Phosphate Recovery from Human Waste via the Formation of
Hydroxyapatite during Electrochemical Wastewater Treatment
Cle
́
ment A. Cid,
†
Justin T. Jasper,
†
and Michael R. Ho
ff
mann
*
Linde-Robinson Laboratories, California Institute of Technology, 1200 E California Blvd, Pasadena, California 91125, United States
*
S
Supporting Information
ABSTRACT:
Electrolysis of toilet wastewater with TiO
2
-coated semi-
conductor anodes and stainless steel cathodes is a potentially viable onsite
sanitation solution in parts of the world without infrastructure for
centralized wastewater treatment. In addition to treating toilet wastewater,
pilot-scale and bench-scale experiments demonstrated that electrolysis can
remove phosphate by cathodic precipitation as hydroxyapatite at no
additional energy cost. Phosphate removal could be predicted based on
initial phosphate and calcium concentrations, and up to 80% total
phosphate removal was achieved. While calcium was critical for phosphate
removal, magnesium and bicarbo
nate had only minor impacts on
phosphate removal rates at concentrations typical of toilet wastewater.
Optimal conditions for phosphate removal were 3 to 4 h treatment at about 5 mA cm
−
2
(
∼
3.4 V), with greater than 20 m
2
m
−
3
electrode surface area to reactor volume ratios. Pilot-scale systems are currently operated under similar conditions, suggesting
that phosphate removal can be viewed as an ancillary bene
fi
t of electrochemical wastewater treatment, adding utility to the
process without requiring additional energy inputs. Further value may be provided by designing reactors to recover precipitated
hydroxyapatite for use as a low solubility phosphorus-rich fertilizer.
KEYWORDS:
Electrochemical precipitation, Phosphorus, Phosphate removal, Wastewater, Onsite sanitation
■
INTRODUCTION
Discharge of phosphorus-containing wastewater to surface
waters can cause algal blooms, leading to growth of toxic
cyanobacteria, hypoxia, and disruption of food webs.
1
,
2
At the
same time, phosphorus is a limited resource with an average
price that has nearly tripled between 2005 and 2015,
3
making
the recovery of phosphorus from waste crucial.
4
Toilet and
domestic wastewater are an important source of phosphorus, as
up to 22% of the world
’
s consumption of phosphorus could be
recovered from human urine and feces.
5
,
6
Recovery of
phosphorus from toilet wastewater or septic systems could
therefore reduce phosphorus pollution as well as reduce
dependency on imported mineral phosphate in countries
where access to a
ff
ordable fertilizers is limited.
7
Enhanced biological phosphorus removal (EBPR) may
provide e
ff
ective phosphorus recovery in centralized wastewater
treatment processes,
8
but in rural communities, small onsite
sanitation systems (e.g., septic tanks, latrines, or cesspools)
make this technology challenging without engineered processes
to maintain the correct microbial population.
9
Phosphorus
recovery in rural communities can be accomplished via forced
precipitation as struvite (NH
4
MgPO
4
·
6H
2
O) or hydroxyapatite
(Ca
5
(PO
4
)
3
OH), but these strategies typically require separa-
tion of urine and feces, addition of chemicals, or use of
sacri
fi
cial electrodes that further complicates and increases the
cost of existing wastewater treatment strategies.
10
−
12
Electrochemical systems have previously been suggested for
phosphorus removal from wastewater. Electrochemical coagu-
lation of phosphate from synthetic wastewater has been
achieved using sacri
fi
cial aluminum or iron anodes,
13
,
14
as
well as magnesium anodes, which allowed for struvite recovery
from ammonium-containing solutions.
15
However, this type of
electrode is depleted by oxidation and needs to be replaced on
a regular basis. Alternatively, an alkaline catholyte chamber
separated from the anode by a cation exchange membrane has
been incorporated into an electrochemical system to
homogeneously precipitate phosphate as hydroxyapatite from
synthetic wastewater.
16
Electrochemical deposition of struvite
directly onto a nickel cathode has been demonstrated in
synthetic solutions containing magnesium, ammonium, and
phosphate, due to the increased pH near the cathode surface.
17
However, these systems provided phosphorus removal alone,
and none of these studies investigated authentic toilet
wastewater or utilized a system that was practical for toilet
wastewater treatment.
