Enzyme and Microbial Technology 51 (2012) 396– 401
Contents lists available at SciVerse ScienceDirect
Enzyme and Microbial Technology
j our na l ho me p age: www.elsev ier .com/ locate /emt
nzyme catalyzed electricity-driven water softening system
ary A. Arugulaa, Kristen S. Brastada, Shelley D. Minteerb, Zhen Hea,∗
Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USADepartment of Chemistry, The University of Utah, Salt Lake City, UT 84112, USA
r t i c l e i n f o
rticle history:eceived 7 May 2012eceived in revised form 17 August 2012ccepted 22 August 2012
eywords:ardnessater softening
nzymatic biofuel celllectricity
a b s t r a c t
Hardness in water, which is caused by divalent cations such as calcium and magnesium ions, presents amajor water quality problem. Because hard water must be softened before use in residential applications,there is great interest in the saltless water softening process because, unlike ion exchange softeners, itdoes not introduce additional ions into water. In this study, a saltless hardness removal driven by bioelec-trochemical energy produced through enzymatic oxidation of glucose was proposed and investigated.Glucose dehydrogenase was coated on a carbon electrode to catalyze glucose oxidation in the presence ofNAD+ as a cofactor/mediator and methylene green as an electrocatalyst. The results showed that electric-ity generation stimulated hardness removal compared with non-electricity conditions. The enzymaticwater softener worked upon a 6 h batch operation per day for eight days, and achieved an average hard-
esalination ness removal of 46% at a high initial concentration of 800 mg/L as CaCO3. More hardness was removed ata lower initial concentration. For instance, at 200 mg/L as CaCO3 the enzymatic water softener removed76.4 ± 4.6% of total hardness. The presence of magnesium ions decreased hardness removal because of itslarger hydrated radius than calcium ions. The enzymatic water softener removed 70–80% of total hard-ness from three actual hard water samples. These results demonstrated a proof-of-concept that enzymecatalyzed electricity generation can be used to soften hard water.
. Introduction
The primary sources of water supply in the U.S. are surfaceaters (e.g., rivers, lakes, streams) and groundwater (deep wells).roundwater often has high levels of hardness, which is causedy a variety of dissolved multivalent metallic ions, predominantlyalcium and magnesium [1,2]. Other cations like aluminum, bar-um, iron, manganese, strontium, and zinc contribute less to waterardness [3]. More than 85% of the U.S. population experience theetrimental effects of hard water both in domestic and industrialsage – hard water often produces a noticeable deposit of pre-ipitate – scaling, especially in hot water pipes, heaters, boilers,itchens, bathtubs, and other units [4,5]. Therefore, prior to residen-ial distribution and consumption, hardness concentration shoulde reduced and hard water should be softened.
Water softening processes comprise approximately 20–30% ofhe industrial and residential water treatment market in the U.S.,ith a specific market share of 2.5 billion dollars (in 2010) for res-
dential applications and 7.3 billion dollars (in 2010) for industrialpplications [6]. Two major methods are typically used to removeardness: lime soda softening and ion exchange softening. Lime
soda softening is used mostly for municipal purposes; it employschemical precipitation, in which lime is added to hard water to pre-cipitate calcium ions as calcium carbonate and magnesium ions asmagnesium hydroxide [7]. The primary drawbacks of the lime sodamethod include the production of a large volume of sludge thatrequires post-treatment; excessive use of chemicals such as lime,lime soda ash, and caustic soda; and the addition of acids for pHadjustment, which increases operating expenses [8].
The ion exchange process is primarily employed for residen-tial water softening. The softening system consists of salt-saturated(e.g., sodium chloride) resin beads and a brine tank to regeneratethe resin bed; in this process, each divalent hardness ion (Ca2+ orMg2+) in the water is replaced by two sodium ions, thereby soft-ening the water [9]. Experimental studies have found the sodiumlevel in softened water was 2.5 times (mean sodium concentration296 mg/L) higher than municipal water [10]. Sodium is an essen-tial mineral, however, excess consumption may be harmful [11,12]and contribute to major health issues such as stroke, hyperten-sion, and high blood pressure [13,14], especially for individualson a sodium-restricted diet [11]. Other residential water softeningmethods include distillation, nanofiltration, electrodialysis, carbon
nanotubes, capacitive deionization, and reverse osmosis, whichconsume a large amount of energy, and operation and maintenanceof these systems can be expensive [15–20]. To avoid introducingadditional salts such as sodium into drinking water during the
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396– 401 397
anion
sd
disemttahphpvuse
iwddwctc(f
2
2
p(ar
Fig. 1. Schematic of an enzymatic water softener. AEM:
oftening process, saltless water softening technologies must beeveloped as an alternative to the ion exchange process.
