Electrochemical Removal of Phosphate Ions from TreatedWastewaterOr Gorni-Pinkesfeld, Hilla Shemer, David Hasson,* and Raphael Semiat
Rabin Desalination Laboratory, Technion-Israel Institute of Technology, Haifa 32000, Israel
ABSTRACT: The objective of this work was to investigate electrochemical removal of phosphate from secondary treatmenteffluent containing calcium and magnesium hardness. Phosphate and hardness removal were carried out using a recentlydeveloped technique based on an electrochemical cation exchange membrane system (ECM system). The ECM systemovercomes major drawbacks of the current electrochemical technology, notably high electrode area requirements. Theparameters investigated were the effects of current density and pH on the phosphate precipitation rate and on the fractionalphosphate removal. The effect of presence or absence of phosphate ion on the coprecipitation of calcium carbonate andmagnesium hydroxide was also studied. Results indicate the feasibility of viable electrochemical phosphate removal by theconvenient ECM technique.
1. INTRODUCTIONMunicipal wastewaters may contain from 5 to 20 mg/L of totalphosphorus, of which 1−5 mg/L is organic and the rest isinorganic.1,2 The Israeli regulations specify maximum phos-phate concentration of 1 mg/L for effluent discharge intosurface waters and 5 mg/L for unlimited irrigation effluent.3
The most widely used phosphate removal techniques arechemical addition of aluminum and iron salts, ion exchangewith struvite precipitation, biological phosphate removal andcombined biological and chemical phosphate removal.4 Use ofelectrocoagulation techniques that generate aluminum and ironions by anodes dissolution has also been proposed.5−7
Phosphate removal using a direct electrochemical processdoes not seem to have been studied.Removal of phosphate from wastewaters can serve three
important functions: recovery of phosphorus for its value as achemical commodity, preventing environmental problemsassociated with algae growth in surface water, and facilitatingsecondary treated wastewaters reuse. A difficulty hinderingwider wastewater reuse is the lack of a reliable antiscalant forpreventing calcium phosphate precipitation in reverse osmosisor nanofiltration purifications.Phosphate removal usually involves precipitation of one or
more of the of the following calcium phosphate salts: Brushite(CaHPO4•2H2O), Monetite (CaHPO4), octacalcium phos-phate (Ca4H(PO4)3•3H2O; OCP), tricalcium phosphate(Ca3(PO4)2), Hydroxyapatite (Ca5(PO4)3OH; HAP) andamorphous calcium phosphate (ACP). The pH level, ionicstrength, temperature and the interactions with othersubstances present in the solution affect the precipitationreaction.8−10 HAP is the thermodynamically most stablecalcium phosphate species at room temperature. Initially aprecursor precipitates which then recrystallizes to formHAP.9,10
Although the possibility of electrochemical scale removal haslong been recognized, industrial application of this technique israther limited. The major limitations of the conventionalelectro-precipitation technology are the high cathode arearequirement, existence of a limiting current density beyond
which the precipitation rate remains unchanged and therequirement of periodical cleaning of the cathode surface. Arecently developed electrochemical technique based on a cationexchange membrane (ECM system) overcomes these limi-tations.11,12 In conventional equipment, the cathode performstwo functions: it generates alkalinity on the boundary layer inimmediate contact with the electrode and also serves as a scaledeposition surface. The basic concept of the ECM system isseparation of the anode and cathode in two separatecompartments using a cationic ion exchange membrane. Inthis case a high alkaline environment is generated throughoutthe whole volume of the cathode compartment. By transferringthe alkaline solution to a separate reaction vessel containingcalcium carbonate and calcium phosphate particles, theprecipitation surface is now the extensive area of the crystalseeds rather than the restricted area of the cathode. Theimproved system enables considerable reduction of the cathodearea, by a factor as high as 10−20.11,12The objective of the present work was to characterize
removal of phosphate together with CaCO3 and Mg(OH)2from a secondary effluent using the ECM system.
2. EXPERIMENTAL SECTION
2.1. Experimental System. Electrochemical removal ofphosphate was studied in the continuous flow once-throughsystem shown in Figure 1. Two electrochemical cells weretested. The initial cell had a total volume of 50 mL and wasseparated into two compartments by a cationic ion-exchangemembrane (Nafion N-424 from Ion Power, Inc.). The anodeconsisted of a 100 × 25 mm2 plate (a so-called “dynamic stableanode”, DSA) while the cathode consisted of a stainless steelplate of the same dimensions. The gap between electrodes was12 mm wide.