Onsite electrochemical wastewater treatment is an appealing
technology for small- and medium-sized treatment and
recycling systems, providing
treatmentwithoutrequiring
construction of traditional wastewater infrastructure.
18
One
promising electrochemical treatment system under develop-
ment by Ho
ff
mann et al.
19
couples stainless steel cathodes to
stable layered
−
layered semiconductor anodes
Received:
September 7, 2017
Revised:
February 1, 2018
Published:
February 5, 2018
Research Article
pubs.acs.org/journal/ascecg
Cite This:
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2018, 6, 3135
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3142
© 2018 American Chemical Society
3135
DOI:
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2018, 6, 3135
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This is an open access article published under a Creative Commons Attribution (CC-BY)
License, which permits unrestricted use, distribution and reproduction in any medium,
provided the author and source are cited.
([Bi
2
O
3
]
z
[TiO
2
]
1
−
z
/Ir
x
Ta
y
O
2
/Ti)
20
−
22
treating the toilet
wastewater in a sequential batch reactor at constant potential
(3.5
±
0.25 V) with a typical residence time of 3
−
4 h. Bench-
scale experiments and long-term
fi
eld-testing have shown
e
ff
ective wastewater disinfection due to generation of
hypochlorous acid from the oxidation of chloride, as well as
reduction of chemical oxygen demand, transformation of trace
organic chemicals, and removal of ammonium via breakpoint
chlorination.
20
,
22
−
25
The purpose of this study was to evaluate the potential for
phosphate removal from human wastewater during electro-
chemical treatment using the same combined anode
−
cathode
system previously shown to provide e
ffi
cient wastewater
treatment.
20
,
23
,
24
Phosphate-containing precipitates were iden-
ti
fi
ed and phosphate removal e
ffi
ciencies were measured in
authentic and synthetic toilet wastewater. Experiments in
synthetic wastewater allowed quanti
fi
cation of the e
ff
ects of
ion composition, bu
ff
ering capacity, current density, and
electrode surface area to volume ratio on phosphate removal
kinetics and equilibria.
■
MATERIALS AND METHODS
Materials.
All reagents were of analytical grade or higher purity.
Solutions were prepared using
≥
18 M
Ω
cm Milli-Q water from a
Millipore system.
Toilet (human) wastewater containing an uncontrolled mixture of
urine, feces, and
fl
ushing water was taken from a previously described
public recycling wastewater treatment system located on the California
Institute of Technology campus (Pasadena, CA) via a macerator
pump.
25
The residence time in the wastewater tank was approximately
160 d. Synthetic wastewater was formulated to replicate the ionic
composition and pH (8.3) of the toilet wastewater (
Table 1
)by
dissolving the following salts in water: NaCl (17.1 mM), NaHCO
3
(4.7 mM), NaH
2
PO
4
·
H
2
O (0.6 mM), Na
2
SO
4
(2.1 mM), MgCl
2
·
6H
2
O (0.8 mM), CaCl
2
·
2H
2
O (1 mM), KCl (3.6 mM), (NH
4
)
2
SO
4
(0.9 mM), NH
4
HCO
3
(12.1 mM), and KOH (2.5 mM). Ion
concentrations were adjusted to test the e
ff
ect of individual ions on
phosphate removal rates.
Electrode arrays consisted of mixed metal oxide anodes
(Bi
2
O
3
]
z
[TiO
2
]
1
−
z
/Ir
x
Ta
y
O
2
/Ti) and stainless steel cathodes (Nano-
pac, Korea) and were similar to those developed by Weres
26
,
27
and
used in previous electrochemical wastewater treatment stud-
ies.
20
,
23
−
25
,
28
Pilot-Scale Experiments.
Pilot-scale experiments were performed
in batch mode in a 40-L acrylic reactor (22 L working volume) mixed
with a circulation pump (10 L min
−
1
), as described previously.
24
,
29
Electrode arrays (7 anodes and 8 cathodes) were sandwiched with a 3
mm separation. The active geometric anodic surface area was 1.8 m
2
,
giving a surface area to e
ff
ective volume ratio of 80 m
2
m
−
3
. Pilot-scale
experiments were conducted using a potentiostatic power supply
coupled with a data logger (Program Scienti
fi
c Instruments, U.S.A.)
with a potential set between 3.3 and 3.5 V. Ion recoveries as precipitate
were calculated in selected experiments by calculating ion masses in
the formed precipitate (
Figure S1
) using the average precipitate
composition and comparing those masses to ion removal from the
aqueous phase.