The recently developed microbial desalination cell (MDC)esalinates saline water via electricity produced by microorgan-
sms and can potentially act as a low-energy and saltless wateroftening technique [21–24]. MDC technology has been previouslymployed to soften hard water [25]; however, the presence oficroorganisms in an MDC is inappropriate for residential applica-
ions. Here, an alternative method is proposed for hardness removalhrough electricity generation using immobilized enzymes on thenode. Well-known since the 1960s, enzyme-based biofuel cellsave been developed to use enzymes as biocatalysts instead ofrecious metals. In an enzymatic biofuel cell, simple sugars or alco-ol are oxidized by the enzymes at the surface of the electrodes toroduce electricity [26,27]. Enzymes offer the advantage of beingery selective in their chemical functions and this allows for these of metabolic pathways or enzyme cascades to deeply oxidizeubstrate/fuel [28–31], therefore, they can produce higher catalyticfficiencies than complex microbial communities.
In this study, a proof-of-concept of an enzymatic water soften-ng process was developed and demonstrated. Enzymes and sugars,
hich locate in the anode compartment and will not get intorinking water, are safer and expected to be more appropriate forrinking water applications than microbial systems. Experimentsere conducted to (1) verify the effect of electricity generation by
omparing the open and closed circuit; (2) examine the stability ofhe enzymatic water softener; (3) investigate the effects of hardnessoncentration in the presence of single and multiple cations; and4) study softening performance using actual hard waters obtainedrom different locations.
. Materials and methods
.1. Chemicals and reagents
Purchased from Sigma–Aldrich, Acros Organics, and Fluka, and used as sup-lied without any pretreatment or further purification, were glucose dehydrogenaseGDH) from Pseudomonas sps (GDH, E.C. 1.1.1.47), d-(+)-glucose, �-nicotinamidedenine dinucleotide hydrate (NAD+) from yeast, methylene green zinc chlo-ide double salt, sodium nitrate, sodium tetraborate, Nafion per fluorinated ion
exchange membrane, CEM: cation exchange membrane.
exchange resin (5 wt%), tetra butyl ammonium bromide 99+%(TBAB), calcium chlo-ride dehydrate, magnesium chloride anhydrous, 10% Pt Black and other standardorganic/inorganic chemicals. All solutions were prepared using Milli-Q®-gradewater. The experiments were performed at an ambient temperature of 22 ± 2 ◦C.
2.2. Preparation of enzymatic anode electrode
The anode electrode was a piece of carbon cloth 1 cm × 1 cm (Zoltek, Inc., St.Louis, USA). To electro-polymerize a thin film of poly (methylene green) ontothe electrode, cyclic voltammetry (CV) was performed using an Ag/AgCl referenceelectrode and platinum wire as a counter electrode on a Gamry Instruments 600Potentiostat from −0.3 V to 1.3 V for 12 sweep segments at a scan rate of 0.05 V/s.The CV was performed in a solution containing 0.4 mM methylene green and 0.1 Msodium nitrate in 10 mM sodium tetraborate. The electrode was gently rinsed anddried overnight under ambient air flow before further modification.
Hydrophobically modified Nafion was prepared for immobilizing enzymes ontothe anode electrode, accordingly to previous literature procedures [32,33], whichhave shown the ability to uniformly encapsulate NAD+-dependent dehydroge-nase enzymes with the cofactor NAD+ [33]. Tetrabutyl ammonium bromide (TBAB)(81 mg) was mixed with a suspension of Nafion (1 mL) at a ratio of the concentrationof TBAB salt three folds to the concentration of sulfonic acid sites in the 5 wt% Nafionsuspension. One mL of the mixed cast solution was transferred to a weigh boat andleft to dry overnight. The membrane (polymer) casting solution was soaked andwashed with 7 mL of Milli-Q®-grade water to remove hydrogen bromide and excessquaternary ammonium bromide. The membrane solution was centrifuged to removethe supernatant and then resuspended in 1 mL alcohol to create an alkyl ammonium-modified Nafion solution. Furthermore, the enzyme/Nafion casting solutions wereprepared by mixing an enzyme-to-polymer ratio of 2:1. One mg/mL enzyme solu-tion was prepared by dissolving 1 mg of GDH in 1 mL of 0.1 M phosphate buffer atpH 7.13. In a microcentrifuge tube, 600 �L of the 1 mg/mL enzyme solution, 300 �Lof the alkyl ammonium-modified Nafion solution, and 0.015 g of NAD+ were mixedand vortexed for 30 s. Then, 100 �L of this casting solution was pipetted onto theelectropolymerized anode electrode and allowed to dry completely for 24 h. As aresult, GDH was immobilized in the TBAB/Nafion mixture casting membrane.
