Electrochemical acidolysis of magnesite to induce struvitecrystallization for recovering phosphorus from aqueous solution
Zhiqiang Zhang a, b, Lu She a, b, Jiao Zhang a, c, *, Zuobin Wang a, b, Pengyu Xiang d,Siqing Xia a, b
a Key Laboratory of Yangtze River Water Environment, Ministry of Education,State Key Laboratory of Pollution Control and Resource Reuse, College ofEnvironmental Science and Engineering, Tongji University, Shanghai, 200092, Chinab Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, Chinac School of Civil and Transportation Engineering, Shanghai Urban Construction Vocational College, Shanghai, 200432, Chinad Zhejiang Weiming Environment Protection Co., Ltd., Wenzhou, 325000, China
h i g h l i g h t s
* Corresponding author. Key Laboratory of Yangtzeof Environmental Science and Engineering, Tongji Un
A novel struvite crystallization method induced by electrochemical acidolysis of cheap magnesite wasinvestigated to recover phosphorus from aqueous solution. Magnesite was confirmed to continuouslydissolve in the anolyte whose pH stabilized at about 2. Driven by the electrical field force, over 90% of thereleased Mg2þ migrated to the cathode chamber via passing through the cation exchange membrane. ThepH of the phosphate-containing aqueous solution in the cathode chamber was elevated to the appropriatepH fit for struvite crystallization. The products were identified as struvite crystals by scanning electronmicroscopy and X-ray diffraction. Increasing the magnesite dosage from 0.83 to 3.33 g L�1 promoted thephosphorus recovery efficiency from 2.2% to 78.3% at 3 d, which was attributed to sufficient Mg2þ supply.Increasing the applied voltage from 3 to 6 V improved the recovery efficiency from 43.6% to 76.4% at 1 d,since the enhanced current density of the electrochemical system markedly accelerated both themagnesite acidolysis and the catholyte pH elevation. The initial catholyte pH between 3 and 5 was found tobenefit the phosphorus recovery due to the final catholyte pH fit for the struvite crystallization.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315308
1. Introduction
Phosphorus as an indispensable nutrient for maintaining lifeactivities, will encounter a global shortage crisis in the near future.As the main source of phosphorus, phosphorus ore is a non-renewable resource. It is predicted that the world's availablephosphorus reserves will be exhausted in the next 50 years(Gilbert, 2009). At the same time, phosphorus is also a majorpollutant in the water environment, which can arouse watereutrophication (Conley et al., 2009). Therefore, in order to solve thecontradiction between the depletion of phosphorus ore resourcesand the eutrophication of water environment, it is of big signifi-cance to develop effective methods for removing and recoveringphosphorus from wastewater.
Struvite crystallization is the most concerned method ofrecovering phosphorus from wastewater, because it can simulta-neously recover nitrogen and phosphorus, and the recoveredproduct is an excellent slow release fertilizer (Marti et al., 2010; Haoet al., 2013; Hug and Udert, 2013). Recovering phosphorus viastruvite crystallization has been investigated for various strongphosphate-containing wastewater, like digested sludge liquid(Antakyali et al., 2005), industrial wastewater (El Diwani et al.,2007), landfill leachates (Kochany and Lipczynska-Kochany, 2009)and swine wastewater (Huang et al., 2016). However, the formationof struvite requires alkaline conditions with pH in the range of8e10 (Ohlinger et al., 1998) and sufficient Mg2þ. The pH value in-fluences the induction of nucleation of struvite crystals, while Mg2þ
concentration is usually the limiting factor for the formation ofstruvite crystals due to the low concentration of Mg2þ in waste-water (Hoevelmann and Putnis, 2016). Additional reagents of alkali(such as NaOH) and magnesium (such as MgCl2, MgSO4, etc.) arerequired to reach the conditions required for struvite crystallization(Barbosa et al., 2016; Merino-Jimenez et al., 2017). The costs of al-kali andmagnesium account for 97% of the total struvite productioncost (Jaffer et al., 2002), resulting in recovery costs of phosphorusfar above the economic value of struvite (Cornel and Schaum, 2009;Cusick et al., 2014; Merino-Jimenez et al., 2017; Lin et al., 2018),which makes the struvite process less economically attractive.