Bench-Scale Experiments.
Bench-scale experiments were con-
ducted to study the role of ionic composition, bu
ff
ering capacity, and
current density on phosphate removal kinetics and equilibria using
anode and cathode pieces cut from a pilot-scale array. The electrode
spacing (3 mm) and electrode surface area to volume ratio (
∼
35 m
2
m
−
3
) were comparable to the pilot-scale system. The electrode array
was either operated potentiostatically (typically 3.5 V between anode
and cathode) or galvanostatically (
∼
10 mA cm
−
2
; 3.75 mA mL
−
1
)
using a battery cycler (Neware, China). Experiments were conducted
in open beakers with magnetic stirring (600 rpm).
The role of wastewater composition was studied by varying calcium,
magnesium, phosphate, and bicarbonate concentrations over the range
of values expected in toilet wastewater (i.e., typical values present in
human waste diluted approximately 10 times by
fl
ushing;
Table S1 and
Figure S2
). The role of bu
ff
ering capacity was studied by adding borate
(0
−
100 mM) to synthetic wastewater at pH 8.3. No ion interactions
with borate were predicted by Visual MINTEQ 3.1 software.
30
The
e
ff
ects of wastewater volume to electrode surface area ratios (
∼
10
−
35
m
2
m
−
3
) were studied by adjusting the solution volume while using the
same size electrodes. The e
ff
ects of current density were investigated
by increasing the current density galvanostatically (
∼
3
−
55 mA cm
−
2
;
1
−
20 mA mL
−
1
). Energy e
ffi
ciency of phosphate removal was
calculated based on the
fi
nal phosphate concentration and the total
amount of electrical energy consumed.
Precipitate Solubility Measurements.
Precipitate scraped from
the stainless-steel cathodes or collected from the pilot-scale reactor
bottom was rinsed with deionized water and dried at 70
°
C overnight
before being ground for analysis. The solubility product constant (
K
sp
)
of the collected precipitate was measured in dilute phosphoric acid
solutions (
∼
0.1 mM) adjusted to pH 6 with sodium hydroxide, as
described previously.
31
Precipitate (0.1 g) was added to vials (25 mL)
capped with minimal headspace. Vials were mixed on a rotisserie for 8
dat22
°
C and solid precipitate remained at the end of the experiment.
The
K
sp
for hydroxyapatite was calculated according to
=
+−−
K
(Ca ) (PO ) (OH )
sp
25
4
33
(1)
Solubility indices (SI), as de
fi
ned by
eq 2
, and ion activity products
(IAP) were calculated using Visual MINTEQ 3.1 software,
30
accounting for ion pairs (e.g., CaPO
4
−
). Equilibrium calculations and
supersaturated conditions for various minerals were determined using
the same software, taking into consideration ion concentrations listed
in
Table 1
.
=−
K
S
I log IAP log
s
p
(2)
Analytical Methods.
X-ray powder di
ff
raction spectra (Philips
PANalytical X
’
Pert Pro X-ray) were collected for crystal phase analysis.
Thermogravimetric analysis was conducted for moisture content
determination and qualitative mineral identi
fi
cation (PerkinElmer STA
6000). Scanning electron microscope imaging and energy dispersive
spectrometry (SEM/EDS; Zeiss 1550VP Field Emission with Oxford
X-Max SDD X-ray) were used for surface topography and elemental
analysis. A
“
site
”
represented an indistinguishable agglomerate of
amorphous or crystallized material.
The chloride, sulfate, nitrate, phosphate, ammonium, potassium,
calcium, and magnesium contents of collected precipitates were
determined by dissolution in 1 M sulfuric acid or 1 M nitric acid and
analysis by ion chromatography (Dionex ICS 2000; AS19G anions,
CS12A cations).
