Vol.:(0123456789) 1 3 Environmental Chemistry Letters https://doi.org/10.1007/s10311-018-0711-1 REVIEW Free radical‑assisted electrocoagulation processes for wastewater treatment Zakaria Al‑Qodah 1  · Mohammad Al‑Shannag 2  · Khalid Bani‑Melhem 3  · Eman Assirey 4  · Mohd Adib Yahya 5  · Ali Al‑Shawabkeh 6 Received: 28 July 2017 / Accepted: 29 January 2018 © Springer International Publishing AG, part of Springer Nature 2018 Abstract The combination of electrocoagulation with another process is a promising approach to enhance the removal efciency of water pollutants. For instance, free radical-assisted electrocoagulation is a new combination showing higher performance. There are diferent combinations depending on the free radical source. This article reviews free radical-assisted electroco- agulation processes. We discuss electrocoagulation mechanisms; ozone-assisted electrocoagulation processes; advanced oxidation-assisted electrocoagulation processes; and ultrasound-assisted electrocoagulation. We present kinetic models used in free radical-assisted electrocoagulation, scale-up of free radical-assisted electrocoagulation and cost estimation. The major points are: most of the available studies have been done at laboratory scale with synthetic wastewater, and lack holistic and systematic approaches to consider the process complexity. The performance of the combined process is improved, and the removal efciency is increased especially with ozone-assisted electrocoagulation, which gives a removal efciency of more than 95%. The use of ultrasound energy with electrocoagulation is advantageous in reducing the problem of electrode passivation. Keywords Electrocoagulation · Combined treatment processes · Advanced oxidation · Ultrasonic energy · Ozone-assisted electrocoagulation Introduction In the last two decades, various wastewater efuents have been treated efectively via electrocoagulation process. It was found that such methods have succeeded to remove many pollutants from water such as lignin, phenol, heavy metal ions and anionic contaminants dyes (Al-Shannag et al. 2013, 2014; Bibi et al. 2017; Butler et al. 2011; Daneshvar et al. 2006; Esfandyari et al. 2015; Murugananthan et al. 2004; Naji et al. 2017; Pulkka et al. 2014; Uğurlu et al. 2008; Wang et al. 2009). In addition, electrocoagulation (EC) processes contribute largely in reducing suspended solids (SS), total dissolved solids (TDS), chemical oxygen demand (COD) and biochemical oxygen demand (BOD) levels in wastewater efuents. In particular, many previous studies demonstrated that electrocoagulation (EC) process repre- sents an alternative approach to remove many heavy metal ions from industrial wastewater such as chromium, copper, nickel, arsenic, zinc, manganese, mercury, cadmium, lead, silver, iron and boron (Akbal and Camcı 2011; Al-Shannag et al. 2015; Al Aji et al. 2012; Amarasinghe and Williams 2007; Bazrafshan et al. 2015; Hashim et al. 2017; Heidmann and Calmano 2008; Kartikaningsih et al. 2016; Mahmad et al. 2015; Merzouk et al. 2009; Mouedhen et al. 2009; Nanseu-Njiki et al. 2009; Singh et al. 2006; Wan et al. 2011). * Mohammad Al-Shannag mohammad_al_shannag@hotmail.com 1 Chemical Engineering Department, Al-Balqa Applied University, Amman 11134, Jordan 2 Chemical Engineering Department, School of Engineering, The University of Jordan, Amman 11942, Jordan 3 Department of Water Management and Environment, Faculty of Natural Resources and Environment, The Hashemite University, Zarqa 13115, Jordan 4 Department of Chemistry, Taibah University, Madinah, Kingdom of Saudi Arabia 5 Faculty of Science and Technology, Universiti Sains Islam Malaysia, Nilai, Malaysia 6 Physics and Basic Sciences Department, Al-Balqa Applied University, Amman 11134, Jordan