Received: June 19, 2013Revised: August 20, 2013Accepted: August 22, 2013Published: August 22, 2013
The second electrochemical cell enabled higher electriccurrents and concomitantly higher pH levels. The cell had atotal volume of 300 mL and was similarly constructed with thesame ion-exchange membrane separating the cationic andanionic compartments. The DSA and stainless steel electrodeshad the dimensions of 400 × 50 mm2 and were held at the wallsof the cell. The gap between the electrodes was 13 mm wide.Feed flow rates in both cells were 100 mL/min and were inlaminar region at Reynolds numbers below 100.The electrochemical reactions generated an alkaline environ-
ment in the cathode compartment and an acidic environmentin the anode compartment.11,12 The anodic solution wasrecirculated from the intermediate anodic vessel to the anodiccompartment. The cathodic solution was recirculated from thecrystallizer vessel to the cathodic compartment. The precip-itation reaction occurred in the crystallizer. Table 1 describesthe experimental conditions performed with the 50 and 300 mLelectrochemical cells.
2.2. Feed Water Quality. The feed solution (Table 2)simulated the composition of secondary effluent treated in theNir-Etzion (Israel) domestic wastewater treatment plant. Thesolution was prepared by dissolving in deionized wateranalytical grade NaCl, Na2SO4, MgCl2•6H2O, KH2PO4,NaHCO3 and CaCl2•2H2O.2.3. Analytical Methods. The calcium, magnesium,
phosphate and alkalinity removal rates were evaluated fromthe difference in concentrations between the feed solution and
the solution leaving the crystallizer. The calcium andmagnesium concentrations were determined by EDTAtitrations. Solution alkalinity was determined by HCl titrationsand phosphate concentration was determined by HACHDR2800 spectrophotometer using method 8048 (detectionlimit of 0.02 mg/L PO4) with PhosVer 3 reagent powderpillows.Repeat experiments were carried out to ascertain the
accuracy of the results. Micrographs of precipitates wereobtained using a high resolution scanning electron microscope(HR-SEMZeiss Ultra Plus).
3. RESULTS AND DISCUSSION3.1. Effect of Current Density on the Precipitation
Rate. The alkaline solution created in the electrochemical cell-induced precipitation of CaCO3, Mg(OH)2 and calcium-phosphate. The experimental data were analyzed to indicatethe rate of precipitation of each of these three species.Results of data measured in the 50 mL cell are presented in
Figures 2−4. Figure 2 shows the rates of phosphateprecipitation (g PO4/h m2 cathode area) at current densitiesranging from 100 A/m2 (pH = 9.4) to 360 A/m2 (pH = 10.2).It is seen that the phosphate precipitation rate reached anasymptotic limit of 22 g/h m2 at a current density in the rangeof 200−250 A/m2. Figure 3 shows the effect of the currentdensity on the fractional removal of the phosphate ion as well
Figure 1. Schematic diagram of the electrochemical ECM precipitation system.
as on the residual phosphate concentration. It is seen that theminimum residual phosphate concentration of 1.2 mg/L wasobtained at the maximum current density of 360 A/m2 and atpH 10.2, achieved with the 50 mL cell. The fractional removalof the phosphate ion was 70% at the current density of 100 A/m2 and reached an asymptotic limit of 87% at the currentdensity of 300 A/m2.Figure 4 shows the effect of current density on the total
precipitation rate of the three species (CaCO3, Mg(OH)2 and
calcium-phosphate). It is seen that the total precipitation rate R(g/h m2) increases linearly with the current density I (A/m2)according to:
= ×R I0.22 (1)
It will be recalled that in the conventional electrochemicaltechnology which is devoid of a separating ion exchange
membrane the precipitation rate reaches an asymptotic limitthat cannot be augmented by increase of the current.11,12 Theabove result provides further confirmation of the advantageousfeature of the ECM systemit enables enhancement of theprecipitation rate by increase of the electric current with noasymptotic limitation.