The electrochemical cathodic production of alkali has beenfound to obviously reduce the alkali cost of struvite crystallization,with simultaneous production of H2 and O2 (Cusick et al., 2014;Merino-Jimenez et al., 2017; Lin et al., 2018). When the phosphate-containing wastewater was used as catholyte, its pH rose to theappropriate range for struvite crystallization due to the reductionreaction of water on the cathode surface (Wang et al., 2010). Thedual-chamber electrolytic cell excelled the single-chamber one atthe recover efficacy of phosphorus, because a separator like a cationexchange membrane (CEM) was used to divide the anode chamberand the cathode chamber in the middle of the electrochemicalsystem to meet the cathodic alkali requirement. However, Hþ
generated at the anode and OH� generated at the cathode couldn'tbe effectively diffused between the two chambers, resulting in theincreasing pH gradient (Lu et al., 2015). Thus, the anolyte acidifi-cation made the system performance gradually decline, and themagnesium cost was still high for struvite crystallization to recoverphosphorus.
MgO and Mg(OH)2 were used as magnesium sources of struvitecrystallization recovering phosphorus due to their low prices andalkali features (Romero-Gueiza et al., 2015; Capdevielle et al., 2016).Their low solubility, however, resulted in excess addition of thereagents and low purity of the struvite. The large-scale use of MgOorMg(OH)2 made their own advantages of cheaper prices no longerapparent (Romero-Gueiza et al., 2015). Some researchers used Mganode as the sacrificial anode to provide magnesium source (Hugand Udert, 2013; Lin et al., 2018), but the energy costs were
pretty high and the Mg anode was also relatively expensive.Magnesite, whose major component is MgCO3, is a natural mineralraw material of magnesium with cheap price and wide sources.After magnesite was dissolved by acid, the released Mg2þ can beused as magnesium source of struvite crystallization. Nevertheless,the dissolution needs to consume amounts of acid, which dis-counted its cost advantage (Gunay et al., 2008; Yu et al., 2017).
Based on the above analyses, the cathodic alkali of the electro-chemical system can provide alkaline conditions for struvite crys-tallization, while the anolyte acidification might provide acidolysisconditions for magnesite. The acidolysis of cheap magnesite in theanode chamber can not only inhibit the anolyte acidification causedby water oxidation on the anode surface, but also online continu-ously release Mg2þ. Driven by the electrical field force, Mg2þ canmigrate to the cathode of stainless steel cloth via passing throughthe CEM. The pH of the phosphate-containing wastewater in thecathode chamber can be elevated due to the formation of hydroxideions by water reduction on the cathode surface. With enough Mg2þ
source and appropriate pH conditions, struvite crystallizationspontaneously took place to recover phosphate and ammoniumfrom the wastewater. Accordingly, it is possible for the electro-chemical method to harvest struvite crystals for recovering phos-phorus from wastewater besides the production of H2 and O2.
To identify the above hypothesis, the characteristics of themagnesite acidolysis and the released Mg2þ migration in theelectrochemical systemwere firstly examined. The feasibility of thestruvite crystallization induced by the electrochemical acidolysis ofmagnesite in the electrochemical systemwas then testified. Finally,the effects of some key factors on the recovery of phosphorus fromaqueous solution were further investigated.
2. Materials and methods
2.1. Materials
The natural magnesite mineral used in this study was purchasedfrom Haicheng Rich Magnesium Refractory Material Factory,Liaoning, China. The main component was MgCO3, and its contentwas as high as 93.87%. The other components included CaO, Fe2O3and SiO2. The magnesite was grinded and then sifted through 200mesh sieve. The grinded magnesite was subjected to X-raydiffraction (XRD) using Bruker D8 Advance. As can be seen from theXRD pattern in Fig. S1, the diffraction peaks position of the MgCO3standard spectrum and the commercial magnesite are almostcompletely matched. The low background value and high peakintensity of the entire peak also indicated the commercial magne-site has high degree of crystallinity and few impurities. All thechemical reagents used in this study were prepared with analyticalpure, which were obtained from Sinopharm Chemical ReagentBeijing Co., Ltd. (China). All the used solutions were prepared bydistilled water.
