Enzyme catalyzed electricity-driven water softening

Enzyme and Microbial Technology 51 (2012) 396–401
Contents lists available at SciVerse ScienceDirect
Enzyme and Microbial Technology
journal homepage: www.elsevier.com/locate/emt
Enzyme catalyzed electricity-driven water softening system
Mary A. Arugula a , Kristen S. Brastad a , Shelley D. Minteer b , Zhen He a,∗
a
b
Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
Department of Chemistry, The University of Utah, Salt Lake City, UT 84112, USA
a r t i c l e
i n f o
Article history:
Received 7 May 2012
Received in revised form 17 August 2012
Accepted 22 August 2012
Keywords:
Hardness
Water softening
Enzymatic biofuel cell
Electricity
Desalination
a b s t r a c t
Hardness in water, which is caused by divalent cations such as calcium and magnesium ions, presents a
major 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, it
does not introduce additional ions into water. In this study, a saltless hardness removal driven by bioelectrochemical 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 of
NAD+ as a cofactor/mediator and methylene green as an electrocatalyst. The results showed that electricity generation stimulated hardness removal compared with non-electricity conditions. The enzymatic
water softener worked upon a 6 h batch operation per day for eight days, and achieved an average hardness removal of 46% at a high initial concentration of 800 mg/L as CaCO3 . More hardness was removed at
a lower initial concentration. For instance, at 200 mg/L as CaCO3 the enzymatic water softener removed
76.4 ± 4.6% of total hardness. The presence of magnesium ions decreased hardness removal because of its
larger hydrated radius than calcium ions. The enzymatic water softener removed 70–80% of total hardness from three actual hard water samples. These results demonstrated a proof-of-concept that enzyme
catalyzed electricity generation can be used to soften hard water.
© 2012 Elsevier Inc. All rights reserved.
1. Introduction
The primary sources of water supply in the U.S. are surface
waters (e.g., rivers, lakes, streams) and groundwater (deep wells).
Groundwater often has high levels of hardness, which is caused
by a variety of dissolved multivalent metallic ions, predominantly
calcium and magnesium [1,2]. Other cations like aluminum, barium, iron, manganese, strontium, and zinc contribute less to water
hardness [3]. More than 85% of the U.S. population experience the
detrimental effects of hard water both in domestic and industrial
usage – hard water often produces a noticeable deposit of precipitate – scaling, especially in hot water pipes, heaters, boilers,
kitchens, bathtubs, and other units [4,5]. Therefore, prior to residential distribution and consumption, hardness concentration should
be reduced and hard water should be softened.
Water softening processes comprise approximately 20–30% of
the industrial and residential water treatment market in the U.S.,
with a specific market share of 2.5 billion dollars (in 2010) for residential applications and 7.3 billion dollars (in 2010) for industrial
applications [6]. Two major methods are typically used to remove
hardness: lime soda softening and ion exchange softening. Lime
∗ Corresponding author. Tel.: +1 414 229 5846; fax: +1 414 229 6958.
E-mail address: [email protected] (Z. He).
0141-0229/$ – see front matter © 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.enzmictec.2012.08.009
soda softening is used mostly for municipal purposes; it employs
chemical precipitation, in which lime is added to hard water to precipitate calcium ions as calcium carbonate and magnesium ions as
magnesium hydroxide [7]. The primary drawbacks of the lime soda
method include the production of a large volume of sludge that
requires post-treatment; excessive use of chemicals such as lime,
lime soda ash, and caustic soda; and the addition of acids for pH
adjustment, which increases operating expenses [8].