2.3. Enzymatic water softener set up
An enzymatic water softener was designed similarly to an MDC, with an anode,cathode, and a middle chamber, as shown in Fig. 1. An anion exchange mem-brane (AMI-7001, Membrane International, Inc., Glen Rock, NJ, USA) separated theanode and the middle chamber, whereas a cation exchange membrane (CMI-7000
Membrane International, Inc.) separated the cathode and the middle chamber. Thechambers were fixed together using gaskets and clamps to prevent water leakage.In the anode chamber, an anode electrode with immobilized enzymes was con-nected to an external circuit through titanium wire. In the cathode chamber, carboncloth with Pt catalyst (0.2 mg/cm2) prepared using 10% Pt according to a previous
3 icrobial Technology 51 (2012) 396– 401
su2cma
2
NbswfwawctsfaUmwpaa
2
laoVCPcua
C
acooQso
3
3
ceo0td
nccoaetws
Fig. 2. Polarization curves of the enzymatic water softener with (black) and without
To examine hardness removal over time, we fed hard water of800 mg/L as CaCO3 in batch mode (on 6-h operation cycle) and mon-itored the system performance for more than 20 days. The current
98 M.A. Arugula et al. / Enzyme and M
tudy [34] was fixed to titanium wire and inserted as a cathode electrode. The liq-id volumes of the anode, middle, and cathode chambers were about 25, 20, and5 mL, respectively. When operating the enzymatic softener, cations such as cal-ium and magnesium ions transfer to the cathode chamber via cation exchangeembrane, and anions like chloride ions transfer to the anode chamber through the
nion exchange membrane.
.4. Operating conditions
The anode chamber was filled with freshly prepared 50 mM glucose and 1 mMAD+ (co-factor) in 20 mM phosphate buffer (pH 7.13), and the cathode cham-er was filled with 2 mM phosphate buffer (pH 7.13). The cathode chamber wasparged with air to supply oxygen to the cathode reaction. Two sources of hardater (synthetic water with single/multiple ions and actual hard water) were used
or hardness removal experiments. Three different concentrations of synthetic hardater (800, 400, and 200 mg/L as CaCO3) were prepared by dissolving respective
mounts of calcium chloride (CaCl2) in a liter of deionized water. Synthetic hardater with multiple cations (Ca2+ and Mg2+) was prepared by dissolving calcium
hloride and magnesium chloride (MgCl2) in a 4:1 ratio to produce a final concen-ration of 200 mg/L as CaCO3. To compare the synthetic and actual hard water at aimilar hardness concentration, actual hard water of 350 mg/L as CaCO3 collectedrom Burnsville (MN, USA) was diluted to a concentration of 200 mg/L as CaCO3. Thectual hard water samples were also collected from three different locations in the.S. and tested separately. During the experiments, the hard water was fed into theiddle chamber in batch-mode operation for 6 h (each batch). Before running theater softening process, the enzymatic water softener operated at an open-circuitotential for 6 h for three cycles. Hardness removal experiments were conductedt an external resistance of 100 �. The liquids in all three chambers were replacedfter each cycle.
