water Article Removal of Cationic Organic Dye from Aqueous Solution by Chemical and Pyrolysis Activated Ulva lactuca Nour Houda M’sakni 1,2,† and Taghreed Alsufyani 1, * ,†   Citation: M’sakni, N.H.; Alsufyani, T. Removal of Cationic Organic Dye from Aqueous Solution by Chemical and Pyrolysis Activated Ulva lactuca. Water 2021, 13, 1154. https:// doi.org/10.3390/w13091154 Academic Editor: Alicia Ronda Gálvez Received: 16 March 2021 Accepted: 19 April 2021 Published: 22 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. 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/). 1 Department of Chemistry, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia; Nour.h@tu.edu.sa 2 Laboratory of the Interfaces and Advanced Materials (LIMA), Science Faculty, Monastir University, Monastir 5019, Tunisia * Correspondence: Taghreed.alsufyani@tu.edu.sa; Tel.: +966-536324255 † These authors contributed equally to this work. Abstract: Ulva lactuca has been used to remove many toxic substances from industrial wastewater. In the present study we tried to optimize the efficiency of U. lactuca as an adsorbent of methylene blue (MB) in aqueous solution. U. lactuca was chemically treated with sulfuric acid (UL-H) and sodium hydroxide (UL-OH) and by a slow pyrolysis process (carbonization process) at high temperature T = 600 ◦ C (UL-T) and compared to the nonactive Ulva (UL-NA) and the water insoluble substance (UL-WIS). Several spectroscopic analyses were carried out to detect the biosorption mechanisms of Ulva to remove MB in solution. The effects of different parameters on the adsorption process were studied, i.e., pH (2–10), mass concentration (1–10 g L −1 ), and contact time (0–120 min). The results showed that the best adsorption of MB by Ulva was at pH = 8, with 5 g L −1 of biomass at 75 min; the best adsorption capacity was 625.0 mg g −1 for UL-OH, which was able to remove more than 89% of MB compared to UL-T, whose removal rate did not exceed 5%. Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) indicated the presence of oxygenated functional groups with a highly porous surface. The kinetic studies confirmed that the majority of treatments follow the pseudo-second-order type. The mathematical models showed that Langmuir model is favorable to UL-OH, UL-WIS, and UL-NA. According to the experimental results, the primary treatment for U. lactuca is a promising environmentally friendly method and an economical strategy for removing MB from aqueous solution. This method can help address the growing demand for adsorbents used in environmental protection processes and the resultant increase in their price. Keywords: Ulva lactuca; activated biomass; water pollution; cationic dye; adsorption kinetic 1. Introduction The daily use of dyes has caused both environmental and nutritional complications. Environmental pollution is caused by the discharge of industrial wastewater due to the manufacture of plastic, paper, textiles, rubber, cosmetics, leather, food, pharmaceuticals, etc., which creates a large amount of toxic dyes, mutagens, and carcinogens. Biosorption is the most practical technique for recycling industrial wastewater rich in dyes due to its simplicity and economy [1,2]. Consequently, attention has turned to low-cost biomass, which may consist of residues that can be disposed of by other industries, e.g., peanut shells [3], rice husks [4], water hyacinth roots [5], spent bleaching earth extracted with hex- ane [6], nonactivated and activated date pits [7], guava seeds [8], the macroalga Sargassum muticum [9], Parthenium plants [10], bacteria and fungi [11], etc. In recent years, algae have become one of the most used sources as a disinfectant for wastewater rich in dyes and heavy metals [12,13]. In the current study, Red sea Ulva lactuca was selected to be an adsorbent of methylene blue (MB) dye due to its low-cost availability over the season and its fast blooming, which Water 2021, 13, 1154. https://doi.org/10.3390/w13091154 https://www.mdpi.com/journal/water