The ion exchange process is primarily employed for residential water softening. The softening system consists of salt-saturated
(e.g., sodium chloride) resin beads and a brine tank to regenerate
the resin bed; in this process, each divalent hardness ion (Ca2+ or
Mg2+ ) in the water is replaced by two sodium ions, thereby softening the water [9]. Experimental studies have found the sodium
level in softened water was 2.5 times (mean sodium concentration
296 mg/L) higher than municipal water [10]. Sodium is an essential mineral, however, excess consumption may be harmful [11,12]
and contribute to major health issues such as stroke, hypertension, and high blood pressure [13,14], especially for individuals
on a sodium-restricted diet [11]. Other residential water softening
methods include distillation, nanofiltration, electrodialysis, carbon
nanotubes, capacitive deionization, and reverse osmosis, which
consume a large amount of energy, and operation and maintenance
of these systems can be expensive [15–20]. To avoid introducing
additional salts such as sodium into drinking water during the
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396–401
397
Fig. 1. Schematic of an enzymatic water softener. AEM: anion exchange membrane, CEM: cation exchange membrane.
softening process, saltless water softening technologies must be
developed as an alternative to the ion exchange process.
The recently developed microbial desalination cell (MDC)
desalinates saline water via electricity produced by microorganisms and can potentially act as a low-energy and saltless water
softening technique [21–24]. MDC technology has been previously
employed to soften hard water [25]; however, the presence of
microorganisms in an MDC is inappropriate for residential applications. Here, an alternative method is proposed for hardness removal
through electricity generation using immobilized enzymes on the
anode. Well-known since the 1960s, enzyme-based biofuel cells
have been developed to use enzymes as biocatalysts instead of
precious metals. In an enzymatic biofuel cell, simple sugars or alcohol are oxidized by the enzymes at the surface of the electrodes to
produce electricity [26,27]. Enzymes offer the advantage of being
very selective in their chemical functions and this allows for the
use of metabolic pathways or enzyme cascades to deeply oxidize
substrate/fuel [28–31], therefore, they can produce higher catalytic
efficiencies than complex microbial communities.
In this study, a proof-of-concept of an enzymatic water softening process was developed and demonstrated. Enzymes and sugars,
which locate in the anode compartment and will not get into
drinking water, are safer and expected to be more appropriate for
drinking water applications than microbial systems. Experiments
were conducted to (1) verify the effect of electricity generation by
comparing the open and closed circuit; (2) examine the stability of
the enzymatic water softener; (3) investigate the effects of hardness
concentration in the presence of single and multiple cations; and
(4) study softening performance using actual hard waters obtained
from different locations.
2. Materials and methods
2.1. Chemicals and reagents
Purchased from Sigma–Aldrich, Acros Organics, and Fluka, and used as supplied without any pretreatment or further purification, were glucose dehydrogenase
(GDH) from Pseudomonas sps (GDH, E.C. 1.1.1.47), d-(+)-glucose, ␤-nicotinamide
adenine dinucleotide hydrate (NAD+ ) from yeast, methylene green zinc chloride double salt, sodium nitrate, sodium tetraborate, Nafion per fluorinated ion
exchange resin (5 wt%), tetra butyl ammonium bromide 99+%(TBAB), calcium chloride dehydrate, magnesium chloride anhydrous, 10% Pt Black and other standard
organic/inorganic chemicals. All solutions were prepared using Milli-Q® -grade
water. 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) onto
the electrode, cyclic voltammetry (CV) was performed using an Ag/AgCl reference
electrode and platinum wire as a counter electrode on a Gamry Instruments 600
Potentiostat 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 M
sodium nitrate in 10 mM sodium tetraborate. The electrode was gently rinsed and
dried overnight under ambient air flow before further modification.