3.2. Calcium Phosphate Polymorph. As mentioned inSection 1 several calcium phosphate polymorphs can crystallizeby alkaline precipitation. Equilibrium conditions of the feedsolution at various pH levels were evaluated using Geochemist’sWorkbench (GWB) software V. 8. Results shown in Figure 5
indicate potential precipitation of two crystal habits exhibitingsupersaturation conditions. Hydroxyapatite (Ca5(PO4)3(OH))becomes supersaturated at pH levels above 5.5 whileoctacalcium phosphate (Ca4H(PO4)3•3H2O) reaches super-saturation at pH levels above 7. These results are in generalagreement with literature data. It is believed that nucleation ofHAP is extremely slow and it requires initial formation of OCPprecursor.9,13,14
3.3. Effect of pH on the Fractional Removal of thePrecipitating Species. The solubility of calcium phosphatesdecreases at increasing pH levels. A series of experiments wascarried out with the two electrochemical cells to determine theresidual phosphate concentration at different pH levels in thecrystallizer. Static equilibrium solubility experiments were alsocarried out to measure phosphate solubility of the investigatedsolution at different pH levels. The static experiments werecarried out using a 250 mL sealed Erlenmeyer flask held in awater bath at 25 °C and agitated for 24 h at 200 rpm.Figure 6 displays residual phosphate concentrations
determined from runs preformed in the two electrochemical
Figure 2. Effect of current density on the phosphate precipitation rate.
Figure 3. Effect of current density on the fractional phosphate removaland on the residual phosphate.
Figure 4. Effect of current density on the total precipitation rate.
Figure 5. Chemical species as predicted by GWB software at differentpH levels.
Figure 6. Effect of pH on the residual phosphate concentration.
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cells and from the equilibrium solubility data measured in theflask experiments. The continuous flow residual phosphate dataare seen to be very close to the equilibrium solubilitymeasurements. This indicates that the residence time of thesolution in the crystallizer (100 min) was sufficient to providephosphate precipitation very close to equilibrium conditions.Figure 6 indicates that a residual phosphate of less than 1 mg/Lcan be achieved by maintaining a pH of 10.5 in the crystallizer.Figure 7 displays the effect of current density on the
composition of the precipitate from data measured with the 300
mL electrochemical cell. As expected, increase of the currentdensity induced an increase in the pH and augmented thefractional removal of the various species.The CaCO3 saturation index is defined by:
=× −
SI log(Ca) (CO )
KCa32
sp(CaCO )3 (2)
The inception of supersaturation (SICa > 0) of the feed solutionof Table 2 is at the pH of 7.2. A significant precipitationpotential of, say SICa = 1.6, will be generated at a pH value aslow as 9. Figure 7 is in qualitative agreement with thisprediction: a significant calcium removal is observed at a pH of10.4 and an increased removal accompanies the pH increase.For Mg(OH)2, the saturation index is defined by:
=×+ −
KSI log
(Mg ) (OH )Mg
2 2
sp(Mg(OH) )2 (3)
The inception of supersaturation (SIMg > 0) of the feed solutionof Table 2 is at the pH of 10.2, so that a significant precipitationpotential of, say SIMg = 1.0, will be generated at a pH of 10.7.Again Figure 7 is in qualitative agreement with this prediction.At pH = 10.4 no magnesium removal was observed since at thispH the solution is close to equilibrium. Magnesium removalwas observed at pH = 11.0 and the removal level increased asthe pH increased.3.4. Effect of Phosphate Ions on the Precipitation
Rate of Alkaline Scale. Effective antiscalants are based onorganic and inorganic polymeric phosphates. Even non-polymeric inorganic phosphates can inhibit somewhat precip-itation of CaCO3 and Mg(OH)2.
15
The influence of the presence of phosphate ions on thecourse of alkaline scales precipitation was tested by comparingthe precipitation degree at different pH levels in the crystallizer,with and without the presence of phosphate ions. The datawere measured with the 300 mL electrochemical cell.
Figures 8 and 9 display the removal level of calcium andmagnesium, respectively, at increasing values of pH induced by
augmenting the current density. The data clearly show that thepresence of phosphate acts to reduce the precipitation level ofboth CaCO3 and Mg(OH)2.Further evidence on the inhibitory effect of phosphate ions
on precipitation of the alkaline species was obtained bydetermining the fractional removal of the various species(phosphate, calcium and magnesium) as a function of time inall pH experiments. The data displayed in Figure 10 for theexperiment preformed at pH of 11.0 illustrates a phenomenonobserved in all pH experiments. Calcium phosphate was foundto be the first precipitating species. Only when most of the
Figure 7. Effect of current density on the fractional removal ofalkalinity, calcium, magnesium and phosphate.