2.2. Experimental set-up
A dual-chamber electrolytic cell was used, which consisted oftwo parts: the anode chamber and the cathode chamber. The twochambers were separated by a CEM (CMI-7000, Membrane Inter-national). The schematic diagram of the device is shown in Fig. 1.The inner cavity of the device was a cylinder with a total workingvolume of 380mL and the size was F10 cm� 8 cm; the size of theCEM was 14 cm� 14 cm and its effective area was 80 cm 2. Thereactor was made of plexiglass and provided with inlet and outletports, gas collection ports, and electrode receptacles. The anodewas constructed using 3mm thick rectangular plate with an activesurface area of 15 cm2 which was made of Ti plate coated with Ir
Z. Zhang et al. / Chemosphere 226 (2019) 307e315 309
oxide and Ru oxide. The cathodewas made of a stainless steel mesh(304) with an active surface area of 80 cm2. The electrodes weresymmetrically placed on the different sides of membranes with thetotal distance of 4 cm. The reactor was bolted and sealed with asilicone gasket in the middle. The electrolyte pHs were monitoredwith a pHmeter (E-201-C, Shanghai Rex Instrument Factory, China)during the whole electrolysis process. Direct current was suppliedby a DC power supply (PS-2002D, Shenzhen Zhaoxin ElectronicInstrument Equipment Co., Ltd., China). The current (I) was calcu-lated by voltage (V) and resistor (R), as I¼ V
R. A 10U was connectedto the circuit in series. The voltage across the resistor was measuredusing a data acquisition system (model 2700, Keithley) at a sam-pling interval of 300 s.
2.3. Electrolysis experiments
The aqueous solution containing (NH4)2SO4 and NH4H2PO4 asthe catholyte, which simulated the concentrations of ammonium(NH4
þ-N) and phosphate (PO43--P) in the sludge digestion solution.
The sulfate salt solution (15mM Na2SO4) was selected as the ano-lyte to increase the conductivity without generating harmful gasessuch as chlorine. Both the volumes of the catholyte and the anolytewere controlled at 300mL. The anolyte was stirred at a rate of500 rpm by a mixer (08-2G, Mei Yingpu instrument and MeterManufacturing Co., Ltd., Shanghai, China) for complete mixing. Theapplied voltage of the system was 3 V, and the magnesite dosagewas 3.33 g L�1. By adding 0.1M NaOH solution and 0.1M H2SO4solution, the initial pH of the anolyte and the catholyte wereadjusted to 7 and 5, respectively. The sampling frequency was every24 h and the sampling volume was 2mL after filtering through a0.45 mm microfiltration membrane. All the experiments were con-ducted at room temperature of 25± 1 �C for 5 d.
The characteristics experiments of the magnesite acidolysis andthe released Mg2þ migration in the electrochemical system werefirstly conducted. To ensure that the precipitation reaction of Mg2þ
Fig. 1. Schematic diagram of the dual-chamber electrolysis reactor for the acidolysis of magn
couldn't take place in the cathode chamber, the catholytewas set asphosphate buffer formulated with 300mM NaH2PO4 and 30mMNa2HPO4, and the initial catholyte pH (pHIC) was controlled to bearound 5.
The feasibility experiments of the struvite crystallizationinduced by the electrochemical acidolysis of magnesite in theelectrochemical system was then carried out. The simulatingaqueous solution containing 10mM (NH4)2SO4 and 10mMNH4H2PO4 was used as the catholyte. The molar ratio of NH4
þ-N andPO4
3--P was controlled at 3:1, and the final concentrations of NH4þ-N
and PO43--P were 30mM and 10mM, respectively.
The influencing experiments of the dosage of magnesite, theapplied voltage and the initial catholyte pH were further per-formed. The simulating aqueous solution containing 10mM(NH4)2SO4 and 10mM NH4H2PO4 was used as the catholyte. Wheninvestigating the effects of the magnesite dosage, the magnesitedosages were 0.83, 1.67, 3.33 and 6.67 g L�1 respectively, otherconditions remain unchanged. When investigating the effects ofthe applied voltage, the applied voltages were 3, 6 and 9 Vrespectively, other conditions remain unchanged. When investi-gating the effects of the initial catholyte pH, the initial catholyte pHwas adjusted to 1, 3, 5 and 7 by adding 0.1M NaOH and 0.1MH2SO4respectively, other conditions remain unchanged.