.5. Measurement and analysis
The voltage data was collected every 3 min by a digital multimeter (2700, Keith-ey Instruments, Inc., Cleveland, OH, USA). The polarization curve was performed by
potentiostat (Reference 600, Gamry Instruments, Warminster, PA) at a scan ratef 0.5 mV/s. The pH was measured with a bench top pH meter (Oakton Instruments,ernon Hills, IL, USA). The total hardness was measured with a digital titrator (Hachompany, Loveland, OH, USA) model 16900 using a ManVer® Hardness Indicatorowder Pillow and titrated with ethylenediamine-tetraacetic acid (EDTA). The con-entrations of calcium and magnesium ions in actual hard waters were measuredsing ion chromatography (IC-1100, Dionex). Columbic recovery (CR) was calculateds:
R = Qoutput
Qinput=
∑It
F × E(1)
where Qoutput is the produced charge, Qinput is the total charge available in thedded organic compounds, I is electric current (A), t is time (s), F is the Faradiconstant (96,485 C/mol), and E is the total mole of electrons (mol) that can be the-retically produced from the input glucose (2 mol of electrons per mole of glucosexidation to gluconic acid). Charge transfer efficiency was calculated as the ratioo/Qr , where Qo is the output of electric charge from the electrical circuit and Qr
tands for the charge from the removed salts (1 mol of CaCl2 removal requires 2 molf electrons).
. Results
.1. Hardness removal stimulated by current generation
Electricity generation in the enzymatic water softener washaracterized by polarization curves. As shown in Fig. 2, in the pres-nce of an immobilized enzyme, the water softener produced anpen-circuit potential of 0.72 V, the maximum current density of.1 mA/cm2 and the maximum power output of 9 �W/cm2. Whenhere was no enzyme on the anode electrode, the maximum currentensity was only 0.008 mA/cm2.
To demonstrate the effect of electricity generation on hard-ess removal, we compared hardness removal under the open andlosed circuit. When the system operated under an open-circuitondition, there was no current generation and the concentrationf hardness decreased by about 2%, from 800 to 760 ± 23 mg/Ls CaCO3 in 6 h. Under a closed-circuit condition, electricity gen-
ration reached an average current of 0.08 mA/cm2. As a result,he hardness concentration decreased to 433 ± 50 mg/L as CaCO3ithin 6 h (Fig. 3), representing 46% removal. These results clearly
how that current generation stimulated hardness removal.
(red) enzyme immobilization on the anode electrode: (A) power production and (B)voltage curves. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of the article.)
3.2. Stability of enzymatic water softener
Bioanode stability is a key factor for an enzymatic system.
Fig. 3. Comparison of the hardness removal between the open and closed circuitswith an initial hardness concentration of 800 mg/L as CaCO3.
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396– 401 399
F(
glf0htip
3
h6ccwsatwtc78cn2i
Fig. 5. Effect of initial hardness concentrations on hardness removal (red) and total
ferent locations in the U.S., with hardness concentrations varyingfrom 282 to 664 mg/L as CaCO3 (Table 1). The initial pHs of thosewaters were generally above 8. The water softener removed 74–86%of hardness within 6 h, and produced 4.45–6.36 C of electric charge
ig. 4. The stability test conducted for 17 batch cycles: (A) current generation andB) corresponding hardness removal.
eneration exhibited a typical batch profile with a peak current fol-owed by decrease. The peak current was stable at ∼0.1 mA/cm2
or eight cycles (eight days), and then started to decrease to.039 mA/cm2 in the following cycles (Fig. 4A). Accordingly, theardness removal decreased from ∼50% to 35% and eventuallyo about 10% (Fig. 4B). This performance shows that there is annstability in either the enzyme, cofactor regeneration, cofactorreconcentration, or poly(methylene green) electrocatalyst layer.
.3. Effect of hardness concentrations
To study the effect of hardness concentrations and subsequentardness removal, a series of experiments were conducted by-h batch cycle for up to seven days using three different con-entrations of synthetic hard water samples with initial hardnessoncentrations of 800, 400, 200 mg/L as CaCO3. More hardnessas removed at a lower initial hardness concentration: the water
oftener removed 69.6 ± 10.6% and 76.4 ± 4.6% of hardness at 400nd 200 mg/L as CaCO3, respectively, both of which were higherhan 46.2 ± 4.5% at 800 mg/L as CaCO3 (Fig. 5). Total electric chargeas calculated for the first five cycles by integrating current with
ime. The highest charge of 7.68 ± 0.11 C was produced with aolumbic recovery (CR) of 3.2% with 400 mg/L as CaCO3, followed by.62 ± 0.06 C with a CR of 3.2% and 5.93 ± 0.11 C with a CR of 2.4% at00 and 200 mg/L as CaCO3, respectively. The charge transfer effi-
iency increased with decreasing initial hardness concentration –early 100% of the charge transfer efficiency was achieved with00 mg/L; while it was 74% and 58% at 400 mg/L and 800 mg/L of
nitial hardness concentrations, respectively.
electric charge production (green) in synthetic hard water. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthe article.)