32
Precipitate carbonate content was determined by
Table 1. Composition of Toilet Wastewater in Onsite
Wastewater Treatment System and Bu
ff
ering Capacity of
Relevant Species
component
value
a
bu
ff
er capacity (
β
i
)
b
Ca
2+
1.0 mM
0
Cl
−
24 mM
0
HCO
3
−
+CO
3
2
−
17 mM
0.79 mequiv L
−
1
pH
−
1
K
+
6.1 mM
0
Mg
2+
0.8 mM
0
Na
+
27 mM
0
NH
4
+
13 mM
2.71 mequiv L
−
1
pH
−
1
PO
4
3
−
T
c
0.6 mM
0.09 mequiv L
−
1
pH
−
1
SO
4
2
−
3.0 mM
0
COD
d
320
−
380 mg O
2
/L
pH
8.3
a
Collected after 180 d of operation.
b
At pH 8.3.
c
Total phosphate.
d
Chemical oxygen demand.
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manometric carbon dioxide measurement following dissolution in acid
(6 M HCl).
33
Samples for aqueous ion concentrations were diluted (10
−
25
×
)
and measured by ion chromatography as described above.
■
RESULTS AND DISCUSSION
Phosphate Removal during Pilot-Scale Treatment.
Electrolysis of collected toilet wastewater in the pilot-scale
onsite treatment system resulted in removal of total phosphate,
magnesium, and calcium over the 5 h treatment cycle (
Figure 1
;
50% total PO
4
3
−
(PO
4
3
−
T
), 89% Mg
2+
, 42% Ca
2+
removed).
Total phosphate removal was similar to predictions based on
initial calcium and phosphate concentrations (see below).
Breakpoint chlorination was achieved in approximately 4 h with
complete ammonia removal
34
and subsequent production of
free chlorine (
Figure S3
). Concurrent with electrolysis, a
greyish precipitate
fl
aked o
ff
the stainless-steel cathodes into
solution (
Figure S1
). Precipitate recovered from the cathodes
and the bottom of the reactor after treatment accounted for
more than 90% of the calcium and total phosphate removed
based on the measured precipitate composition. Pilot-scale
phosphorus removal was therefore primarily attributed to
electrochemically induced precipitation.
Characterization of Precipitated Hydroxyapatite.
Precipitate collected from th
e pilot-scale electrochemical
reactor was primarily composed of hydroxyapatite
(Ca
5
(PO4)
3
OH), based on X-ray di
ff
raction spectroscopy
(
Figure 2
). The crystallinity of the precipitate was found to
be signi
fi
cantly higher than hydroxyapatite formed by
homogeneous precipitation in synthetic dairy manure waste-
water,
35
as evidenced by resolution of peaks at 2
θ
values of 28
°
,
29
°
,31
°
, and 32
°
.
In addition to phosphate (30
±
2% by mass) and calcium (18
±
1% by mass), the precipitate was composed of chemically
bound water (8
−
20% by thermogravimetry;
Figure S4
),
magnesium (6
±
1%), carbonate (6
±
1%), silicate (9
±
3%), and undissolvable material (3
−
6%;
Table 2
). Magnesium
and carbonate are commonly observed to substitute for calcium
and hydroxide, respectively, in hydroxyapatite
35
−
38
and may
a
ff
ect precipitation kinetics. Silicate is known to substitute for
phosphate in hydroxyapatite (Ca
10
(PO
4
)
6
−
x
(SiO
4
)
x
(OH)
2
−
x
)
39
and was only observed in reactors sealed with silicon grease.
Chloride, sulfate, nitrate, ammonium, potassium, and sodium
were not present in collected precipitates in signi
fi
cant amounts
(less than 1% by mass), as expected for hydroxyapatite.
SEM/EDS mapping of collected precipitate revealed a
homogeneous distribution of elements with phosphorus,
calcium, and magnesium in all deposits (
Figure S5
). Scanning
of several particles showed ratios of Ca/P = 1.5
±
0.3, Mg/P =
1.0
±
0.2, and O/P = 5.0
±
1.6. The low Ca/P ratios observed
as compared to pure hydroxyapatite (Ca/P = 1.67) could be
explained by substitution of magnesium for calcium and silicate
for phosphate ((Ca+Mg)/(Si+P) = 1.7
±
0.2).
The measured
K
sp
of the electrochemically deposited
hydroxyapatite (5.0
±
0.5
×
10
−
47
) was signi
fi
cantly larger
than literature values for pure hydroxyapatite (3.04
±
0.25
×
Figure 1.