For clarity, the detailed conditions of the above experimentsetups are summarized in Table S1.
2.4. Analyses and calculations
Samples collected from the experiments were first filteredthrough a 0.45 mm microfiltration membrane and then stored in a2mL centrifuge tube for later use. The concentrations of PO4
3--Pwere analyzed using spectrophotometry (UV-2700, Shimadzu) andthe concentrations of Mg2þ concentrations were analyzed by ICP-AES (720 ES, Agilent). After the electrochemical experimentended, the precipitates on the stainless steel were firstly rinsed
esite inducing struvite crystallization to recover phosphorus from the aqueous solution.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315310
with deionized water, then dried at room temperature, and finallyscraped for collection. The obtained precipitates were examined byZeiss Ultr55 scanning electron microscopy (SEM), and then thecrystal structure of the precipitates was subjected to XRD using aBruker D8 advanced diffractometer, and the diffraction pattern wasanalyzed using Jade 6.5.
For the characteristics experiments of the magnesite acidolysisand the releasedMg2þmigration in the electrochemical system, themigration efficiency of Mg2þ in this section of experiments wascalculated using Eq. (1).
MEMg�t ¼CMg�c�t � Vc
CMg�a�t � Va þ CMg�c�t � Vc(1)
where CMg-a-t (mg L�1) was the Mg2þ concentrations in the anolyteat time t; CMg-c-t (mg L�1) was the Mg2þ concentrations in thecatholyte at time t; Va (L) and Vc (L) were the volumes of the anolyteand the catholyte, respectively.
Since the volume of the anolyte was equal to that of the cath-olyte in the experiments, Eq. (1) could be simplified as Eq. (2).
MEMg�t ¼CMg�c�t
CMg�a�t þ CMg�c�t(2)
For the feasibility experiments of the struvite crystallization
Fig. 2. Variations of (a) electrolyte pH, (b) Mg2þ concentration in electrolyte and Mg2þ migrapplied voltage 3 V, initial catholyte pH 5, magnesite dosage 3.33 g L�1; NH4
þ-N 30mM and
Fig. 3. Variations of (a) electrolyte pH and (b) phosphorus recovery efficiency, total releaconditions: applied voltage 3 V, initial catholyte pH 5, magnesite dosage 3.33 g L�1; NH4
þ-N
induced by the electrochemical acidolysis of magnesite and theinfluencing experiments of the dosage of magnesite, the appliedvoltage and the initial catholyte pH in the electrochemical system,the phosphorus recovery was reflected by the PO4
3--P removal bystruvite crystallization in the cathode chamber. Thus, the phos-phorus recovery efficiency (Rp) was calculated using Eq. (3).
RP ¼ CP�0 � CP�t
CP�0� 100% (3)
where CP-0 (mg L�1) and CP-t (mg L�1) were the PO43--P concentra-
tions in the catholyte at time 0 and t, respectively.The total released concentration of Mg2þ (CMg-t, mg L�1) in the
anode chamber reflected the acidolysis amount of the magnesite,which was calculated based on Eqs. (4) and (5).
DPt ¼ ðCP�0 � CP�tÞ � Vc (4)
where DPt (mg) was the total amount of PO43--P removed between
0 and t; CP-0 (mg L�1) and CP-t (mg L�1) were the PO43--P concen-
trations at time 0 and t, respectively.
ating efficiency in the electrochemical system with running time under the conditions:PO4
3--P 10mM in the aqueous solution.
sed Mg2þ concentration under feasibility experiment conditions over time under the30mM and PO4
3--P 10mM in the aqueous solution.