Figure 8. Effect of current density on calcium removal in the absence(black) and in the presence (gray) of phosphate ions.
Figure 9. Effect of current density on magnesium removal in theabsence (black) and in the presence (gray) of phosphate ions.
Figure 10. Sequence of phosphate, calcium and magnesiumprecipitations at pH = 11.0.
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phosphate was removed, calcium carbonate started toprecipitate. Magnesium was the last precipitating species.Evidence of the effect of phosphate presence on the
morphology of the crystalline precipitates was also observedin SEM micrographs. Marked differences in crystal morphologyare seen in micrographs of crystals precipitated in the absence(Figure 11a) and in the presence (Figure 11b) of phosphateions. Crystals formed in the absence of phosphate ions haveneedle-like shapes similar to those observed by Holt et al.16
Crystals formed in the presence of phosphate ions resemblethin elongated twigs.The practical implication of the above result is that the
presence of phosphate ions in hard water alters the mechanismof precipitation of its alkaline scale species.3.5. Current Efficiency and Energy Consumption.
3.5.1. Current Efficiency. Current efficiency φ expresses thefractional percentage of the electric current that accomplishesthe desired chemical reactions. The efficiency is lowered byunwanted processes such as ion leakage through the membraneand undesired side reactions. In the system investigated in thisstudy three species are precipitated (CaCO3, Mg(OH)2 andHAP). Each of these reactions decreases the solution alkalinityby an equivalent amount of the precipitate. The currentefficiency φ is given by the ratio of the OH alkalinity consumedin the three precipitation reactions to the OH generated by theelectric current i:
φ =Δ + Δ + Δ − Δ m m m m
i F/Ca HAP Mg Alk
(4)
where ΔmCa is the equivalent removal rate of Ca as CaCO3,ΔmHAP is the equivalent removal rate of Ca as HAP, ΔmMg isthe equivalent removal rate of Mg as Mg(OH)2 and ΔmAlk isthe difference between the rate of feed solution alkalinity andthe rate of outlet solution alkalinity. The data displayed in
Table 3 show that in most experiments current efficiencies werehigh, exceeding 85%.
3.5.2. Energy Consumption. The energy consumption in anelectrochemical system depends on several parameters, notablydistance between electrodes, electrodes overpotential andsolution conductivity. The specific energy consumption isgiven by:
= ×E
V iQ (5)
where, Q (m3/h) is the feed flow to the anodic and cathodiccompartments, V is the voltage (V) and i is current intensity(A). The data in Table 3 indicate that the specific energyconsumption in the various experiments ranged from 0.7 to 2.9KWh/m3.From a practical point of view the specific energy for a viable
electrochemical phosphate removal process should be less than0.5−1.0 KWh/m3. Such conditions were realized in some of theexperiments carried out here with simple laboratory equipment.It is reasonable to assume that an electrochemical cell ofoptimum design would enable phosphate removal meetingeconomic criteria.
4. CONCLUSIONSThe present study examined electrochemical phosphateremoval from wastewaters containing calcium and magnesiumhardness in the improved ECM system. The precipitatedspecies in the alkaline environment of the crystallizer arecalcium phosphate, usually in the form of HAP, calciumcarbonate and magnesium hydroxide.Phosphate removal was found to increase with current
density reaching an asymptotic limit of 87% at a solution pH of10.5, achieved by current density of 300 A/m2. The sequence ofthe reactions was initial precipitation of phosphate followed by
Figure 11. SEM micrograph of crystals formed in the absence (a) and presence (b) of phosphate ions.
Table 3. Energy Consumption and Current Efficiency
calcium carbonate precipitation terminating with magnesiumhydroxide precipitation. The presence of phosphate was alsofound to significantly reduce the fractional removal of calciumand to a lesser extent the fractional removal of magnesium.Current efficiency was high, exceeding 85%, and energy
consumption was in the range of 0.7−2.9 KWh/m3. Resultsindicate the feasibility of viable electrochemical phosphateremoval by the ECM technique.
■ ACKNOWLEDGMENTSThis paper forms part of the work performed by O.G.P. for herresearch thesis. Thanks are due to the Israel Water Authorityfor sponsoring this research.
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