3.4. Effect of multiple cations
To study the removal of multiple divalent hardness cations,we prepared a synthetic hard water sample of 200 mg/L as CaCO3spiked with both CaCl2 and MgCl2 at a ratio of 4:1 [35]. The totalcharge for the first five cycles was 5.11 ± 0.07 C and 60.4 ± 3.1%of the hardness was removed with this dual-cation hard water(Fig. 6). For comparison, an actual hard water sample (Burnsville,MN) was diluted to 200 mg/L as CaCO3 and tested in the water soft-ener: 74.0 ± 3.0% of the hardness was removed and 5.82 ± 0.12 C ofelectric charge was produced.
3.5. Hardness removal from actual hard water
Operation of the enzymatic water softener to remove hardnessions in actual hard water was examined in a batch-fed operation.The samples of actual hard water were collected from three dif-
Fig. 6. Total electric charge (green) and hardness removal (red) with an initial con-centration of 200 mg/L as CaCO3 in three different hard waters, single-cation (Ca2+)synthetic hard water, dual-cation (Ca2+ and Mg2+) synthetic hard water and actualhard water. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of the article.)
400 M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396– 401
Table 1Characteristics of the hard water sampled from three different locations in the U.S., and its removal and electricity generation in 6-h operation in the enzymatic watersoftener.
Location Sampling site Initial pH Final pH CHa Ca:Mgb Hardness removal Total charge (C)
Mt Joy, PA Groundwater, sampled fromkitchen faucets
8.14 7.65 282 1:1 74.0% 4.45 ± 0.06
Burnsville, MN Mixed ground and surfacewater, sampled from kitchenfaucets
8.75 6.91 350 8:1 86.0 ± 1.7% 5.01 ± 0.13
Roswell, NM Groundwater, sampled from acommercial building
8.25 6.82 664 7.5:1 82.3 ± 3.7% 6.36 ± 0.16
(6
4
cGrgaoaogdebf
cttgpPthdmobtatiaimtatnh
trn2ndc
a Hardness concentration (mg/L as CaCO3).b The ratio of element mass.
Table 1). The softened water exhibited a lower pH varying between.82 and 7.65.
. Discussion
The results demonstrated a proof-of-concept that bioelectro-hemical energy produced by an enzymatic bioanode employingDH immobilized in a TBAB modified Nafion membrane can
emove hardness from water. As the driving force, electrons areenerated from glucose oxidation to gluconic acid, in which NADHcts as electron shuttles through redox reactions. To decrease thever-potential of NADH oxidation, poly(methylene green) was useds an electrocatalyst for NADH [36]. The comparison between thepen and closed circuits provides solid evidence that electricityeneration plays an important role in removing hardness. The slightecrease in hardness under the open circuit was likely due to ionxchange or ion diffusion [37]. Hardness removal can be improvedy increasing current generation, which will be the focus of ouruture study.
The water softener maintained a stable performance for eightycles within eight days. The polarization tests demonstrated thathe anodic potential increased by 0.4 V (from −0.2 to 0.2 V), whilehe cathodic potential decreased by 0.1 V (from 0.3 to 0.2 V), sug-esting that the bioanode was a major limiting factor to the overallerformance. That is supported by our previous study that thet/C cathode could achieve stable performance for more thanhree months [34]. The decrease in enzyme catalytic activity mightave led to the decreased performance of the system after eightays compared with 30 days of typical two-chambered enzy-atic fuel cells [38]; but frequently the decrease in performance
f NAD-dependent dehydrogenase-based bioanodes is the insta-ility of the cofactor during cycling or the long term stability ofhe NADH electrocatalyst. Better system performance could bechieved by improving the stability of the enzyme, the cofactor, andhe electrocatalyst. Our next investigation will focus on improv-ng the enzyme’s stability by adopting different immobilizationnd stabilization strategies, such as using covalent and crosslink-ng attachment to single-walled carbon nanotubes, porous chitosan
embranes, or other entrapment techniques and using enzymeshat are NAD+ independent, like glucose oxidase, to avoid co-factornd electrocatalyst instability [39–41]. The use of nanomaterials inhe present water softener will need to consider the (potentially)egative effects of those materials on human and environmentalealth during the disposal of an enzymatic electrode.