Fig. 4. (a) SEM image and (b) XRD diffractogram of the recovered precipitate under feasibility experiment conditions: applied voltage 3 V, initial catholyte pH 5, magnesite dosage3.33 g L�1; NH4
þ-N 30mM and PO43--P 10mM in the aqueous solution.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315 311
CMg�t ¼DPt �MMg � RMg=P
MP � Vaþ ðCMg�a�t � CMg�a�0Þ
þ ðCMg�c�t � CMg�c�0Þ � Vc
Va(5)
where MMg and MP were the molar masses of magnesium element(24 gmol�1) and phosphorus element (31 gmol�1), respectively;
Fig. 5. Effects of magnesite dosage on (a) phosphorus recovery efficiency, (b) total releasedapplied voltage 3 V, initial catholyte pH of 5; NH4
þ-N 30mM and PO43--P 10mM in the aque
RMg/P is the removedmolar ratio of Mg2þ to PO43--P, whichwas equal
to 1 when PO43--P was removed as struvite crystals; Va (L) was the
anolyte volume; CMg-a-t (mg L�1) and CMg-a-0 (mg L�1) were theMg2þ concentrations in the anolyte at time 0 and t, respectively;CMg-c-t (mg L�1) and CMg-c-0 (mg L�1) were the Mg2þ concentrationsin the catholyte at time 0 and t, respectively.
Mg2þ concentration, (c) electrolyte pH and (d) current density under the conditions:ous solution.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315312
3. Results and discussion
3.1. Characteristics of the magnesite acidolysis and the Mg2þ
migration in the electrochemical system
The characteristics of the magnesite acidolysis and the Mg2þ
migration in the electrochemical system under migration experi-ment conditions are shown in Fig. 2. According to Fig. 2a, the initialpH of the anolyte and the catholyte were 6.76 and 4.78, respec-tively. On the one hand, since the electrochemical reaction as Eq. (6)occurred on the anode surface, the anolyte pH decreased to about2 at 1 d, and kept stable during the next 4 d. The acidic anolytecould make the acidolysis of the magnesite naturally took place. Onthe other hand, the electrochemical reaction as Eq. (7) occurred onthe cathode surface at the same time. The catholyte pH wasmaintained at about 5 via the pH buffering effect of the buffer so-lution in the cathode chamber, ensuring that the precipitation re-action of Mg2þ couldn't take place in the cathode chamber. Thus,the migration of Mg2þ from the anode chamber to the cathodechamber in electrochemical system was clearly verified.
2H2O� 4e�/4Hþ þ O2[ (6)
4H2Oþ 4e�/4OH� þ 2H2[ (7)
From Fig. 2b, the Mg2þ concentration in the anode chamber wasmaintained at a low level of 10.0mg L�1, while the Mg2þ concen-tration in the cathode chamber gradually increased. At 5 d, the
Fig. 6. Effects of applied voltage on (a) phosphorus recovery efficiency, (b) total releasedmagnesite dosage 3.33 g L�1, initial catholyte pH 5; NH4
þ-N 30mM and PO43--P 10mM in th
Mg2þ concentration in the cathode chamber reached 172.5mg L�1,meaning that about 90% of Mg2þ in the electrochemical systemwaspresent in the cathode chamber. Thus, theMg2þmigration from theanode chamber to the cathode chamber was demonstrated, whichwould provide sufficient Mg2þ for the subsequent electrochemicalstruvite crystallization recovering phosphorus from the aqueoussolution in the cathode chamber.
3.2. Feasibility of the struvite crystallization induced by theelectrochemical acidolysis of magnesite in the electrochemicalsystem
The variations of phosphorus recovery efficiency, the releasedMg2þ and electrolyte pH under feasibility experiment conditionsover time were shown in Fig. 3. From Fig. 3a, the acidic anolyte andthe alkaline catholyte caused by the electrolysis reactions providedconditions for the acidolysis of magnesite and the crystallization ofstruvite, respectively. From Fig. 3b, both the phosphorus recoveryefficiency and the released Mg2þ concentration increased withtime, and the phosphorus recovery efficiency reached 96.7% at the5 d. The measured Mg2þ concentration in the cathode chamber wasvery low, because the Mg2þ migrating from the anode chamber tothe cathode chamber quickly reacted with NH4
þ and PO43� to form
struvite crystals in the cathode chamber. It indicates that thephosphorus recovery efficiency of the system was majorly decidedby the released Mg2þ concentration.
