterial toxins) have been observed insurface waters worldwide. These cyanotoxins are produced bycyanobacteria (also referred to as blue-green algae due to theirability to undergo photosynthesis) in blooms in the United States,China, Australia, and other countries. Cyanobacteria producetoxins which can contaminate surface waters, particularly ineutrophic surface waters. Because many of these surface waters areused as sources for drinking water, the presence of thesecyanobacteria and their toxins in the water may endanger humanhealth. Cyanotoxins are widely considered to be a potential hazardto the health of humans and some marine animals.

The most frequently found cyanobacteria are microcystis,anabaena, and planktothrix strains (Sivonen, 1990), while theirmajor cyanotoxins include microcystins, saxitoxins, anatoxins, andcylindrospermopsin. Saxitoxin (STX), also known as paralyticshellfish toxin (PST), are a group of chemically related neurotoxicalkaloids produced by blue-green algae. To date, 57 analogs of PSTs

* Corresponding author at: Department of Civil and Environmental Engineering,

Utah State University, Logan, Utah 84322, USA.

E-mail addresses: honglan@mst.edu (H. Shi), Terry.timmons@dnr.mo.gov

(T. Timmons), craig.adams@usu.edu (C. Adams).

1568-9883/$ – see front matter � 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.hal.2012.05.008

have been identified (Wiese et al., 2010). Saxitoxin (STX) is one ofthe most toxic variants of the PSTs (Watts et al., 1966; IPCS, 1984),albeit often not the most prevalent. In Australia, STX is used as astandard measure of toxicity of this class of chemicals, with aninformal guideline of 3 mg (STX equivalents)/L in drinking water(NHMRC, 2001; Orr et al., 2004), while a maximum STXconcentration of 1 mg/L in drinking water is currently requiredby the New Zealand Ministry of Health (Orr et al., 2004).

PSTs have been increasingly reported in fresh and brackishwater in many countries worldwide, including Denmark (Kaasand Henriksen, 2000), Thailand (Kungsuwan et al., 1997), Brazil(Molica et al., 2005; Clemente et al., 2010), Venezuela (Sevciket al., 2003), Paraguay (Sevcik et al., 1993, 2003), Bangladesh(Zaman et al., 1997), Australia (Humpage et al., 1994; Negri et al.,1997), China (Liu et al., 2006), United States (Carmichael, 1997;Landsberg et al., 2006; Deeds et al., 2008; Abbott et al., 2009),Mexico (Berry and Lind, 2010), and Germany (Ballot et al., 2010).The concentrations of STX in these countries range up to 15 mg/Lin natural waters (with intracellular STX of from 5 to 3400 mg (STXequivalents)/g dry weight of cells). STX is also a food contaminant,especially in fish, and has also been classified as a chemicalweapon as well as a drug candidate (mainly local anesthetics)(Llewellyn, 2006).

Control of saxitoxins in water treatment has been studied bya number of research groups (Newcombe and Nicholson, 2002,2004; Kayal et al., 2008; Zamyadi et al., 2010). STX and other

Fig. 1. Molecule structure and computed speciation of STX in aqueous solution of different pH levels.

H. Shi et al. / Harmful Algae 19 (2012) 61–6762

saxitoxins are both stable and soluble in water (Alfonzo et al.,1994). A study by Rositano et al. (2001) concluded thatsaxitoxins were not readily susceptible to ozonation. Similarly,Orr et al. (2004) showed that ozonation alone, as well as theozone/peroxide advanced oxidation process, was not effective atremoving STX from drinking water. Different activated carbonshave been identified that do effectively remove STX and theanalogs from water (Bailey et al., 1999; Newcombe et al., 2002;Orr et al., 2004; Ho et al., 2009). Newcombe et al. (2002) alsoreported that destruction of saxitoxins by chlorination is pHdependent, with the destruction efficiency increasing rapidly asthe pH increases from 6 to 9.