Mg
2+
,Ca
2+
,PO
4
3
−
T
, and ammonia (NH
4
+
+NH
3
) percent
removal during electrochemical treatment (3.3 V; 50 A) of toilet
wastewater ([Cl
−
] = 80 mM) in pilot-scale reactor. Initial ion
concentrations are indicated in the legend.
Figure 2.
X-ray di
ff
raction spectrum of collected precipitate. Overlay
of pure hydroxyapatite with highest peak normalized to 600 au (ICSD
no. 24240 and PDF no. 01-073-1731) is in red sticks.
Table 2. Collected Precipitate Composition
component
% mass
detection method
Ca
2+
18
−
19%
IC; SEM-EDS
Mg
2+
5
−
7%
IC; SEM-EDS
PO
4
3
−
T
27
−
32%
IC; SEM-EDS
CO
3
2
−
6
±
1%
acid digestion
SiO
4
2
−
a
9
±
3%
SEM-EDS, assuming Si is SiO
4
H
2
O8
−
20%
vacuum oven; TGA
organics; undigested
material
3
−
6%
fi
lter acid-dissolved precipitate
solution
total
72
−
103%
a
SiO
4
2
−
was only detected in samples collected from a silicon-grease
sealed electrochemical reactor.
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3137
10
−
59
),
31
likely due to incorporation of magnesium, carbonate,
and silicate.
40
For example, incorporation of a similar mass
percentage of carbonate (i.e.,
∼
4% by mass) into hydrox-
yapatite can increase hydroxyapatite
’
s
K
sp
by more than 8
orders of magnitude.
37
Electrochemically deposited hydrox-
yapatite solubility may also have been lower than that of pure
hydroxyapatite, since it was not completely crystalline (
Figure
2
).
Although no precipitation was observed before electrolysis,
41
the collected toilet wastewater (
Table 1
) was supersaturated
with respect to aragonite and calcite (SI
≈
0.9 and 1.1),
disordered and ordered dolomite (SI
≈
1.7 and 2.2),
42
α
and
β
tricalcium phosphate (SI
≈
2.2 and 2.9), and tetracalcium
phosphate (SI
≈
2.3). Toilet wastewater was also super-
saturated with respect to pure hydroxyapatite (SI
≈
12), which
was the thermodynamically favored mineral phase. However,
toilet wastewater was slightly below saturation with respect to
the measured solubility of the
electrochemically formed
precipitate (SI =
−
0.2).
Phosphate Removal Equilibria and Kinetics.
Phos-
phate Removal in Synthetic versus Authentic Wastewater.
Synthetic wastewater was used to determine the e
ff
ect of
wastewater composition ([Ca
2+
], [Mg
2+
], [HCO
3
−
], and
[PO
4
3
−
]
T
), bu
ff
ering capacity, and current density on
phosphate removal (
Figure S6
). Despite the lack of organic
matter in synthetic wastewaters, which may reduce hydrox-
yapatite formation rates,
43
calcium, magnesium, and total
phosphate removal was found to be comparable to that
observed with authentic toilet wastewater (
Figure 3
). The XRD
spectrum of a stainless steel cathode after consecutive synthetic
wastewater electrolysis cycles also exhibited similar peaks as the
precipitate formed in the pilot-scale reactor (
Figures S7
and
2
).
The majority of the phosphate removed was recovered as a
precipitate on the cathode (70
−
100%), indicating that
phosphate removal was primarily due to hydroxyapatite
formation. Synthetic wastewater was therefore taken to be a
good proxy for genuine toilet wastewater for these experiments.
Extent of Phosphate Removal.
In synthetic wastewater,
percent phosphate removal at equilibrium (
∼
3
−
4 h) could
typically be predicted (
Figure 4
) based on initial calcium and
phosphate concentrations by solving the simultaneous equa-
tions for the hydroxyapatite solubility product (
eq 1
) and the
mass balance for calcium and phosphate removal (
eq 3
)ata
cathodic pH of about 9.4 (
Table S1
). The cathodic pH was
estimated assuming that the solution at the cathode surface was
equilibrated (SI = 1) with re
spect to electrochemically
precipitated hydroxyapatite (
K
sp
=5
×
10
−
47
) and that
[Ca
2+
] and [PO
4
3
−
] at the cathode were the same as measured
in solution at the end of the experiment (when ion
concentrations had stabilized).