The product features of the struvite crystallization induced by
Mg2þ concentration, (c) electrolyte pH and (d) current density under the conditions:e aqueous solution.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315 313
the electrochemical acidolysis of magnesite in the electrochemicalsystem were analyzed to confirm the struvite production andexamine its purity. The SEM image (Fig. 4a) shows that the pre-cipitation belonged to regular orthorhombic particles. The sizes ofthe crystals ranged from 30 to 200 mm in length. XRD analysis(Fig. 4b) shows that the peak position was basically consistent withthe standard spectrum of struvite. The matching product analyzedby Jade 6.5 software was also struvite (Zhou et al., 2015). Moreover,the high peak intensity, the sharp peak shape and the flat baselineof the XRD diffraction peaks indicates that the obtained precipita-tionwas of high crystallinity and purity (Moulessehoul et al., 2017).
The above results convincingly proved the feasibility of thestruvite crystallization induced by the electrochemical acidolysis ofmagnesite for recovering phosphorus from aqueous solution in theelectrochemical system. To enhance the phosphorus recovery effi-ciency, however, it is necessary to further investigate the effects ofsome key factors on the struvite crystallization induced via theelectrochemical acidolysis of magnesite in the electrochemicalsystem.
3.3. Effects of key factors on the struvite crystallization recoveringphosphorus in the electrochemical system
3.3.1. Magnesite dosageThe effects of the magnesite dosage on the struvite crystalliza-
tion recovering phosphorus in the electrochemical system are showin Fig. 5. With rising magnesite dosage, the phosphorus recoveryefficiency, the released Mg2þ concentration and the catholyte pHincreased, the anolyte pH nearly kept stable, while the currentdensity increased first and then decreased. The stable anolyte pH
Fig. 7. Effects of initial catholyte pH on (a) phosphorus recovery efficiency, (b) total releaseapplied voltage 3 V, magnesite dosage of 3.33 g L�1; NH4
þ-N 30mM and PO43--P 10mM in th
denotes that the continuously produced hydrogen ions in theanode chamber were used for the acidolysis of magnesite, thusmagnesium ions were released from magnesite. Accordingly,increasing the magnesite dosage significantly increased thereleased Mg2þ concentration, and further improved the phos-phorus recovery efficiency. This result was consistent with theprevious studies that sufficient Mg2þ concentration was requiredfor the high phosphorus recovery efficiency (Yu et al., 2017; Kimet al., 2018).
The variation of the current density was related with thedeposition of struvite crystals on the cathode stainless steel mesh,which increased the internal resistance of the electrochemicalsystem (Hug and Udert, 2013). Nevertheless, the deposition ofstruvite crystals on the cathode were beneficial to their volumegrowth and product harvest (Lei et al., 2017), as shown in Fig. S2.Therefore, periodic harvesting of the cathode crystals would beessential for maintaining optimum electrode kinetics (Cusick andLogan, 2012).
3.3.2. Applied voltageThe effects of the applied voltage on the struvite crystallization
recovering phosphorus in the electrochemical system are show inFig. 6. With rising applied voltage, the phosphorus recovery effi-ciency, the released Mg2þ concentration and the electrolyte pHincreased first and then decreased, while the current densityincreased. In the early process period (<3 d) of the system,increasing the applied voltage could improve the phosphorus re-covery efficiency, because the pH gradient difference between thetwo chambers was increased (Fig. 6c). According to the Faraday'slaw of electrolysis (Lin et al., 2018), the higher the applied voltage is,
d Mg2þ concentration, (c) electrolyte pH and (d) current density under the conditions:e aqueous solution.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315314
the greater the electrochemical reaction rate on the electrode is,and the faster the generation rate of Hþ and OH� is. When theirproduction rates exceeded their consumption rates, a big pH dif-ference between the two chambers occurred. In the later processperiod (4~5 d) of the system, however, higher applied voltagescaused the catholyte pH became higher, which exceeded the pHrange that favored the struvite crystallization. Thus, the phos-phorus recovery efficiency was not as high as at the low voltage. Forthe actual wastewater with strong pH buffering ability, increasingthe applied voltage would be a goodmethod to increase the pH in ashort time without additional alkali addition.