Activated carbon surface chemistry and pore structure havebeen shown to play an important role in removal of organiccompounds in drinking water treatment (Kilduff and Wigton,1999; Li et al., 2002a,b; Quinlivan et al., 2005; Zhang et al., 2010).Non-electrostatic adsorption mechanisms are often the mostcommon type of adsorption mechanism for synthetic organicchemicals (SOCs) on activated carbon. Further, they most oftencontrol the removal of neutral organics from water, via carbonadsorption, in water treatment. However, when an organiccompound has a cationic or anion character, it becomes morepolar, and may be more poorly adsorbed by activated carbon vianon-specific physisorption, while electrostatic mechanisms couldpossibly be enhanced.

The molecular structure of STX has several amine groups thatcan potentially gain protons and, thereby, become cationicdepending on the solution pH. As a result, up to 10 differentspecies of STX exist, depending on the pH of the solution (Fig. 1)(Hilal et al., 1995). The neutrally charged Species 1, whichpredominates between pH 9 and 12, is the species of STX thatwould most likely have the highest adsorption onto most activatedcarbons, based on non-electrostatic interactions. Due to thespeciation behavior of STX throughout the pH range common towater treatment, it has been hypothesized that pH would have astrong effect on the treatability of STX in drinking water treatment.The purpose of this research was to test this hypothesis, todetermine quantitatively the effect(s) of pH on sorption capacity,and to provide guidance to the water industry with respect to

treatability of STX. The sorption of STX by PAC was investigated atdifferent pH levels in both buffered-deionized (DI) water andsurface water with a variety PAC doses. The effects of pH onequilibrium, and on the kinetics of adsorption, were investigated,as was the impact of natural organic matter (NOM) on the STXadsorption.

NOM has been shown to hinder the removal of SOCs onactivated carbon generally by two different mechanisms. In thefirst mechanism, NOM may directly compete for adsorption siteson the surface of the carbon, thereby reducing the surface areaavailable for SOC (e.g., STX) adsorption (Zhang et al., 2011). Thisgenerally involves the smaller, lower molecular weight fractionof the NOM which is able to diffuse into the smaller porespredominant in activated carbon. The second mechanisminvolves blockage or diffusional hindrance of meso- and/ormicro-pores in the activated carbon by the large (highermolecular weight) NOM so that the surface area within themicropores is less available for smaller, lower molecular weightSOC adsorption (Zhang et al., 2011).

The adsorption efficiency of three different types of powderedactivated carbon (PAC) that have different surface and poreproperties, as a function of pH, were also compared. While theresult of this study focused on STX-contaminated water treatment,the other PSTs response could differ with respect to the effects ofpH on PAC treatment.

2. Materials and methods

2.1. Chemicals and standard preparation

HPLC-grade acetic acid, HPLC-grade acetonitrile, ACS-certifiedgrade ammonium phosphate (dibasic), phosphoric acid, andsodium phosphate (dibasic) were purchased from Fisher Scientific(Pittsburgh, PA, USA). ACS-reagent grade periodic acid and sodium1-heptanesulfonate were purchased from Sigma–Aldrich (St. Louis,MO, USA). The STX standard solution in 0.003 M HCl waspurchased from the Institute for Marine Biosciences (NationalResearch Council of Canada, Ottawa, Ontario, Canada). Deionized(DI) water was produced with a Millipore Simplicity 185 water

H. Shi et al. / Harmful Algae 19 (2012) 61–67 63

system (Billerica, MA). The surface water was collected from apond, and was filtered through 0.45-mm nylon membrane filterand stored at 4 8C for later use.

STX standard solutions for calibration were prepared by thedilution of stock standard solution with 0.05 M acetic acid inamber vials. A five-point calibration curve was routinely obtainedover a concentration range of up to 25 mg/L.

Three activated carbons were studied: WPH (Calgon CarbonCorporation), HydroDarco B (HDB, NORIT Americas Inc.), and AquaNuchar (AN, Meadwestvaco Corporation). WPH was used for themost removal studies at four pH levels, and with varied contacttimes. All three PACs were compared at two contact times at asingle pH (described below).