Hydrophobically modified Nafion was prepared for immobilizing enzymes onto
the anode electrode, accordingly to previous literature procedures [32,33], which
have shown the ability to uniformly encapsulate NAD+ -dependent dehydrogenase 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 concentration
of TBAB salt three folds to the concentration of sulfonic acid sites in the 5 wt% Nafion
suspension. One mL of the mixed cast solution was transferred to a weigh boat and
left to dry overnight. The membrane (polymer) casting solution was soaked and
washed with 7 mL of Milli-Q® -grade water to remove hydrogen bromide and excess
quaternary ammonium bromide. The membrane solution was centrifuged to remove
the supernatant and then resuspended in 1 mL alcohol to create an alkyl ammoniummodified Nafion solution. Furthermore, the enzyme/Nafion casting solutions were
prepared by mixing an enzyme-to-polymer ratio of 2:1. One mg/mL enzyme solution was prepared by dissolving 1 mg of GDH in 1 mL of 0.1 M phosphate buffer at
pH 7.13. In a microcentrifuge tube, 600 ␮L of the 1 mg/mL enzyme solution, 300 ␮L
of the alkyl ammonium-modified Nafion solution, and 0.015 g of NAD+ were mixed
and vortexed for 30 s. Then, 100 ␮L of this casting solution was pipetted onto the
electropolymerized anode electrode and allowed to dry completely for 24 h. As a
result, 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 membrane (AMI-7001, Membrane International, Inc., Glen Rock, NJ, USA) separated the
anode and the middle chamber, whereas a cation exchange membrane (CMI-7000
Membrane International, Inc.) separated the cathode and the middle chamber. The
chambers were fixed together using gaskets and clamps to prevent water leakage.
In the anode chamber, an anode electrode with immobilized enzymes was connected to an external circuit through titanium wire. In the cathode chamber, carbon
cloth with Pt catalyst (0.2 mg/cm2 ) prepared using 10% Pt according to a previous
398
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396–401
study [34] was fixed to titanium wire and inserted as a cathode electrode. The liquid volumes of the anode, middle, and cathode chambers were about 25, 20, and
25 mL, respectively. When operating the enzymatic softener, cations such as calcium and magnesium ions transfer to the cathode chamber via cation exchange
membrane, and anions like chloride ions transfer to the anode chamber through the
anion exchange membrane.
2.4. Operating conditions
The anode chamber was filled with freshly prepared 50 mM glucose and 1 mM
NAD+ (co-factor) in 20 mM phosphate buffer (pH 7.13), and the cathode chamber was filled with 2 mM phosphate buffer (pH 7.13). The cathode chamber was
sparged with air to supply oxygen to the cathode reaction. Two sources of hard
water (synthetic water with single/multiple ions and actual hard water) were used
for hardness removal experiments. Three different concentrations of synthetic hard
water (800, 400, and 200 mg/L as CaCO3 ) were prepared by dissolving respective
amounts of calcium chloride (CaCl2 ) in a liter of deionized water. Synthetic hard
water with multiple cations (Ca2+ and Mg2+ ) was prepared by dissolving calcium
chloride and magnesium chloride (MgCl2 ) in a 4:1 ratio to produce a final concentration of 200 mg/L as CaCO3 . To compare the synthetic and actual hard water at a
similar hardness concentration, actual hard water of 350 mg/L as CaCO3 collected
from Burnsville (MN, USA) was diluted to a concentration of 200 mg/L as CaCO3 . The
actual hard water samples were also collected from three different locations in the
U.S. and tested separately. During the experiments, the hard water was fed into the
middle chamber in batch-mode operation for 6 h (each batch). Before running the
water softening process, the enzymatic water softener operated at an open-circuit
potential for 6 h for three cycles. Hardness removal experiments were conducted
at an external resistance of 100 . The liquids in all three chambers were replaced
after each cycle.
2.5. Measurement and analysis
The voltage data was collected every 3 min by a digital multimeter (2700, Keithley Instruments, Inc., Cleveland, OH, USA). The polarization curve was performed by
a potentiostat (Reference 600, Gamry Instruments, Warminster, PA) at a scan rate
of 0.5 mV/s. The pH was measured with a bench top pH meter (Oakton Instruments,
Vernon Hills, IL, USA). The total hardness was measured with a digital titrator (Hach
Company, Loveland, OH, USA) model 16900 using a ManVer® Hardness Indicator
Powder Pillow and titrated with ethylenediamine-tetraacetic acid (EDTA). The concentrations of calcium and magnesium ions in actual hard waters were measured
using ion chromatography (IC-1100, Dionex). Columbic recovery (CR) was calculated
as:
It
Qoutput
=
CR =
F ×E
Qinput
(1)
where Qoutput is the produced charge, Qinput is the total charge available in the
added organic compounds, I is electric current (A), t is time (s), F is the Faradic
constant (96,485 C/mol), and E is the total mole of electrons (mol) that can be theoretically produced from the input glucose (2 mol of electrons per mole of glucose
oxidation to gluconic acid). Charge transfer efficiency was calculated as the ratio
Qo /Qr , where Qo is the output of electric charge from the electrical circuit and Qr
stands for the charge from the removed salts (1 mol of CaCl2 removal requires 2 mol
of electrons).