3.3.3. Initial catholyte pHThe effects of the initial catholyte pH on the struvite crystalli-
zation recovering phosphorus in the electrochemical system areshow in Fig. 7. With rising initial catholyte pH, the phosphorusrecovery efficiency, the released Mg2þ concentration and thecatholyte pH increased first and then decreased, the anolyte nearlykept stable, while the current density decreased. The phosphorusrecovery efficiencies at pHIC 3 and 5 were significantly higher thanthose at pHIC 1 and 7, since the final catholyte pH of 8e10 was morefit for the struvite crystallization (Doyle and Parsons, 2002). Theinitial catholyte pH exerted some effects on the pH elevation pro-cess of the catholyte, and further affected the struvite crystalliza-tion. The initial catholyte pH benefitting the phosphorus recoverywas between 3 and 5, which are the common pH area of mostanaerobic digestion solutions (Dai et al., 2016).
The current density decreased with increasing initial catholytepH due to the increasing overpotential presence caused by theincreasing pH gradient difference between the two chambers. Thehigher the initial catholyte pH was, the bigger the overpotentialwas, and the smaller the current density was. This should be acommon phenomenon of electricity loss caused by electrochemicalrecovery of precipitates (Cusick et al., 2014). In addition, under allthe four initial catholyte pH conditions, the current densitiesgradually decreased over time. At the beginning period, the rapiddrop of the current density was mainly due to the overpotentialpresence. Moreover, the gradual deposition of struvite crystals onthe cathode increased the reactor's internal resistance, which alsoresulted in the reduction of the current density (Hug and Udert,2013).
4. Conclusions
In this study, a novel electrochemical acidolysis of cheapmagnesite inducing struvite crystallization was identified torecover phosphorus from the aqueous solution. With enough Mg2þ
source and appropriate pH conditions in the cathode chamber,struvite crystallization spontaneously took place to recover phos-phate and ammonium from the aqueous solution. Increasing thedosage of magnesite could promote the phosphorus recovery viaproviding sufficient Mg2þ. Increasing the applied voltage couldincrease the phosphorus recovery via obviously increasing thecurrent density of the electrochemical system. However, once thecurrent density exceeded a limitation, the struvite crystallizationwould be inhibited. The initial catholyte pH between 3 and 5 wasconducive to the phosphorus recovery due to the final catholyte pHfit for the struvite crystallization.
Acknowledgements
This work was supported by the National Key R&D Program ofChina [No. 2017YFC0403400]; the Fundamental Research Funds forthe Central Universities; and the Foundation of Key Laboratory ofYangtze River Water Environment, Ministry of Education (Tongji
University), China [No. YRWEF201805].
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.chemosphere.2019.03.106.
References
Antakyali, D., Schmitz, S., Krampe, J., Rott, U., 2005. Nitrogen removal frommunicipal sewage sludge liquor through struvite precipitation for application ina mobile plant. J. Residuals Sci. Technol. 2, 221e226.
Barbosa, S.G., Peixoto, L., Meulman, B., Alves, M.M., Pereira, M.A., 2016. A design ofexperiments to assess phosphorous removal and crystal properties in struviteprecipitation of source separated urine using different Mg sources. Chem. Eng. J.298, 146e153.
Capdevielle, A., Sykorova, E., Beline, F., Daumer, M., 2016. Effects of organic matteron crystallization of struvite in biologically treated swine wastewater. Environ.Technol. 37, 880e892.
Cornel, P., Schaum, C., 2009. Phosphorus recovery from wastewater: needs, tech-nologies and costs. Water Sci. Technol. 59, 1069e1076.
Cusick, R.D., Logan, B.E., 2012. Phosphate recovery as struvite within a singlechamber microbial electrolysis cell. Bioresour. Technol. 107, 110e115.
Cusick, R.D., Ullery, M.L., Dempsey, B.A., Logan, B.E., 2014. Electrochemical struviteprecipitation from digestate with a fluidized bed cathode microbial electrolysiscell. Water Res. 54, 297e306.
Dai, X., Li, X., Zhang, D., Chen, Y., Dai, L., 2016. Simultaneous enhancement ofmethane production and methane content in biogas from waste activatedsludge and perennial ryegrass anaerobic co-digestion: the effects of pH and C/Nratio. Bioresour. Technol. 216, 323e330.