Characteristics of each carbon were obtained from varioussources. The BET surface area and the pore size distribution ofdifferent samples of these same carbons were previouslydetermined using an Autosorb-1-MP (Quantachrome Corp., FL,USA) by Dr. Detlef Knappe’s laboratory (personal communication,North Carolina State University, Raleigh, NC, USA). The oxygencontent of different samples of these same carbons was alsopreviously determined by Huffman Laboratories (Golden, CO)using ASTM D5622 (Knappe, 2011). The authors’ research groupalso determined the BET surface area using an Autosorb-1-MP(Quantachrome Corp, FL, USA) (Jain et al., 2004). These data are allshown in Table 1.

Micropores, mesopores, and macropores are generally consid-ered to have diameters of <2 nm, 2–50 nm, and >50 nm,respectively. The percentage for each pore volume range ispresented for each carbon in Table 1. Sorption of saxitoxin, othercyanotoxins, pesticides, and many other smaller molecules are,therefore, capable of utilizing surface areas within micropores aswell as the larger pores. Larger molecules, including much of thenatural organic matter (NOM), however, are generally too large toutilize the micropores. In fact, NOM is generally considered tointerfere with sorption of smaller adsorbates via the twomechanisms discussed above (i.e., direct competition for sorptionsites and pore blockage).

WPH PAC (with which a majority of the experiments for thismanuscript were conducted) is a bituminous-coal based PAC witha larger BET surface area of about 900–1000 m2/g, and mostlymicropores (66% by volume) with very few macropores (Table 1).HDB is a lignite-coal based PAC with a BET surface area of about500 m2/g, that contains a predominance of mesopores and about20% of both micro- and macro-pores (Table 1). AN is a wood-basedPAC with an even larger BET surface area of about 1500 m2/g, thatcontains mostly mesopores (66% by volume) and micropores (33%)(Table 1). The point-of-zero charge (PZC) for these carbons isranked (Table 1) (Knappe, 2011): AN < WPH < HDB. The oxygencontent of these PACs increased in the order (Table 1) (Knappe,2011): WPH < AN < HDB.

2.2. HPLC analysis of STX

A post-column derivatization liquid chromatography withfluorescence detection (FLD) method was used for analysis of

Table 1Characteristics of three powdered activated carbons (PAC) types used in this study.

PAC Source BET surface

areaa (m2/g)

BET surface

areab (m2/g)

Mic

vol

cm

HydroDarco B (HDB) Lignite coal 507 510 0.1

WPH Bituminous coal 901 1027 0.3

AquaNuchar (AN) Wood 1464 1567 0.3

a Detlef Knappe, North Carolina State University, personal communication.b Jain et al. (2004).

the cyanotoxins following a published method (Yasukatsu, 1995)with some modifications. High pressure liquid chromatography(HPLC)/FLD was conducted using a Waters (Milford, MA, USA)HPLC system, including a 717 plus Autosampler, 600 Controller,2475 Multi l Fluorescence Detector, and with Empower software.The HPLC column was a reverse phase Keystone Betabasic-C8column, with a dimension of 250 mm � 4.6 mm and a particle sizeof 5 mm (Thermo Electron Corporation). The post-column deriva-tization setup included a Shimadzu Model LC-10AD pump with amodified flow path for delivering both oxidation reagent andacidifying reagent simultaneously. The post-column reactiontubing was a 10-m Teflon tubing with 0.5-mm i.d. The reactiontubing was kept in a water bath maintained at a temperature of80 8C. Due to the instability of the periodic acid in the post-columnderivatization solution, a cooler with ice was used to keep thisreagent at a low temperature.

The HPLC mobile phase contained 30 mM of ammoniumphosphate with 2 mM of sodium 1-heptanesulfonate (as an ionpairing reagent) at pH 7.1, plus 4.8% acetonitrile filtrated through a0.45-mm membrane filter after preparation.