The enzymatic water softener performed better at a lower ini-ial hardness concentration, at which the charge transfer efficiencyendered a higher removal rate compared with a higher initial hard-ess concentration. The charge transfer efficiency of nearly 100% at
00 mg/L suggests that almost all electrons were used to drive hard-ess removal (transporting Ca2+), or almost all Ca2+ removed wasue to electricity generation. The decreased charge transfer effi-iency at higher hardness concentrations indicates the hardness
was partially removed by processes other than current generation.Enhanced ion exchange or diffusion at higher hardness concentra-tions could contribute to hardness removal; however, we did notobserve a strong effect of those processes under the open-circuitcondition; the exact reasons require further investigation. Unlikeour previous study of microbial-based water softening process [25],the hardness concentration did not significantly affect electricitygeneration in this study and three concentrations yielded similartotal coulombs. According to our experience, a Pt cathode with suf-ficient oxygen supply could meet the demand of electron transferin this type of reactor; therefore, we believe the anode limits theperformance of the present enzymatic water softener. Based on theglucose concentration, it was estimated that the coulombic recov-ery was less than 4%; thus, the organic supply was sufficient andthe conversion of organic to electrons restricted the overall perfor-mance. The conversion is determined by enzyme, co-factors, andelectrocatalysts.
At the initial concentration of 200 mg/L as CaCO3, a comparisonof hardness removal between the synthetic water containing singleand dual cations revealed that the presence of Mg2+ decreased theremoval of total hardness, presumably due to the faster migrationof calcium ions, which have a small hydrated ionic radius of 6 Athan magnesium with a larger hydrated ionic radius of 8 A. Typi-cally, ion selectivity of a cation is determined by its higher valence;for cations of similar valence, a smaller cation has larger hydratedradius than a larger cation [42,43]. The diluted actual hard waterat 200 mg/L as CaCO3 removed hardness more efficiently than thedual-cation water, but lower than the single-cation water, likelydue to a higher ratio of Ca2+ to Mg2+. The IC analysis of actual hardwater samples showed that ratio of calcium to magnesium was∼8:1, higher than 4:1 used for the dual-cation water; more Ca2+
ions in the actual hard water samples promoted the removal oftotal hardness.
Future development of enzymatic softener will focus on opti-mizing the configuration, and improving enzymatic catalyticactivities through an enzyme cascade. The configuration of thereactor, especially the dimensions of the middle chamber, alsoaffects hardness removal. The width of the middle chamber, orthe distance between the two ion exchange membranes, can affectthe internal resistance (e.g., electrolyte resistance) of the reactorand thus the current generation that directly relates to hardnessremoval. The enzymatic softener removes hardness in a processsimilar to an electrodialysis (ED). The distance between membranepairs in an ED is usually smaller than 1 mm for minimizing inter-nal resistance; in the present enzymatic softener, this distance was9 mm, significantly larger than an ED. Therefore, a smaller gapbetween membrane pair must be considered in the next design.GDH oxidation of glucose to gluconic acid is an incomplete oxida-
tion reaction that only liberates 2 electrons. If an enzyme cascade ispresent then glucose may be oxidized completely to CO2, therebyreleasing 24 electrons/mol for use in the system. That will improvefuel efficiency and thus reduce fuel cost: the cost of glucose or
icrobi
scstoc
5
wstsCaalmgeb
A
GfaSpm
R
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[[
[
[
[
[
[
M.A. Arugula et al. / Enzyme and M
ugar is clearly higher than sodium salts used in ion exchange pro-esses. Another approach to reduce fuel cost is to use less purifiedubstrate, which may have higher requirement for enzyme’s selec-ivity. Other factors are critical for hardness removal, such as effectf pH, buffering capacity, chloride ions, time period of each cycle,oncentration of substrate in the anode chamber and temperature.
