Citation: Marin, N.M. Maize Stalk Obtained after Acid Treatment and Its Use for Simultaneous Removal of Cu 2+ , Pb 2+ , Ni 2+ , Cd 2+ , Cr 3+ and Fe 3+ . Polymers 2022, 14, 3141. https:// doi.org/10.3390/polym14153141 Academic Editor: Alberto Romero Garc í a Received: 2 July 2022 Accepted: 28 July 2022 Published: 2 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Maize Stalk Obtained after Acid Treatment and Its Use for Simultaneous Removal of Cu 2+ , Pb 2+ , Ni 2+ , Cd 2+ , Cr 3+ and Fe 3+ Nicoleta Mirela Marin 1,2 1 National Research and Development Institute for Industrial Ecology ECOIND, Street Podu Dambovitei No. 57-73, District 6, 060652 Bucharest, Romania; nicoleta.marin@incdecoind.ro 2 Science and Engineering of Oxide Materials and Nanomaterials, Faculty of Chemical Engineering and Biotechnologies University POLITEHNICA of Bucharest, Gh. Polizu 1-7, 011061 Bucharest, Romania Abstract: In this research, eco-friendly material represented by maize stalk (MS) obtained after acid treatment was employed for simultaneous removal of Cu 2+ , Pb 2+ , Ni 2+ , Cd 2+ , Cr 3+ and Fe 3+ (M X+ ) from simulated textile aqueous matrix and tannery wastewater produced by the leather industry. The acid treatment of MS was done with 4 M HCl. The influence of experimental parameters was evaluated in order to optimize the adsorption process for simulated textile matrix. The contact time 10–60 min and initial concentration of 0.5–1 mg/L M X+ influence were studied by batch method. Additionally, the adsorption data of M X+ onto MS was fitting by kinetic and isotherm models. The results obtained showed that the 60 min was necessary to reach adsorption equilibrium of the MS. The adsorption capacity of MS was 0.052 mg Cu 2+ /g of MS, 0.024 mg Pb 2+ /g of MS, 0.042 mg Ni 2+ /g of MS, 0.050 mg Cd 2+ /g of MS, 0.056 mg Fe 3+ /g of MS and 0.063 mg Cr 3+ /g of MS at pH = 4.2. The Langmuir model described the adsorption process very well. The MS showed huge selectivity for Cr 3+ and Fe 3+ in the presence of Cu 2+ , Pb 2+ , Ni 2+ and Cd 2+ . The adsorption of M X+ from liquid phases were analyzed by spectrometric adsorption method (AAS). The solid phases of MS before and after adsorption by TG and SEM analysis were characterized. When MS was used for removal of M X+ from tannery wastewater, two major issues were investigated: First, the decrease of M X+ content from highly polluted and difficult to treat tannery wastewaters by improve its quality and in the second part, specific recovery of M X+ from MS mass increasing the economic efficiency of metals production based on green technology. Keywords: copper; lead; nickel; iron; chromium; cadmium; natural polymer; low cost treatment 1. Introduction Nowadays, the textile and leather industry produces significant volumes of wastew- ater with high concentrations of metals. Heavy metals are used in the production of pigments [1]. Over time, the demand for clothing and footwear has increased considerably. Thus, these industries contribute positively to the well-being of mankind but also have a negative effect on the environment if proper treatment operations are not applied [2]. Therefore, the monitoring of organic compounds and metal ions must be done before wastewater is discarded in an environmental aquatic medium [3]. For this, development of environmentally ecofriendly technologies could solve this problem. Biomaterials are a valuable alternative for the retention of metal ions and other organic compounds from effluents resulting from the painting and tannery process, respectively. Recently, several papers reported the capacity of bioadsorbents to remove heavy metals and other pollutants from polluted wastewater [4–11]. Biomaterials have been explored raw and after chemical modifications in order to enhance adsorption proprieties. Modifications of bio- materials can improve adsorption proprieties but can produce a secondary pollution if the dangerous chemical is applied for this aim. Taking into consideration this aspect is necessary to search for chemicals which induce a minimum impact to aquatic lives. Additionally, the most Polymers 2022, 14, 3141. https://doi.org/10.3390/polym14153141 https://www.mdpi.com/journal/polymers