For post-column derivatization, the oxidizing reagent was7 mM of periodic acid in 50 mM sodium phosphate buffer at a pH of9.0. The acidifying reagent was a 0.5 M acetic acid solution. Boththe oxidizing reagent and acidifying reagent were filtered througha 0.45-mm membrane filter. The sample injection volume was50 mL. The separation mobile phase flow rate was 0.8 mL/min withboth oxidation and acidifying reagent flow rates at 0.4 mL/min. Thepost-column reaction temperature was maintained at 80 8C in awater bath. The FLD detector excitation wavelength was 330 nm,and the emission wavelength was 390 nm. The retention time forSTX was approximately 18.6 min. The calibration curve for STX waslinear in a concentration range of 1–25 mg/L (R2 > 0.9995), with aninstrument detection limit less than 0.1 mg/L.

2.3. Adsorption experiments

The majority of the experiments were conducted using WPHPAC. The experiments to compare different PAC types on theiradsorption efficiency were conducted using all three PACs: WPH,HDB, and AN. For each of these experiments, the initial STXconcentration was 25 mg/L; PAC dosages were 0, 1, 2, 5, 10, 20, 40,and 80 mg/L; with a sampling time of 2 h.

Each PAC was dried in an oven at 105 8C overnight prior to use.An 800 mg/L PAC stock suspension solution was prepared bystirring the PAC in DI water for at least 20 min. The water sampleswere buffered with 10 mM phosphate to pH of 5.7, 7.5, 8.2, and10.7. The adsorption experiments were initiated by adding 200 mLof 1.25 mg/L STX stock solution and 8.8 mL of buffered watersolution to 12-mL glass vials. Next, a total volume of 1 mL of thePAC suspension and deionized water were added into eachtreatment vial to make a total final volume of 10 mL, with25 mg/L STX and varied PAC dosages. The vials were quickly placedin LABQUAKE tumbler clips, and tumbled continuously in the darkat 8 rpm in a temperature controlled chamber at 22 8C. 1.5-mLsamples were taken from each vial at specified times and

ropore

umea

Mesopore

volumea

Macropore

volumea

Oxygena (%) Zero point

of chargea

(pH units)3/g % cm3/g % cm3/g %

40 22 0.386 61 0.112 18 10.9 10.6

17 66 0.140 29 0.023 5 5.8 6.1

91 32 0.807 66 0.029 2 9.4 4.9

H. Shi et al. / Harmful Algae 19 (2012) 61–6764

transferred into centrifuge tubes. After centrifugation at 1000 rpmfor 5 min to remove the PAC, the clear supernatant was thentransferred into HPLC autosampler vials, and stored at 4 8C untilHPLC analysis (conducted within 72 h of sample collection). Nodegradation of STX was observed in control tests.

Adsorption experiments were conducted at a pH of 5.7, 7.05,8.2, and 10.7 to study the effect of WPH PAC dosages (i.e., 0, 1, 2, 5,10, 20, 40, 80 mg/L) and equilibration times (i.e., 0, 0.5, 1, 2, 4, 7,24 h). Adsorption of STX by PAC was tested in buffered-DI water.

3. Results and discussion

3.1. Buffered-deionized (DI) water systems

The removal efficiency of STX with WPH PAC at four pH levelswas first tested using deionized (DI) water to exclude other factorsin water that may affect the experimental results. The pH wascontrolled with 10 mM of phosphate buffer. For each pH, PACdosages ranged from 1 to 80 mg/L to determine STX removalefficiency for different PAC dosages.

Overall, STX removal increased with increasing pH in DI water(Fig. 2). For example, at a PAC dosage of 10 mg/L, STX removal at pH5.7, 7.05, 8.7, and 10.7 at 2 h was <10, 48, 51, and 77%, respectively,and, at 24 h, it was <10, 79, 97 and >99%, respectively. Larger PACdosages and longer contact times provided greater STX removals.