−=−
−−
++
5
3
([PO ]
[PO ] ) ([Ca ] [Ca ] )
4
3
T,0
4
3
T,fin
2
0
2
fin
(3)
However, for low ratios of calcium to total phosphate,
phosphate removal was greater than predicted. This may have
been due to precipitation of calcium-phosphate minerals poor
in calcium, such as amorphous calcium phosphate (Ca
3
(PO
4
)
2
·
n
H
2
O;
K
sp
= 2.49
×
10
−
7
), dicalcium phosphate dihydrate
(CaHPO
4
·
2H
2
O;
K
sp
= 1.26
×
10
−
7
), and others.
43
Other
deviations between predicted and observed percent phosphate
removal could be explained by the presence of magnesium or
variations in the applied current density (see below).
Based on
eqs 1
and
3
, high phosphate removal is predicted at
high initial calcium concentrations and high initial ratios of
calcium to phosphate concentrations (
Figure 5
). Reliance on
high calcium concentrations for e
ffi
cient phosphate removal is a
limitation of this technology. However, urine in toilet
wastewater typically contains su
ffi
cient calcium to achieve
Figure 3.
Percent PO
4
3
−
T
,Ca
2+
,andMg
2+
remaining during
potentiostatic electrochemical treatment (3.6 V;
∼
18 mA cm
−
2
)of
genuine toilet wastewater (
fi
lled markers) and synthetic wastewater
(empty markers) with similar ionic compositions. [PO
4
3
−
]
T,0
≈
0.5
mM; [Ca
2+
]
0
≈
1.3 mM; [Mg
2+
]
0
≈
1.3 mM. Error bars represent
±
one standard deviation of three replicates.
Figure 4.
Measured vs predicted percent total phosphate removal
following galvanostatic electrolysis (4 h; 10 mA cm
−
2
). Error bars
represent
±
standard deviation of three replicates. Experiments are
referenced by letter and are described in
Table S1
.
ACS Sustainable Chemistry & Engineering
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2018, 6, 3135
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greater than 50% phosphate removal (i.e.,
∼
1 mM following
∼
10
×
dilution by
fl
ushing).
44
Electrochemical Phosphate Precipitation Rates.
In syn-
thetic wastewater initial electrochemical phosphate precipita-
tion rates (
k
ini
) were determined based on calcium and
phosphate concentrations during the
fi
rst 3 h of treatment
(
Figure S6
). Initial phosphate precipitation rates increased from
about 0.05 to 0.25 mM h
−
1
with the product [Ca
2+
][PO
4
3
−
]
(
Figure 6
), as expected based on a homogeneous hydrox-
yapatite precipitation model (
eq 4
) previously developed by
Inskeep and Silvertooth.
45
In their study, Inskeep and
Silvertooth found that the precipitation rate of hydroxyapatite
had a
fi
rst-order dependence on calcium and phosphate and did
not directly depend on [OH
−
] (the in
fl
uence of pH was
accounted for by PO
4
3
−
).
=≈
+−
t
kk
d[HAP]
d
[Ca ][PO ]
f
2
4
3
ini
(4)
Above [Ca
2+
][PO
4
3
−
] values of 0.4 mM
2
, however, initial
phosphate removal rates were constant at about 0.25 mM h
−
1
,
suggesting that initial precipitation was mass limited only at low
calcium and phosphate concentrations. In all cases, though,
removal rates were su
ffi
cient to reach equilibrium within 3 to 4
h, which is a typical treatment cycle for disinfection and
ammonium removal during onsite electrochemical wastewater
treatment in the system developed by Ho
ff
mann et al.
21
,
28
E
ff
ect of Magnesium on Phosphate Removal.
Adsorption
of magnesium onto actively growing crystals during homoge-
neous hydroxyapatite precipitation and subsequent substitution
of magnesium for calcium has been shown to reduce
hydroxyapatite growth rates and increase hydroxyapatite
solubility.
36
,
46
,
47
However, e
ff
ects were generally signi
fi
cant
only at concentrations above 1 mM,
48
which is the maximum
magnesium concentration expected in toilet wastewater
assuming about 10 times dilution by
fl
ushing water.
