Enhanced struvite production via membrane capacitive deionization and electrolysis: interference of humic acid and calcium

This study examined struvite crystal formation for nutrient recovery in the presence of coexisting cations and organic substances. Coupled membrane capacitive deionization (MCDI) with sacrificial magnesium anode electrolysis was performed to produce struvite crystals from synthetic wastewater containing ammonium, phosphate, sodium, calcium, and humic acid (HA). In this way, MCDI was employed to adsorb coexisting cations onto the activated carbon (AC) electrode and retain <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>NH</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> ions in the effluent. The MCDI effluent was the input for Mg-based electrolysis, which used an AZ31B Mg alloy as the anode to release Mg²⁺ ions. Various initial Nitrogen/Phosphate (N/P) molar ratios were prepared (i.e. 1:1, 5:1, and 10:1) in the absence and presence of 10 mM competitor ions and 100 mg⋅L^-1 of HA. The results showed that MCDI can retain <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>NH</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> approximately 84.19 and 78.07%, respectively, which is equivalent to the ion electrosorption capacity of the AC electrode, approximately 15.53 mg⋅g^-1 for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>NH</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> and 21.45 mg⋅g^-1 for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math>. Interestingly, the addition of co-ions, such as Na⁺ and Ca²⁺, can remain 2-5 times <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>NH</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> in the effluent. After Mg-based electrolysis, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>NH</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> ions increased by approximately 87.32% and 63.65%, respectively. In the presence of Na⁺, coupling MCDI with the electrolysis process produced up to 0.6195 g of high-purity struvite. Conversely, HA and Ca²⁺ significantly reduced crystal purity and altered morphology. Under an N/P ratio of 10:1, without co-ions or humic acid, the energy consumption for struvite recovery was 15.88 Wh g^-1.