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Environmental Processes
Is there a precipitation sequence in municipal wastewater induced by electrolysis? Yang Lei, Jorrit Christiaan Remmers, Michel Saakes, Renata van der Weijden, and Cees J.N. Buisman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02869 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018
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Environmental Science & Technology
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Is there a precipitation sequence in municipal wastewater
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induced by electrolysis?
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Yang Lei†, ‡, Jorrit Christiaan Remmers‡, Michel Saakes†, Renata D. van der
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Weijden*, †, ‡, Cees J.N. Buisman†, ‡
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†
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The Netherlands
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‡
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6700AA Wageningen, The Netherlands
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*
Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900CC Leeuwarden,
Sub-department Environmental Technology, Wageningen University and Research, P.O. Box 17,
Corresponding author, email:
[email protected] 10
Abstract
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Electrochemical wastewater treatment can induce calcium phosphate precipitation on the
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cathode surface. This provides a simple yet efficient way for extracting phosphorus from
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municipal wastewater without dosing chemicals. However, the precipitation of amorphous
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calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO3) and brucite
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(Mg(OH)2). To increase the content of ACP in the products, it is essential to understand the
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precipitation sequence of ACP, calcite and brucite in electrochemical wastewater treatment.
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Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic
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solubility product and highest saturation index in the wastewater, it has the potential to
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precipitate first. However, this is not observed in electrochemical phosphate recovery from
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raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high
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bicarbonate concentration in the wastewater result in formation of calcite. In case of
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decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in
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the wastewater was used for ACP formation instead of calcite. The formation of of brucite,
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however, was only affected when the current density was decreased, or the size of cathode
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was changed. Overall, the removal of Ca and Mg is much more affected by current density
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than the surface area of cathode, whereas for P removal, the reverse is true. Because of these
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dependencies, though there is no definite precipitation sequence among ACP, calcite and
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brucite, it is still possible to influence the precipitation degree of these species by relative low
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current density and high surface area or by targeting phosphorus-rich wastewaters. 1
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Introduction
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Municipal wastewater is a significant source of contaminants but can be an important source
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of nutrients as well, i.e., phosphorus (P).1-3 P often is considered to be the principal stimulant
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of eutrophication. However, it is also a limited and essential resource.4 To bridge the gap of P
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being too much as a pollutant and too little as a resource, it is suggested to remove and reuse
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P from wastewater.4, 5
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Phosphate can be removed from the wastewater by precipitation as useful P products, such as
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struvite (MgNH4PO4·6H2O) and calcium phosphate.6-9 The most stable phase of calcium
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phosphate is hydroxyapatite (Ca5(PO4)3OH, HAP), which owns the highest Ca/P molar ratio
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(1.67).10 The typical Ca concentration in domestic wastewater is 20-120 mg/L,11 whereas the
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P-PO4 concentration varies from less than 1.0 (effluent) to 10 mg/L (influent). The P in the
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downstream wastewater from an anaerobic digestion system can be even higher.12 In most
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cases, the aqueous Ca concentration is high enough to precipitate the coexisting phosphate in
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the wastewater.
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However, to achieve efficient precipitation, the wastewater pH needs to be raised above 10.13,
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14
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However, this method has some drawbacks. First, the products achieved by conventional
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chemical precipitation have poor settle ability. Moreover, after precipitation the pH of
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wastewater needs to be reduced by dosing acid, as the typical pH (> 10) required for chemical
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precipitation is higher than the discharge standard (6 < pH < 9).
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Electrochemical induced calcium phosphate precipitation opens door for avoiding such
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problems. In electrochemical systems, a local high pH near cathode can be achieved by water
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electrolysis,14, 17, 18 as shown in eq 1:
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Cathode: 4H 2 O + 4e − → 4OH − + 2H 2 ↑
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The formation of OH− ions will create pH gradients between the cathode and the bulk solution.
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While the pH gradients are not wanted in most electrochemical systems,17 in terms of calcium
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phosphate precipitation, the high local pH can be very useful. As is well-known, the solution
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pH plays a crucial role in the precipitation of calcium phosphate.9, 19 Moreover, the solubility
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of calcium phosphate minerals is pH dependent and a high pH usually means high
The most simple way of increasing wastewater pH is adding sodium hydroxide.15,
16
(1)
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thermodynamic driving force for calcium phosphate precipitation.19
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Anode: 2H 2 O → 4H + + O 2 ↑ + 4e −
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In the electrochemical system, although the local pH is increased, the changes of pH in bulk
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solution is small. This is because an equal number of H+ and OH− are produced at the anode
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and cathode, respectively (eq 1 and eq 2). Moreover, the presence of buffers such as inorganic
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carbon in wastewater may contribute to a stable bulk solution pH.20 Hence, a post-reduction
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of pH of bulk solution is not needed. Furthermore, as the precipitation only takes place in the
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vicinity of the cathode and the formed precipitates can easily be collected from the cathode,14
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a post-separation of precipitates from the bulk solution is avoided as well. Therefore, in
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principle, P can be removed and separated simultaneously from the wastewater without
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dosing chemicals in the electrochemical system.