These results can be explained by considering the variouselectrostatic and non-electrostatic interactions that would beexpected to dominate at different pH levels which would affectboth the PAC surface characteristics and the speciation of STX.First, at a pH of 10.7, WPH had a net negative surface charge, basedon its pHPZC of 6.1 (Table 1). At pH 10.7, STX was in its neutral form

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

pH 5.7

pH 8.2

Fig. 2. Adsorptive removal of STX on WPH PAC at different pH levels in

(Fig. 1), thereby suggesting that non-electrostatic interactionsdominated and with a high degree of adsorption (Fig. 2). Thesenon-electrostatic adsorbate–adsorbent interactions include bothdispersive (e.g., van der Waal’s, predominantly instantaneousdipole–dipole interactions) and H-bonding interactions (Moreno-Castilla, 2004). Additionally, hydrophobic solute–solvent effectsmay also play a lesser role, that is, the tendency of a molecule oflower polarity attempting to escape (or being squeezed out of) thehighly polar aqueous phase. Because STX is relatively soluble inwater (though less so in its neutral form at a pH of 10.7), it is likelythat dispersive interactions were the dominant adsorptionmechanism at a pH of 10.7.

At a pH of 8.2, the surface of WPH was still negatively charged,but STX shifted to approximately 75% cationic and only 25% neutral(Fig. 1). The adsorption results observed were a slight decrease inthe adsorption of STX on the PAC at all contact times and PAC doses(Fig. 2). However, with 75% cationic speciation of STX at a pH of 8.2,electrostatic interactions would be expected to dominate for thatfraction, while non-electrostatic (e.g., dispersive interactions)would likely continue to contribute for the neutral fraction. Thedecrease in capacity of STX on the PAC, with the shift toelectrostatic interactions, suggests that there were fewer sites(generally surface oxygen complexes of phenolic or carboxylicnature (Moreno-Castilla, 2004)) available for electrostatic inter-actions than for general non-electrostatic interactions.

With a decrease in the pH to 7.05, the surface of the WPH stillmaintained a slight net negative charge (pHPZC = 6.1), while STXshifted to a mix of mono-cationic and di-cationic species(approximately 65 and 35%, respectively; Fig. 1). With a smallerdegree of the anionic nature of the PAC surface, and completecationic speciation of STX, electrostatic interactions appeared to

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

pH 7.05

pH 10.2

buffered-DI water as a function of contact time and PAC dosage.

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr1

10

100

001011

STXRe

maining,%

PAC Dose, mg/L

0.5 hr

1 hr

2 hr

4 hr

7 hr

24 hr

pH 5.7pH 7.05

pH 8.2 pH 10.7

Fig. 3. Adsorptive removal of STX on WPH PAC at different pH levels in natural water containing high levels of natural organic matter (NOM) as a function of contact time and

PAC dosage.

H. Shi et al. / Harmful Algae 19 (2012) 61–67 65

dominate, but with a smaller total capacity due to the fixednumber of surface complexes with which to interact. Additionally,as the solubility of STX increases as it becomes more ionic atlower pH, hydrophobic driving force pushing STX from theaqueous phase is reduced concurrently reducing the apparentcapacity on the carbon.

Finally, at a pH of 5.7, WPH had a slightly net positive chargebased on its pHPZC of 6.1, and STX was predominantly di-cationic(Fig. 1). Thus, at a pH of 5.7, electrostatic repulsion appeared to bethe dominant mechanism that resulted in the negligible adsorp-tion observed (except at the excessively high PAC dosage of 80 mg/L). These results strongly suggest that the adsorption of STX onactivated carbon involved a complex mix of electrostatic and non-electrostatic interactions due to the pH effects on both theadsorbate (STX) and adsorbent (WPH PAC).

3.2. Natural water systems

The effects of competition with NOM were investigated usingnatural water collected from a local pond near Rolla, MO. The pH ofthe water was adjusted, as needed, by buffering with 10 mM ofphosphate. The dissolved organic carbon (DOC) concentration ofthe natural water was very high, specifically 28 � 2 mg/L as C,which served well for examining NOM competition effects.