3. Results
Fig. 2. Polarization curves of the enzymatic water softener with (black) and without
(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.
To examine hardness removal over time, we fed hard water of
800 mg/L as CaCO3 in batch mode (on 6-h operation cycle) and monitored the system performance for more than 20 days. The current
3.1. Hardness removal stimulated by current generation
Electricity generation in the enzymatic water softener was
characterized by polarization curves. As shown in Fig. 2, in the presence of an immobilized enzyme, the water softener produced an
open-circuit potential of 0.72 V, the maximum current density of
0.1 mA/cm2 and the maximum power output of 9 ␮W/cm2 . When
there was no enzyme on the anode electrode, the maximum current
density was only 0.008 mA/cm2 .
To demonstrate the effect of electricity generation on hardness removal, we compared hardness removal under the open and
closed circuit. When the system operated under an open-circuit
condition, there was no current generation and the concentration
of hardness decreased by about 2%, from 800 to 760 ± 23 mg/L
as CaCO3 in 6 h. Under a closed-circuit condition, electricity generation reached an average current of 0.08 mA/cm2 . As a result,
the hardness concentration decreased to 433 ± 50 mg/L as CaCO3
within 6 h (Fig. 3), representing 46% removal. These results clearly
show that current generation stimulated hardness removal.
Fig. 3. Comparison of the hardness removal between the open and closed circuits
with an initial hardness concentration of 800 mg/L as CaCO3 .
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396–401
399
Fig. 5. Effect of initial hardness concentrations on hardness removal (red) and total
electric charge production (green) in synthetic hard water. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
the 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 CaCO3
spiked with both CaCl2 and MgCl2 at a ratio of 4:1 [35]. The total
charge 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 softener: 74.0 ± 3.0% of the hardness was removed and 5.82 ± 0.12 C of
electric charge was produced.
Fig. 4. The stability test conducted for 17 batch cycles: (A) current generation and
(B) corresponding hardness removal.
generation exhibited a typical batch profile with a peak current followed by decrease. The peak current was stable at ∼0.1 mA/cm2
for eight cycles (eight days), and then started to decrease to
0.039 mA/cm2 in the following cycles (Fig. 4A). Accordingly, the
hardness removal decreased from ∼50% to 35% and eventually
to about 10% (Fig. 4B). This performance shows that there is an
instability in either the enzyme, cofactor regeneration, cofactor
preconcentration, or poly(methylene green) electrocatalyst layer.
3.5. Hardness removal from actual hard water
Operation of the enzymatic water softener to remove hardness
ions in actual hard water was examined in a batch-fed operation.
The samples of actual hard water were collected from three different locations in the U.S., with hardness concentrations varying
from 282 to 664 mg/L as CaCO3 (Table 1). The initial pHs of those
waters 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
3.3. Effect of hardness concentrations
To study the effect of hardness concentrations and subsequent
hardness removal, a series of experiments were conducted by
6-h batch cycle for up to seven days using three different concentrations of synthetic hard water samples with initial hardness
concentrations of 800, 400, 200 mg/L as CaCO3 . More hardness
was removed at a lower initial hardness concentration: the water
softener removed 69.6 ± 10.6% and 76.4 ± 4.6% of hardness at 400
and 200 mg/L as CaCO3 , respectively, both of which were higher
than 46.2 ± 4.5% at 800 mg/L as CaCO3 (Fig. 5). Total electric charge
was calculated for the first five cycles by integrating current with
time. The highest charge of 7.68 ± 0.11 C was produced with a
columbic recovery (CR) of 3.2% with 400 mg/L as CaCO3 , followed by
7.62 ± 0.06 C with a CR of 3.2% and 5.93 ± 0.11 C with a CR of 2.4% at
800 and 200 mg/L as CaCO3 , respectively. The charge transfer efficiency increased with decreasing initial hardness concentration –
nearly 100% of the charge transfer efficiency was achieved with
200 mg/L; while it was 74% and 58% at 400 mg/L and 800 mg/L of
initial hardness concentrations, respectively.
