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Environmental Processes
Calcium Carbonate Packed Electrochemical Precipitation Column: New Concept of Phosphate Removal and Recovery Yang Lei, Santosh Narsing, Michel Saakes, Renata van der Weijden, and Cees J.N. Buisman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03795 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019
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Calcium Carbonate Packed Electrochemical Precipitation Column: New
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Concept of Phosphate Removal and Recovery
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Yang Lei*,†, ‡, Santosh Narsing †, Michel Saakes †, Renata D. van der Weijden *, †, ‡, and Cees J.
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N. Buisman †, ‡
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† Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900CC
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Leeuwarden, The Netherlands
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‡ Department of Environmental Technology, Wageningen University and Research, P.O. Box
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17, 6700AA Wageningen, The Netherlands
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* Corresponding authors,
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E-mail:
[email protected] (Yang);
[email protected] (Renata)
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ABSTRACT
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Phosphorus (P) is a vital micro-nutrient element for all life forms. Typically, P can be extracted from
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phosphate rock. Unfortunately, the phosphate rock is a non-renewable resource with a limited
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reserve on the earth. High levels of P discharged to water bodies lead to eutrophication. Therefore,
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P needs to be removed and is preferably recovered as an additional P source. A possible way to
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achieve this goal is by electrochemically induced phosphate precipitation with coexisting calcium
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ions. Here, we report a new concept of phosphate removal and recovery, namely a CaCO3 packed
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electrochemical precipitation column, which achieved improved removal efficiency, shortened
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hydraulic retention time and substantially enhanced stability, compared with our previous
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electrochemical system. The concept is based on the introduction of CaCO3 particles, which
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facilitates calcium phosphate precipitation by buffering the formed H+ at the anode, releasing Ca2+,
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acting as seeds, and establishing a high pH environment in the bulk solution in addition to the
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vicinity of the cathode. It was found that the applied current, the CaCO3 particle size, the CaCO3
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bed height, and the feed rate affect the removal of phosphate. Under optimized conditions
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(particle size : 50% removal over 125 days with little maintenance. The specific energy consumptions of this
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system lie between 29 and 61 kWh/Kg P. The experimental results demonstrate the promising
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potential of the CaCO3 packed electrochemical precipitation column for P removal and recovery
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from P-containing streams.
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INTRODUCTION
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How to feed 10 billion people is one of the main challenges of the 21st century. In any way, the
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use of phosphorus (P) fertilizer is crucial. P, as an essential element for all living organisms,
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accounts for 2-4% weight of most dried cells and plays a vital role in fundamental biochemical
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reactions (i.e., gene expression).1 Typically, one adult consumes 35.2 kg phosphate rock per year.2
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In general, P fertilizer can be processed from mined phosphate rock. Unfortunately, phosphate
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rock, as a finite resource, is subject to exhaustion in a few hundred years under the current mining
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and usage rate.3 On the other hand, the use of P fertilizer and other P products have brought much
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P to water bodies, leading to a worldwide environmental problem namely eutrophication.4, 5
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We need to address the P issues being too little as a resource (fertilizer) yet too much as a pollutant
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(eutrophication) by recycling the P in waste streams.4, 5 Pioneers have recognized the importance
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of P recycling and developed many approaches for achieving this goal.6, 7 While each approach has
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its merits and drawbacks, the principle is the same, that is converting P from soluble form to solid
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phase, which can then be separated from waste streams for potential reuse.
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In the realm of P recovery, struvite process is a well-developed method.8 This method has the
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advantage that phosphate (PO4-P) and ammonium (NH4-N) are removed simultaneously. Also, the
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recovered product can be used as a slow release fertilizer. However, the struvite process needs
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well-controlled solution conditions,9 which typically means a high phosphate concentration, a Mg:
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NH4: PO4 molar ratio close to 1:1:1, and pH between 8 and 9.10 As such, due to the low Mg2+
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concentration relative to PO43− and NH4+ in most nutrient-rich waste streams, the dosing of a Mg
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source is required.10 The struvite process is used in practice but is not widely adopted.
