* Corresponding author. 1944-3994/1944-3986 © 2021 Desalination Publications. All rights reserved. Desalination and Water Treatment www.deswater.com doi: 10.5004/dwt.2021.27455 232 (2021) 39–54 August Optimization of ammonium adsorption from landfll leachate using montmorillonite/hematite nanocomposite: response surface method based on central composite design Hassan Hashemi a , Shima Bahrami a , Zahra Emadi b , Hamideh Shariatipor a , Majid Nozari c, * a Research Center for Health Sciences, Shiraz University of Medical Sciences, Shiraz, Iran, emails: h_hashemi@sums.ac.ir (H. Hashemi), bahramishima31@gmail.com (S. Bahrami), Hamideh.shariaty@gmail.com (H. Shariatipor) b Department of Environmental Health, School of Public Health, Shahrekord University of Medical Sciences, Shahrekord, Iran, email: Zahra.emadi1987@yahoo.com c Department of Environmental Health Engineering, Faculty of Public Health, Kerman University of Medical Sciences, Kerman, Iran, Tel. +98-9383921819; email: nozari.m@kmu.ac.ir Received 26 December 2020; Accepted 18 May 2021 abstract In this study, the adsorption of ammonium ions from landfill leachate (LL) was examined using montmorillonite/hematite nanocomposite (M/HNC). Parameters affecting ammonium adsorp- tion such as pH 3–11, adsorbent dosage 0.05–0.25 g/L, mixing speed 75–175 rpm, and contact time 25–125 min were assessed. The central composite design (CCD)–based response surface meth- odology was employed to design the experiments and find optimal conditions. M/HNC was syn- thesized under green conditions and structurally investigated using X-ray powder diffraction, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller, and scanning electron micros- copy techniques. M/HNC was synthesized with a particle size of about 26 nm, magnetic properties, and a high specific surface area. The efficiency of ammonium adsorption from LL was increased by enhancing parameters such as pH, adsorbent dosage, mixing speed, and contact time. The pH of 8.05, adsorbent dosage of 0.167 g/L, mixing speed of 117.42 rpm, and contact time of 81.99 min was pre- sented as optimal conditions according to the results of the CCD method. In addition, the efficiency of ammonium adsorption from LL using M/HNC was 80% at optimal conditions. The data of ammo- nium adsorption on M/HNC fitted well with pseudo-second-order kinetic and Langmuir isotherm models. It should be noted that the interactive effects of the parameters had an increasing impact on the ammonium adsorption efficiency. Based on the results of the present study, M/HNC had an effective potential for the adsorption of ammonium from LL. Keywords: Ammonium; Landfill leachate; Montmorillonite/hematite nanocomposite; Central composite design 1. Introduction Wastes are recognized as a threat to human survival, living organisms, and natural resources that are necessary for human life [1]. In fact, poor waste management can cause soil, water, and atmospheric pollution [2]. Moreover, urban solid waste can block drains, accumulate stagnant water which spreads insects, and cause floods during the rainy seasons [3]. Precipitation, rainfall, and water flow from solid waste can cause the production of heavy wastewater, which is known as landfill leachate (LL) [4–6]. Depending on the type and density of wastes, age and hydrology of landfill, and climate, LL includes different substances such as heavy