Fig. 6. Total electric charge (green) and hardness removal (red) with an initial concentration 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 actual
hard water. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
400
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396–401
Table 1
Characteristics 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 water
softener.
Location
Sampling site
Initial pH
Final pH
CH a
Ca:Mgb
Hardness removal
Total charge (C)
Mt Joy, PA
Groundwater, sampled from
kitchen faucets
Mixed ground and surface
water, sampled from kitchen
faucets
Groundwater, sampled from a
commercial building
8.14
7.65
282
1:1
74.0%
4.45 ± 0.06
8.75
6.91
350
8:1
86.0 ± 1.7%
5.01 ± 0.13
8.25
6.82
664
7.5:1
82.3 ± 3.7%
6.36 ± 0.16
Burnsville, MN
Roswell, NM
a
b
Hardness concentration (mg/L as CaCO3 ).
The ratio of element mass.
(Table 1). The softened water exhibited a lower pH varying between
6.82 and 7.65.
4. Discussion
The results demonstrated a proof-of-concept that bioelectrochemical energy produced by an enzymatic bioanode employing
GDH immobilized in a TBAB modified Nafion membrane can
remove hardness from water. As the driving force, electrons are
generated from glucose oxidation to gluconic acid, in which NADH
acts as electron shuttles through redox reactions. To decrease the
over-potential of NADH oxidation, poly(methylene green) was used
as an electrocatalyst for NADH [36]. The comparison between the
open and closed circuits provides solid evidence that electricity
generation plays an important role in removing hardness. The slight
decrease in hardness under the open circuit was likely due to ion
exchange or ion diffusion [37]. Hardness removal can be improved
by increasing current generation, which will be the focus of our
future study.
The water softener maintained a stable performance for eight
cycles within eight days. The polarization tests demonstrated that
the anodic potential increased by 0.4 V (from −0.2 to 0.2 V), while
the cathodic potential decreased by 0.1 V (from 0.3 to 0.2 V), suggesting that the bioanode was a major limiting factor to the overall
performance. That is supported by our previous study that the
Pt/C cathode could achieve stable performance for more than
three months [34]. The decrease in enzyme catalytic activity might
have led to the decreased performance of the system after eight
days compared with 30 days of typical two-chambered enzymatic fuel cells [38]; but frequently the decrease in performance
of NAD-dependent dehydrogenase-based bioanodes is the instability of the cofactor during cycling or the long term stability of
the NADH electrocatalyst. Better system performance could be
achieved by improving the stability of the enzyme, the cofactor, and
the electrocatalyst. Our next investigation will focus on improving the enzyme’s stability by adopting different immobilization
and stabilization strategies, such as using covalent and crosslinking attachment to single-walled carbon nanotubes, porous chitosan
membranes, or other entrapment techniques and using enzymes
that are NAD+ independent, like glucose oxidase, to avoid co-factor
and electrocatalyst instability [39–41]. The use of nanomaterials in
the present water softener will need to consider the (potentially)
negative effects of those materials on human and environmental
health during the disposal of an enzymatic electrode.
The enzymatic water softener performed better at a lower initial hardness concentration, at which the charge transfer efficiency
rendered a higher removal rate compared with a higher initial hardness concentration. The charge transfer efficiency of nearly 100% at
200 mg/L suggests that almost all electrons were used to drive hardness removal (transporting Ca2+ ), or almost all Ca2+ removed was
due to electricity generation. The decreased charge transfer efficiency at higher hardness concentrations indicates the hardness
was partially removed by processes other than current generation.