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Alternatively, P removal and recovery as calcium phosphate was proposed.11 This process has the
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advantage that the addition of Ca is usually not necessary as Ca2+ is an abundant ion in most waste
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streams.12, 13 It is worth mentioning that even when Ca addition is required, the cost of dosing Ca
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is cheaper than dosing Mg.14, 15 Moreover, calcium phosphate is the key component of mined
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phosphate rock and therefore, it can be used directly as a raw material for producing P fertilizer in
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the existing production process.12 Furthermore, calcium phosphate has also recently been shown
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to be an effective fertilizer when compared to conventional fertilizers and struvite.16
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Electrochemical processes have gained increasing interest as next-generation wastewater
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treatment technologies in the last decades.17, 18 On top of wastewater treatment; electrochemical
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methods also show excellent potential in resource recovery from waste streams.19,
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coauthors established and validated an electrochemical approach for P removal and recovery.13,
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21, 22
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of wastewater, and no need for a solid-liquid separation process.13,
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membrane-free system. However, the P removal efficiency in this system is low, and the retention
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time is long. For example, to reach 50% removal efficiency, a run-time of about 24 h at 20 mA
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(current density of 4.0 A/m2) is needed.23
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One of the critical reasons for the low removal efficiency and long retention time in this
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membrane-free electrochemical approach is the recombination of anode produced protons (H+)
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with cathode generated hydroxide ions (OH−). While the H+-OH− recombination can be avoided by
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using ion exchange membranes, the introduction of membranes will bring other problems, such
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as a complex configuration, the fouling of membranes, the associated maintenance effort, and the
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increased operating cost.
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Here, in this paper, we propose a simple yet highly efficient approach to overcome direct H+-OH−
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recombination. In our approach, we fill a column-shaped electrochemical precipitation reactor
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with calcium carbonate (CaCO3) particles. The CaCO3 particles, which are in contact with/or close
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to the anode, react with electrochemically produced H+,24 and thus limit the neutralization
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between H+ and OH−. Additionally, two extra benefits are achieved. Along with the consumption
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of H+ by CaCO3 particles, Ca2+ ions are released into the bulk solution. Meanwhile, the
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electrochemically generated OH− ions are accumulated in the bulk solution. As a result, a high pH
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environment can be established in the bulk solution as well as in the vicinity of the cathode.24
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Therefore, homogenous precipitation of calcium phosphate may occur in the bulk solution, in
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Lei and
This approach showed advantages: no need to dose calcium source, no adjustment of the pH
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Moreover, it is a
Environmental Science & Technology
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addition to its precipitation on the cathode surface. Furthermore, the CaCO3 particles may work
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as crystallization seeds, allowing calcium phosphate nucleation and growth on their surface at a
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much lower driving force and induction time.25
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The goal of this research is to identify the possibility, the efficiency, and the mechanism of the
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CaCO3 particles packed electrochemical precipitation column towards phosphate removal and
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recovery. We first looked at the possibility of electrochemically splitting of CaCO3 particles. We
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then explored the effects of particle size, feed rate, and electrical current on the removal of
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phosphate in this system. Additionally, we examined the feasibility of this system for treating low
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P-containing streams and real domestic wastewater and further evaluated the stability of this
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system for long-term operation. The CaCO3 packed electrochemical precipitation column may
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offer a robust yet efficient approach towards P removal and recovery from various wastewaters.
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MATERIALS AND METHODS
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Materials. Na2HPO4·2H2O, NaOH and Na2SO4 were purchased from VWR chemicals (Belgium).
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Ca(NO3)2·4H2O was received from Merck (Germany). The CaCO3 particles were supplied by a
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drinking water company where CaCO3 solids were produced from the water softening process.
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These particles were fractioned through mesh sives as