44
As expected, electrochemical treatment of synthetic waste-
water with 1 mM calcium, 0.6 mM phosphate, and varying
magnesium concentrations up to 1 mM showed no signi
fi
cant
change in initial phosphate removal rates (
Figure 7
a) or percent
phosphate removal (
Figure 8
a). In fact, at calcium concen-
trations below 1 mM with 0.5 mM total phosphate, phosphate
removal percentages were higher than predicted based on
calcium concentrations alone in the presence of 1 mM
magnesium (
Figure 8
b, compare experiments B, C, and D
Figure 5.
Predicted percent total phosphate removal. Predictions are
based on solving the simultaneous
eqs 1
and
3
at varying initial total
phosphate and calcium concentrations and a cathodic pH of 9.4.
Figure 6.
Initial rate constants (
k
ini
) for the formation of
hydroxyapatite during galvanostatic electrolysis (10 mA cm
−
2
)asa
function of [Ca
2+
]
0
[PO
4
3
−
]
0
.The
fi
tequationwasdetermined
empirically using Igor Pro 6.37 (Wavemetrics). Error bars represent
±
standard deviation of three replicates. Experiments are referenced by
letter and are described in
Table S1
.
Figure 7.
Initial phosphate removal rate following galvanostatic
electrolysis (4 h; 10 mA cm
−
2
unless noted otherwise) as a function of
(a) [Mg
2+
]
0
; (b) [HCO
3
−
]
0
; (c) electrolysis current density,
j
; and (d)
electrode surface area to volume ratio. Error bars represent
±
standard
deviation of three replicates. Experiments are referenced by letter and
are described in
Table S1
. (b) bu
ff
ering capacity
β
(meq L
−
1
pH
−
1
)is
noted in brackets.
ACS Sustainable Chemistry & Engineering
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ACS Sustainable Chem. Eng.
2018, 6, 3135
−
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3139
with 1 mM Mg
2+
to F, K, and G with 0 mM Mg
2+
). This may
have been due to magnesium compensating for the de
fi
ciency
in calcium. Magnesium is therefore not expected to hamper
electrochemical phosphate removal at concentrations typical of
toilet wastewater.
E
ff
ect of Bicarbonate on Phosphate Removal.
Toilet
wastewater stored in onsite treatment systems will produce
bicarbonate due to hydrolysis of urea.
49
Previous studies have
reported reductions in homogeneous hydroxyapatite precip-
itation by more than 40% with addition of carbonate, due to
increased solubility of carbonate-substituted hydroxyapa-
tite.
35
,
43
,
50
Bicarbonate may also reduce hydroxyapatite
precipitation by increasing the bu
ff
ering capacity (
β
)of
wastewater, inhibiting the increased cathodic pH that initiates
precipitation.
51
As expected, phosphate removal was signi
fi
cantly reduced at
high bicarbonate concentrations (i.e., 57
±
3% removal at 60
mM HCO
3
−
vs
∼
70
−
75% removal at 16 to 30 mM HCO
3
−
;
Figure 8
c). Phosphate removal rates were also slightly reduced
at 60 mM bicarbonate (
Figure 7
b), although the di
ff
erence was
not signi
fi
cant (i.e., 0.13
±
0.04 mM h
−
1
at 60 mM HCO
3
−
vs
0.17
−
0.23 mM h
−
1
at 16
−
30 mM HCO
3
−
).
The e
ff
ect of bicarbonate on phosphate removal was
attributed to the increased solubility of carbonate-substituted
hydroxyapatite, as bicarbonate is not predicted to increase
bu
ff
ering capacities su
ffi
ciently to a
ff
ect phosphate removal at
concentrations typical of toilet wastewater (i.e., <100 mM).
This assertion was supported by experiments in bu
ff
ered
synthetic wastewater with bu
ff
ering capacities ranging from 3.6
to 25 mequiv L
−
1
pH
−
1
(0
−
100 mM borate) at pH 8.3.
Bu
ff
ering capacity (
β
) was calculated by
eq 5
:
∑
β
=
+
+
+
CK
K
2.3
[H ]
([H])
ii
i
a,
a,i
2
(5)
where
C
i
and
K
a,
i
are the concentration and acid dissociation
constant of species
i
, respectively. Phosphate removal rates
were only a
ff
ected at bu
ff
ering capacities of 14.2 mequiv L
−
1
pH
−
1
and above (50
−
100 mM borate;
Figure S8
). This was
considerably higher than the bu
ff
ering capacity of toilet
wastewater at elevated bicarbonate concentrations (i.e., 5.6
mequiv L
−
1
pH
−
1
at 60 mM HCO
3
−
; 7.4 mequiv L
−
1
pH
−
1
at
100 mM HCO
3
−
).