Enhanced ion exchange or diffusion at higher hardness concentrations could contribute to hardness removal; however, we did not
observe a strong effect of those processes under the open-circuit
condition; the exact reasons require further investigation. Unlike
our previous study of microbial-based water softening process [25],
the hardness concentration did not significantly affect electricity
generation in this study and three concentrations yielded similar
total coulombs. According to our experience, a Pt cathode with sufficient oxygen supply could meet the demand of electron transfer
in this type of reactor; therefore, we believe the anode limits the
performance of the present enzymatic water softener. Based on the
glucose concentration, it was estimated that the coulombic recovery was less than 4%; thus, the organic supply was sufficient and
the conversion of organic to electrons restricted the overall performance. The conversion is determined by enzyme, co-factors, and
electrocatalysts.
At the initial concentration of 200 mg/L as CaCO3 , a comparison
of hardness removal between the synthetic water containing single
and dual cations revealed that the presence of Mg2+ decreased the
removal of total hardness, presumably due to the faster migration
of calcium ions, which have a small hydrated ionic radius of 6 A
than magnesium with a larger hydrated ionic radius of 8 A. Typically, ion selectivity of a cation is determined by its higher valence;
for cations of similar valence, a smaller cation has larger hydrated
radius than a larger cation [42,43]. The diluted actual hard water
at 200 mg/L as CaCO3 removed hardness more efficiently than the
dual-cation water, but lower than the single-cation water, likely
due to a higher ratio of Ca2+ to Mg2+ . The IC analysis of actual hard
water 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 of
total hardness.
Future development of enzymatic softener will focus on optimizing the configuration, and improving enzymatic catalytic
activities through an enzyme cascade. The configuration of the
reactor, especially the dimensions of the middle chamber, also
affects hardness removal. The width of the middle chamber, or
the distance between the two ion exchange membranes, can affect
the internal resistance (e.g., electrolyte resistance) of the reactor
and thus the current generation that directly relates to hardness
removal. The enzymatic softener removes hardness in a process
similar to an electrodialysis (ED). The distance between membrane
pairs in an ED is usually smaller than 1 mm for minimizing internal resistance; in the present enzymatic softener, this distance was
9 mm, significantly larger than an ED. Therefore, a smaller gap
between membrane pair must be considered in the next design.
GDH oxidation of glucose to gluconic acid is an incomplete oxidation reaction that only liberates 2 electrons. If an enzyme cascade is
present then glucose may be oxidized completely to CO2 , thereby
releasing 24 electrons/mol for use in the system. That will improve
fuel efficiency and thus reduce fuel cost: the cost of glucose or
M.A. Arugula et al. / Enzyme and Microbial Technology 51 (2012) 396–401
sugar is clearly higher than sodium salts used in ion exchange processes. Another approach to reduce fuel cost is to use less purified
substrate, which may have higher requirement for enzyme’s selectivity. Other factors are critical for hardness removal, such as effect
of pH, buffering capacity, chloride ions, time period of each cycle,
concentration of substrate in the anode chamber and temperature.
5. Conclusions
This study demonstrated the feasibility of an enzyme-based
water softening system as a potential alternative for ion exchange
systems for residential applications. The enzymatic softener effectively removed hardness at different initial concentrations in both
synthetic and actual hard waters without input of electrical energy.
Current generation through enzymatic oxidation of glucose played
a major role in hardness removal. The removal efficiency was
affected by the composition of hard waters; magnesium especially
lowered the removal of total hardness. The performance of an enzymatic water softener needs to be improved by increasing current
generation (e.g., lowering external resistance for high current generation and better enzyme activities) and bioanode stability (e.g., a
better preparation procedure).
Acknowledgements
This project was financially supported by a Bradley Catalyst
Grant from the UW-Milwaukee Research Foundation and private
funds from Mr. Mark Murphy. Kristen Brastad was supported by
a grant from A. O. Smith Corporation. We thank Ms. Michelle
Schoenecker (UW-Milwaukee) for her assistance with manuscript
proofreading, and the anonymous reviewers for their helpful comments.
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