. Conclusions
This study demonstrated the feasibility of an enzyme-basedater softening system as a potential alternative for ion exchange
ystems for residential applications. The enzymatic softener effec-ively removed hardness at different initial concentrations in bothynthetic and actual hard waters without input of electrical energy.urrent generation through enzymatic oxidation of glucose played
major role in hardness removal. The removal efficiency wasffected by the composition of hard waters; magnesium especiallyowered the removal of total hardness. The performance of an enzy-
atic water softener needs to be improved by increasing currenteneration (e.g., lowering external resistance for high current gen-ration and better enzyme activities) and bioanode stability (e.g., aetter preparation procedure).
cknowledgements
This project was financially supported by a Bradley Catalystrant from the UW-Milwaukee Research Foundation and private
unds from Mr. Mark Murphy. Kristen Brastad was supported by grant from A. O. Smith Corporation. We thank Ms. Michellechoenecker (UW-Milwaukee) for her assistance with manuscriptroofreading, and the anonymous reviewers for their helpful com-ents.
eferences
[1] Frederick KD. America’s water supply: status and prospects for the future.Consequences 1995;1(1):14–23.
[2] Crittenden JC, Harza MW. Water treatment principles and design. 2nd editionHoboken, NJ: J. Wiley; 2005.
[3] WHO. Hardness in drinking-water (WHO/HSE/WSH/10.01/10/Rev/1); 2011.Available at http://www.who.int/water sanitation health/dwq/chemicals/hardness.pdf [accessibility verified April 30, 2012].
[4] Burris MA. Soft water, hard choice? Government Engineering; 2004,July–August. p. 20–21. Available at http://www.govengr.com/ArticlesJul04/soft.pdf [accessibility verified April 30, 2012].
[5] Cameron BA. Detergent considerations for consumers: laundering in hardwater – how much extra detergent is required? The Journal of Extension2011;49:4RIB6.
[6] SBI Energy, Water and Air Purification Systems & Products: Residen-tial & Commercial; 2010, October. Available at http://www.sbireports.com/Water-Air-Purification-2809842/ [accessibility verified April 30, 2012].
[7] Bergman R. Membrane softening versus lime softening in Florida: a cost com-parison update. Desalination 1995;102:11–24.
[8] Randtke SJ, Hoehn RC. Removal of DBP precursors by enhanced coagulationand lime softening. Denver, CO: American Water Works Association ResearchFoundation and American Water Works Association; 1999. Available athttp://www.waterrf.org/ProjectsReports/PublicReportLibrary/RFR90783 1999814.pdf [accessibility verified April 30, 2012].
[9] Skipton SO. Drinking water treatment: water softening (ion exchange); October2008. Available at http://www.ianrpubs.unl.edu/live/g1491/build/g1491.pdf[accessibility verified April 30, 2012].
10] Yarows SA, Fusilier WE, Weder AB. Sodium concentration of water from soft-
eners. Archives of Internal Medicine 1997;157(2):218–22.
11] Das G, Janine F. The effects of home water softeners: added sodium may behazardous to your health. Journal of Environmental Health 1980;50(7).
12] Das G. You and your drinking water: health implication for the use of cationexchange water softeners. Journal of Clinical Pharmacology 1988;28:683–90.
[
[
al Technology 51 (2012) 396– 401 401
13] He FJ, MacGregor GA. Effect of modest salt reduction on blood pressure: ameta-analysis of randomized trials. Implications for public health. Journal ofHuman Hypertension 2002;16(November (11)):761–70 [cited 2012 March 9].Available from: http://www.ncbi.nlm.nih.gov/pubmed/12444537.
14] Du Cailar G, Ribstein J, Mimran A. Dietary sodium and target organ damage inessential hypertension. American Journal of Hypertension 2002;15:222–9.
15] Ghizellaoui S, Chibani A, Ghizellaoui S. Use of nanofiltration for partial softeningof very hard water. Desalination 2005;179:315–22.
16] Kabay N, Demircioglu M, Ersoz E, Kurucaovali I. Removal of calcium and mag-nesium hardness by electrodialysis. Desalination 2002;149:343–9.
17] Tofighy MA, Mohammadi T. Permanent hard water softening using carbonnanotube sheets. Desalination 2011;268:208–13.
18] Gabrielli C, Maurin G, Francychausson H, Thery P, Tran T, Tlili M. Electrochemi-cal water softening: principle and application. Desalination 2006;201:150–63.
19] Seo SJ, Jeon H, Lee JK, Kim GY, Park D, Nojima H, et al. Investigation on removal ofhardness ions by capacitive deionization (CDI) for water softening applications.Water Research 2010;44:2267–75.
20] Cuda P, Pospísil P, Tenglerová J. Reverse osmosis in water treatment for boilers.Desalination 2006;198:41–6.