Doyle, J.D., Parsons, S.A., 2002. Struvite formation, control and recovery. Water Res.36, 3925e3940.
El Diwani, G., El Rafie, S., El Ibiari, N.N., El-Aila, H.I., 2007. Recovery of ammonianitrogen from industrial wastewater treatment as Struvite slow releasing fer-tilizer. Desalination 214, 200e214.
Gunay, A., Karadag, D., Tosun, I., Ozturk, M., 2008. Use of magnesit as a magnesiumsource for ammonium removal from leachate. J. Hazard Mater. 156, 619e623.
Hao, X., Wang, C., van Loosdrecht, M.C., Hu, Y., 2013. Looking beyond struvite for P-recovery. Environ. Sci. Technol. 47, 4965e4966.
Hoevelmann, J., Putnis, C.V., 2016. In situ nanoscale imaging of struvite formationduring the dissolution of natural brucite: implications for phosphorus recoveryfrom wastewaters. Environ. Sci. Technol. 50, 13032e13041.
Huang, H., Zhang, P., Zhang, Z., Liu, J., Xiao, J., Gao, F., 2016. Simultaneous removal ofammonia nitrogen and recovery of phosphate from swine wastewater bystruvite electrochemical precipitation and recycling technology. J. Clean. Prod.127, 302e310.
Hug, A., Udert, K.M., 2013. Struvite precipitation from urine with electrochemicalmagnesium dosage. Water Res. 47, 289e299.
Kim, J.H., An, B.M., Lim, D.H., Park, J.Y., 2018. Electricity production and phosphorousrecovery as struvite from synthetic wastewater using magnesium-air fuel cellelectrocoagulation. Water Res. 132, 200e210.
Kochany, J., Lipczynska-Kochany, E., 2009. Utilization of landfill leachate parametersfor pretreatment by Fenton reaction and struvite precipitation–a comparativestudy. J. Hazard Mater. 166, 248e254.
Lei, Y., Song, B., van der Weijden, R.D., Saakes, M., Buisman, C.J.N., 2017. Electro-chemical induced calcium phosphate precipitation: importance of local pH.Environ. Sci. Technol. 51, 11156e11164.
Lin, X., Han, Z., Yu, H., Ye, Z., Zhu, S., Zhu, J., 2018. Struvite precipitation from biogasdigestion slurry using a two chamber electrolysis cell with a magnesium anode.J. Clean. Prod. 174, 1598e1607.
Marti, N., Pastor, L., Bouzas, A., Ferrer, J., Seco, A., 2010. Phosphorus recovery bystruvite crystallization in WWTPs: influence of the sludge treatment lineoperation. Water Res. 44, 2371e2379.
Merino-Jimenez, I., Celorrio, V., Fermin, D.J., Greenman, J., Ieropoulos, I., 2017.Enhanced MFC power production and struvite recovery by the addition of seasalts to urine. Water Res. 109, 46e53.
Moulessehoul, A., Gallart-Mateu, D., Harrache, D., Djaroud, S., de la Guardia, M.,Kameche, M., 2017. Conductimetric study of struvite crystallization in water as afunction of pH. J. Cryst. Growth 471, 42e52.
Z. Zhang et al. / Chemosphere 226 (2019) 307e315 315
Alvarez, J., Chimenos, J.M., 2015. Reagent use efficiency with removal of nitro-gen from pig slurry via struvite: a study on magnesium oxide and related by-products. Water Res. 84, 286e294.
Wang, C.C., Hao, X.D., Guo, G.S., van Loosdrecht, M.C.M., 2010. Formation of purestruvite at neutral pH by electrochemical deposition. Chem. Eng. J. 159,280e283.
Yu, R., Ren, H., Wu, J., Zhang, X., 2017. A novel treatment processes of struvite withpretreated magnesite as a source of low-cost magnesium. Environ. Sci. Pollut.Res. 24, 22204e22213.
Zhou, Z., Hu, L., Ren, W., Zhao, Y., Jiang, L., Wang, L., 2015. Effect of humic substanceson phosphorus removal by struvite precipitation. Chemosphere 141, 94e99.
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