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Proof of principle of electrochemical induced phosphate precipitation has been demonstrated
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by using well-defined solutions for both struvite
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study indicated that P could precipitate as calcium phosphate on cathode in a wide pH range,
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even under acidic conditions (i.e., pH 4.0).14 The effects of essential operation conditions and
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water matrices on this process were studied as well.20, 22 It was found that the presence of
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natural organic matter is beneficial to the removal of P.22 However, the efficiency of this
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system in raw wastewater is not fully addressed yet.
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Given the complexity of wastewater composition, it is likely more than calcium phosphate
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will precipitate. To reduce the formation of unwanted species, we need to understand the
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precipitation mechanism, not only of calcium phosphate but also of associated byproducts.
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Particularly, the relative precipitation tendency among products and byproducts is of great
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importance. If such precipitation sequence exists, we may be able to selectively precipitate P.
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In case of simultaneous precipitation and the absence of a precipitation sequence,
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pretreatment may be needed.
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The objective of this study is to evaluate the performance of electrochemical P removal and
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recovery in raw wastewater. Specifically, we focus on understanding the precipitation
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sequence of all possible solids that may precipitate in the wastewater, which can be important
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in preventing or reducing the formation of unwanted byproducts and recovering P as useful
(2)
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and calcium phosphate
14
. Our previous
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products.
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Materials and methods.
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Materials. The anode was Pt coated Ti disc (Ø 80 mm, thickness 1 mm). Square Ti plate with
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sizes of 6 × 6 cm, 4 × 4 cm and 2 × 2 cm were used as the cathode. The anode and the cathode
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were perpendicularly welded to a Pt-coated Ti rod and pure Ti rod (Ø 3 mm, length 120 mm),
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respectively. Both electrodes were acquired from MAGNETO Special Anodes BV (Schiedam,
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The Netherlands).
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Wastewater sampling and processing. Raw domestic wastewater was collected from the
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influent of a local wastewater treatment plant (Leeuwarden, The Netherlands). After sampling,
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all the wastewater was physically filtered through a combined sieve filter (325 µm) and stored
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in 4°C fridge.
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Wastewater electrolysis. An undivided glass type electrochemical cell with a volume of 1.0
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L was used for all experiments. The electrodes were horizontally located, with the anode at
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the top and the cathode at the bottom of the cell. The distance between electrodes was 30 mm.
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Unless specified otherwise, the electrolysis current was held constant at 8.3 A m−2 using a
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power supply (ES 015-10, Delta Electronika, The Netherlands). After each test, the cathode
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was removed and dried in air at room temperature for one day. The next day, the precipitates
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on the electrode surface were collected by light scraping and then the cathode was cleaned by
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immersion into acidic solution (1.0 M HNO3) for another day. After acid washing, the cathode
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was rinsed with deionized water. The anode was also cleaned by acid weekly.
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Analytical methods. The concentration of cations (Na+, NH4+) and anions (SO42− Cl−, NO3−)
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were measured using ion chromatography (Compact IC 761, Metrohm). Concentrations of Ca,
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Mg, K and P ions were quantified by ICP-AES. Inorganic carbon and total organic carbon
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(TOC) concentration were accessed by a TOC analyzer (Shimadzu). Given that 98% of the IC
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was present as bicarbonate at pH 8.0 (See Figure S1), the initial concentration of bicarbonate
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was expressed as inorganic carbon concentration. The wastewater pH was measured by a
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daily-calibrated pH meter (Metter Toledo). The morphologies of precipitates and the
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corresponding element composition were characterized by scanning electron microscopy
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(SEM, JEOL-6480LV) combined with energy dispersive spectroscopy (EDS, Oxford 4
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Instruments). The samples for SEM-EDS analysis were coated with gold and detected using
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carbon film as the background. Because of this, the carbon and gold content were excluded
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from the element composition. The solid phases of precipitates were analyzed by X-ray
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diffraction (XRD). The phase quantification of solid species in the precipitates was acquired
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by using HighScore Plus program.
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Calculations.
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We used Visual MINTEQ 3.1 (available at https://vminteq.lwr.kth.se/download/) and
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Hydra-Medusa (available at https://www.kth.se/che/medusa/) to calculate the supersaturation
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index (SI) of potential precipitates and the fractions of Ca, Mg, P and N in the wastewater
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versus pH, respectively. The SI is defined as eq 3:
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SI = log ( )
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Where IAP and Ksp refer to the ion activity of the associated lattice ions and the
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thermodynamic solubility product, respectively.
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Results and discussion
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Thermodynamic insights. We previously confirmed that the electrochemical induced
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calcium phosphate precipitation was due to the high pH near the cathode surface.14 While the
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local pH was not recorded, we assumed the local pH near the cathode could range from the
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bulk solution pH 8.0 to as high as 12.0. Such a high local pH was recorded in a biofilm by
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using a micro pH sensor.18 Based on the wastewater composition (Table S1), at least sixteen
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precipitates including calcium phosphate, magnesium precipitates, and carbonate precipitates
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may form and precipitate from the wastewater in response to high local pH, as indicated by
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the SI of these species (Table 1). It should be noted that for the calculations of SI and ion
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fractions the organic contents was not considered. However, it is worth mentioning the
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formation of struvite is not favorable according to the thermodynamic calculations, as struvite
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is undersaturated (SI