21] Cao XX, Huang X, Liang P, Xiao K, Zhou YJ, Zhang XY, et al. A new method forwater desalination using microbial desalination cells. Environmental Scienceand Technology 2009;43:7148–52.
22] Jacobson KS, Drew DM, He Z. Efficient salt removal in a continuously operatedupflow microbial desalination cell with an air cathode. Bioresource Technology2010;102:376–80.
23] Jacobson KS, Drew DM, He Z. Use of a liter-scale microbial desalination cellas a platform to study bioelectrochemical desalination with salt solution orartificial seawater. Environmental Science and Technology 2011;45:6690–6.
24] Luo H, Xu P, Roane TM, Jenkins PE, Ren Z. Desalination cells for improved per-formance in wastewater treatment, electricity production, and desalination.Bioresource Technology 2012;105:60–6.
25] Brastad K, He Z. Water softening using microbial desalination cell technology.Desalination, under review.
26] Bullen RA, Arnot TC, Lakeman JB, Walsh FC. Biofuel cells and their development.Biosensors and Bioelectronics 2006;21:2015–45.
27] Cooney MJ, Svoboda V, Lau C, Martin G, Minteer SD. Enzyme catalysed biofuelcells. Energy & Environmental Science 2008;1:320–33.
28] Sokic-Lazic D, de Andrade A, Minteer SD. Utilization of enzyme cascades forcomplete oxidation of lactate in an enzymatic biofuel cell. Electrochimica Acta2011;56:10772–5.
29] Sokic-Lazic D, Arechederra RL, Treu BL, Minteer SD. Oxidation of biofuels: fueldiversity and effectiveness of fuel oxidation through multiple enzyme cascades.Electroanalysis 2010;22(7–8):757–64.
30] Arechederra R, Minteer SD. Kinetics and transport analysis of immobilized oxi-doreductases that oxidize glycerol and its oxidation products. ElectrochimicaActa 2010;55:7679–82.
31] Xu S, Minteer SD. Enzymatic biofuel cell for oxidation of glucose to CO2. ACSCatalysis 2012;2:91–4.
32] Klotzbach TL, Watt M, Ansari Y, Minteer SD. Improving the microenvironmentfor enzyme immobilization at electrodes by hydrophobically modifying chi-tosan and Nafion polymers. Journal of Membrane Science 2008;311:81–8.
33] Moore CM, Akers NL, Minteer SD. Improving the environment for immobi-lized dehydrogenase enzymes by modifying Nafion with tetraalkylammoniumbromides. Biomacromolecules 2004;5:1241–7.
34] Xiao L, Damien J, Luo J, Jang H, Huang J, He Z. Crumpled graphene particles formicrobial fuel cell electrodes. Journal of Power Sources 2012;208:187–92.
plete oxidation. Electrochemical and Solid-State Letters 2009;12:F26.37] Mehanna M, Saito T, Yan J, Hickner M, Cao X, Huang X, et al. Using microbial
desalination cells to reduce water salinity prior to reverse osmosis. Energy &Environmental Science 2010;3:1114–20.
38] Akers NL, Moore CM, Minteer SD. Development of alcohol/O2 biofuel cells usingsalt-extracted tetrabutylammonium bromide/Nafion membranes to immobi-lize dehydrogenase enzymes. Electrochimica Acta 2005;50:2521.
39] Wang X, Li D, Watanabe T, Shigemori Y, Mikawa T, Okajima T, et al. A glucose/O2
biofuel cell using recombinant thermophilic enzymes. International Journal ofElectrochemical Science 2012;7:1071–8.
40] Klotzbach TL, Watt MM, Ansari YA, Minteer SD. Effect of hydrophobic modifi-cation of chitosan and Nafion on transport properties, ion exchange capacities,and enzyme immobilization. Journal of Membrane Science 2006;282:276–83.
41] Lau C, Martin G, Minteer SD, Cooney MJ. Development of a chitosan scaffoldelectrode for fuel cell applications. Electroanalysis 2010;22:793–8.
42] Muyibi SA, Evison LM. Moringa oleifera seeds for softening hard water. WaterResearch 1995;29:1099–105.
43] Buhlmann P, Pretsch E, Bakker E. Carrier-based ion-selective electrodes andbulk optodes. 2. Ionophores for potentiometric and optical sensors. ChemicalReviews 1998;98:1593–688.
