The Use of Ultrasound for the Electrochemical Synthesis of Magnesium Ammonium Phosphate Hexahydrate (Struvite)

As concern increases over climate change, resource depletion, water, air and soil pollution, loss of biodiversity, the agricultural sector is under tremendous strain. Due to the exponential increase in global population, the world has seen a great rise in agricultural productivity1. With the clear global intensification of the agricultural sector, slurry management has now become a serious problem. Agricultural pollution is mainly due to the use of excess industrial fertilizers, nutrients and pesticides, and the presence of harmful bacteria, viruses and other micro-organisms induced by poor manure management in turns causing pollutants to enter soil and waterways (and airways too)1-4. Consequently, the sector has set some urgent priorities, for example by minimizing storage and encouraging the treatment of wastewater and solid slurries, by reducing energy consumption, and by creating on-site renewable energy systems1. Moreover, most of the agricultural manures contain useful nutrients such as nitrogen (N), phosphorus (P) and potassium (K), and efficiently extracting them to produce added-value products such as the slow-release fertilizer, magnesium ammonium phosphate hexahydrate (MgNH4PO4*6H2O) or struvite5, could reduce significantly environmental impacts and save on commercial fertilizers’ costs. Thus, it is important to implement novel, efficient and clean environmental and energy technologies with the view of recycling important nutrients from agricultural sludges and wastes. There are various methods to extract these important nutrients, such as chemical, biological and physical3,4. This present work highlights the recovery of these nutrients by electrochemical precipitation of struvite in the presence of ultrasound at various ultrasonic frequencies and intensities using magnesium (Mg) electrodes. It is shown that the combination of electrochemistry and ultrasound6 (20 kHz – 1 MHz), also known as sonoelectrochemistry 7-9, can facilitate and accelerate the production of struvite from “model” solutions, as long as the sonoelectrochemical reactor is optimised. Buckwell and E. Nadeu, Nutrient Recovery and Reuse (NRR) in European agriculture. A review of the issues, opportunities, and actions. RISE Foundation, Brussels, 2016. A. Sutton, C.M. Howard, J.W. Erisman, G. Billen, A. Bleeker, P. Grennfelt, H. van Grinsven, B. Grizzetti, (Eds.). The European Nitrogen Assessment. Cambridge University, Press Cambridge. 2011. M. Mehta, W.O. Khunjar, V. Nguyen, S. Tait and D.J. Batstone, Technologies to Recover Nutrients from Waste Streams: A Critical Review, Critical Reviews in Environmental Science and Technology, 45:4, 385-427, 2015. Huang, P. Zhang, Z. Zhang, J. Liu, J. Xiao and F. Gao, Simultaneous Removal of Ammonia Nitrogen and Recovery of Phosphate from Swine Wastewater by Struvite Electrochemical Precipitation and Recycling Technology, Journal of Cleaner Production, 127, 302-310, 2016. Moussa, G. Maurin, C. Gabrielli, and M. Ben Amor, Electrochemical Precipitation of Struvite, Electrochem. Solid-State Lett. 9(6), C97-C101, 2006. Yasui, Acoustic Cavitation and Bubble Dynamics, B.G. Pollet & M. Ashokkumar (Eds), SpringerBriefs, 10.1007/978-3-319- 68237-2, 2018. G. Pollet (Ed), Power ultrasound in electrochemistry: from versatile laboratory tool to engineering solution, John Wiley & Sons, Hoboken, NJ, USA (2012) ISBN: 978-0-470-97424-7. G. Pollet, A short introduction to Sonoelectrochemistry. Electrochem. Soc. Interface 2018, 27, 41–4. G. Pollet, M. Ashokkumar, Introduction to Ultrasound, Sonochemistry and Sonoelectrochemistry; B.G. Pollet, M. Ashokkumar (Eds); SpringerBriefs: Berlin, Germany, 2019; in press.