Comparison of adsorption results for buffered DI water (Fig. 2)and for natural water (Fig. 3) showed that the greatest effects ofNOM on STX adsorption occurred at pH 7.05, with much smalleramounts of STX removed with NOM present, than for buffered-DIwater. Generally, the NOM charge would be expected to benegative throughout this and a higher pH range (Newcombe and

Drikas, 1997). As compared to high pH levels, the net surfacecharge of WPH was slightly less negative, and STX was slightlymore cationic in nature. The reason for the pronounced decreasein STX removal in the presence of natural water, at a pH of 7.05,but not at 8.2 or 10.2, is not completely clear. It is hypothesizedthat STX–NOM solution-phase interactions may have beenenhanced at this pH due to the increasingly cationic nature ofthe STX in the presence of the negatively charged NOM. It ishypothesized that these solution-phase interactions may havebeen hindering the adsorption of STX on the carbon surface.

At a pH of 5.7, little adsorptive removal was observed for STXin the presence of NOM (Fig. 3). As with the buffered-DI waterresults, the PAC surface was slightly net positive, STX waspositive, and NOM was slightly net negative. Thus, STX was stillexpected to be attracted to the aqueous phase and (slightly)repulsed by the PAC surface. STX–NOM interaction in solutionwould also likely to further reduce STX adsorption, as would anyNOM sorption on the PAC.

3.3. Effect of PAC type

Three different kinds of widely used PACs were examined andcompared in buffered-DI water, including WPH (bituminous-coal based), HDB (lignite-coal based), and AN (wood-ash based).The properties of these carbons are presented in Table 1. Thesorption experiments were performed at a pH of 8.2 in buffered-DI water at 22 8C.

The experimental results showed that the sorption capacity forSTX followed the trend (Fig. 4): HDB < WPH < AN. While, thisorder of removal and sorption capacity corresponds with the order

1

10

100

001011

STXRe

maining,%

PAC Dose, mg /L

HDB-2 hrHDB-24 hrWPH-2 hrWPH-24 hrAN-2 hrAN-24 hr

Fig. 4. Comparison of STX adsorption on WPH, HDB, and AN PACs at pH 8.2 with

contact times of 2 and 24 h.

H. Shi et al. / Harmful Algae 19 (2012) 61–6766

of BET surface areas (Table 1), normalization of the removal data toBET surface area showed that BET surface area effects accountedfor only a part of the performance trend noted. Another significantfactor was hypothesized to be related to the surface charge on thePACs. The pHPZC for AN and WPH (4.9 and 6.1, respectively) areboth less than the solution pH, and, hence, both PACs would have anet negative charge at pH 8.2. The pHPZC for HDB, however, is 10.9,and greater than solution pH so that the surface of HDB would havea net positive charge at a pH of 8.2. Because STX is approximately75% cationic and 25% neutral at a pH of 8.2 (Fig. 1), STX would beexpected to be repulsed by the HDB surface and attracted to theWPH and AN surfaces, which is consistent with the observed trendof HDB having the least STX removal.

The carbon oxygen content trend was not consistent with theSTX removal trend for the three carbons. Specifically, WPH had thelowest oxygen content (Table 1).

4. Conclusions

This study conclusively showed that pH (as well as PAC type,dosage, and contact time; and NOM concentration) has a largeimpact on the adsorptive efficiency of PAC for STX treatment. Theseeffects can be understood based on well-known non-electrostaticand electrostatic interactions. Furthermore, at an intermediatecommon pH for water treatment of 8.2, the relative performancesof three PAC were (from lowest to highest capacity): lignite-coal-based HDB < bituminous-coal-based WPH < wood-based AN. Theresults of this study suggest that the choice of PAC should beclosely matched to the objectives of the PAC treatment. Further-more, these results reinforce that water quality conditions alsoplay a critical role in PAC sorption performance and must be wellunderstood and/or studied in laboratory or field experiments tooptimize water treatment system performance.

Acknowledgements

This research work was supported by the Missouri Departmentof Natural Resources (MDNR). The authors thank Dr. Detlef Knappe(North Carolina State University) for providing carbon characteri-zation data critical to this work, as well as insightful comments onthe results.[SS]

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