materials Article Nano-Porous-Silicon Powder as an Environmental Friend Marwa Nabil 1, * , Kamal Reyad Mahmoud 2 , Raghda Nomier 3 , El-Maghraby El-Maghraby 3 and Hussien Motaweh 3   Citation: Nabil, M.; Mahmoud, K.R.; Nomier, R.; El-Maghraby, E.-M.; Motaweh, H. Nano-Porous-Silicon Powder as an Environmental Friend. Materials 2021, 14, 4252. https:// doi.org/10.3390/ma14154252 Academic Editor: Avelino Núñez-Delgado Received: 25 June 2021 Accepted: 19 July 2021 Published: 30 July 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 Electronic Materials Researches, Advanced Technology and New Materials Research Institute, City for Scientific, Research and Technology Applications, New Borg El-Arab City 21934, Egypt 2 Department of Physics, Faculty of Science, Kafrelsheikh University, Kafr El Sheikh 33516, Egypt; kamalreyad@gmail.com 3 Department of Physics, Faculty of Science, Damanhour University, Damanhur 22511, Egypt; raghda.mhmd91@yahoo.com (R.N.); maghrabym@yahoo.com (E.-M.E.-M.); prof_motaweh@yahoo.com (H.M.) * Correspondence: marwamoh2000@yahoo.com Abstract: Nano-porous silicon (NPS) powder synthesis is performed by means of a combination of the ultra-sonication technique and the alkali chemical etching process, starting with a commercial silicon powder. Various characterization techniques {X-ray powder diffraction, transmission electron microscopy, Fourier Transform Infrared spectrum, and positron annihilation lifetime spectroscopy} are used for the description of the product’s properties. The NPS product is a new environmentally friendly material used as an adsorbent agent for the acidic azo-dye, Congo red dye. The structural and free volume changes in NPS powder are probed using positron annihilation lifetime (PALS) and positron annihilation Doppler broadening (PADB) techniques. In addition, the mean free volume (VF), as well as fractional free volume (Fv), are also studied via the PALS results. Additionally, the PADB provides a clear relationship between the core and valence electrons changes, and, in addition, the number of defect types present in the synthesized samples. The most effective parameter that affects the dye removal process is the contact time value; the best time for dye removal is 5 min. Additionally, the best value of the CR adsorption capacity by NPS powder is 2665.3 mg/g at 100 mg/L as the initial CR concentration, with an adsorption time of 30 min, without no impact from temperature and pH. So, 5 min is the enough time for the elimination of 82.12% of the 30 mg/L initial concentration of CR. This study expresses the new discovery of a cheap and safe material, in addition to being environmentally friendly, without resorting to any chemical additives or heat treatments. Keywords: microporous materials; positron annihilation spectroscopy; X-ray diffraction 1. Introduction The major threat, for the time being, which must be dealt with on a global level is toxic and carcinogenic environmental pollutants. In particular, the new technologies developed for the easier decolorization of different compound types have attracted widespread interest [1]. Many industries produce residual dyes (i.e., dye intermediates, textile, paper, and pharmaceutical industries, etc.). Wastewater treatment systems have to deal with a wide range of organic pollutants. Pollution with dyes is undesirable, as many of the dyes released are toxic and carcinogenic [2]. In order to remove the wastewater color, several physical and chemical experiments have been performed. Therefore, it was found that the process of de-pigmentation using physical adsorption technology is the most effective and economically appropriate [3]. So, the adsorption technique is one of the best techniques for water reuse, as a result of its economic cost, simple design, ease of operation and non- toxicity [4]. Accordingly, many porous adsorbent materials, such as activated carbon [5], peat, chitin, and silica, are used for testing the possibility of dye removal [6]. However, intraparticle diffusion associated with porous adsorbents may lessen the rate and capacity of adsorption [7]. Therefore, the adsorption process is a surface process; its adsorption Materials 2021, 14, 4252. https://doi.org/10.3390/ma14154252 https://www.mdpi.com/journal/materials