E
ff
ect of Current Density and Treatment Volume on
Phosphate Removal.
Increasing current density increases the
rate of proton consumption at the cathode and, depending on
the bu
ff
ering capacity of the wastewater, can therefore increase
the pH near the cathode,
52
favoring hydroxyapatite precip-
itation.
As expected, initial phosphate removal rates (
Figure 7
c) and
total phosphate removal (
Figure 8
d) increased from about 50%
with an initial rate of about 0.1 mM h
−
1
at 2.6 mA cm
−
2
to
greater than 75% with an initial rate of about 0.25 mM h
−
1
at 15
mA cm
−
2
. Increases in surface area to synthetic wastewater
volume ratio augmented the rate of phosphate removal (
Figure
7
d) but did not change signi
fi
cantly a
ff
ect the amount of energy
required per volume of wastewater (
Figure S9
, inset). For
example, achieving 60% total phosphate removal required 30
±
5 kWh m
−
3
at all surface area to volume ratios tested, but
occurred after about 7 h at 10 m
2
m
−
3
and after only 2 h at 34
m
2
m
−
3
.
Design and Operation Considerations.
During pilot-
scale experiments, electrochemical phosphate precipitation
resulted in scaling on the cathode (
Figure S1
), which
subsequently fell into solution as approximately 1 cm
2
fl
akes.
Although cathodic scaling did not adversely a
ff
ect wastewater
treatment over short-term tests (i.e., less than 200 treatment
cycles), complete cathode coverage by the precipitate during
long-term operation may be problematic. Sustainable phos-
phate removal therefore requires electrode maintenance to
remove and collect deposited precipitate. Although electrodes
can be cleaned manually, this process could also be
accomplished automatically by periodically polarizing the
hydroxyapatite-coated stainless steel plates anodically. In
addition, post-treatment hydroxyapatite collection could be
automated, for example by incorporating a funnel into the
bottom of electrochemical reactors, providing a phosphorus-
rich precipitate that could be used as a fertilizer at minimum
cost.
53
In addition to human waste in onsite toilet treatment
systems, electrochemical treatment would likely be e
ff
ective for
other phosphate-rich waste streams including agricultural
wastes, such as animal husbandry wastewater. Dairy manure
waste has a similar composition to toilet wastewater,
35
and in
addition to phosphate removal, electrochemical treatment
provides disinfection, nitrogen removal, and chemical oxygen
demand reduction with no additional electrochemical energy
costs.
■
ASSOCIATED CONTENT
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website
at DOI:
10.1021/acssusche-
meng.7b03155
.
Figure 8.
Measured percent total phosphate removal following
galvanostatic electrolysis (4 h; 10 mA cm
−
2
unless noted otherwise)
as a function of (a) [Mg
2+
]
0
; (b) [Ca
2+
]
0
; (c) [HCO
3
−
]
0
; and (d)
electrolysis current density,
j
. Error bars represent
±
standard
deviation of three replicates. Experiments are referenced by letter
and are described in
Table S1 and Figure S2
.
ACS Sustainable Chemistry & Engineering
Research Article
DOI:
10.1021/acssuschemeng.7b03155
ACS Sustainable Chem. Eng.
2018, 6, 3135
−
3142
3140
Additional Figures S1
−
S9 and Table S1 as discussed in
the text. (
PDF
)
■
AUTHOR INFORMATION
Corresponding Author
*
E-mail:
mrh@caltech.edu
.
ORCID
Cle
́
ment A. Cid:
0000-0002-7293-035X
Justin T. Jasper:
0000-0002-2461-5283
Author Contributions
†
C.A.C. and J.T.J. contributed equally to this work.
Notes
The authors declare no competing
fi
nancial interest.
■
ACKNOWLEDGMENTS
This research was supported by the Bill and Melinda Gates
Foundation (Grants OPP 1069500 and OPP 1111246) and a
Resnick Sustainability Postdoctoral Fellowship to J.T.J.
■
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