The Use of Response Surface Methodology to Determine Governing Factors Driving a Nanodiamond-Based Wastewater Treatment Method Erick Butler , Oliver Mulamba, Summer Webb, and Andy Pimentel School of Engineering, Computer Science and Mathematics, West Texas A&M University, Canyon, TX 79015; erick.ben.butler@gmail.com (for correspondence) Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12929 The purpose of this study was to determine and character- ize the factors involved in the treatment of wastewater con- taining an azo dye, Congo red, using nanodiamond (ND). A Box Behnken study was developed by the authors that con- sidered the dye wastewater dilution, catalyst dose, and light source. Statistical analysis, XRD, and SEM analysis con- firmed that ND loses its effectiveness when the concentration is below 0.025 g/100 mL of Congo red. It was also deter- mined that at these catalyst doses ND is primarily driven by adsorption, confirmed by observing high treatment efficiency within 30 min of each experimental run. While one cannot absolutely confirm this is the case at much higher doses of ND, it was clear that adsorption plays a major role in color removal by the carbon allotrope. An additional EDS analysis shows that ND specifically adsorbs of sodium, lead, chlorine, and sulfur from Congo red. V C 2018 American Institute of Chemi- cal Engineers Environ Prog, 00: 000–000, 2018 Keywords: response surface methodology, wastewater treatment, nanodiamond, Congo red INTRODUCTION Humans have significant contact with dyes everyday. This is very evident when reviewing the constituents present in food, beverages, and clothing. Unfortunately, the production of these substances has led to high volumes of wastewater, which some have estimated that nearly 20% of it is directly discharged into the environment [1]. The effects of this discharge can impact both human health and the environment. Dyes create deleteri- ous effects to the human body because they can be toxic and carcinogenic (cancer-causing) [2]. The effects on the environ- ment range from increasing pollution, develop euthrophication on a water body, or damage an ecosystem [3]. Literature has cited many treatment methods to handle these types of pollutants. An incomplete list includes coagu- lation, flocculation, membrane processes, ion exchange, and oxidation [4]. One of the more recent treatment methods in at least the last 20 years is heterogeneous photocatalysis. Heterogeneous photocatalysis is a process by which a semi- conductor comes into contact with a light source at a given wavelength forming ions at the surface of the substance that are capable of degrading pollutants within water [5]. Prior to describing the process, it is important to first describe the mechanisms that make-up a semiconductor. Understanding the structure of the semiconductor is important to because it helps the research determine the conditions necessary to degrade pollutants within a wastewater. Semiconductors are divided into bands of varying energy levels. There are two categories of energy levels within a semiconductor—the valence band and the conduction band [6]. During photocatalysis, energy from an incoming light source excites electrons and moves them from the lower energy valence band to the higher energy conduction band. Subsequently, the loss of a negatively charged electron (e – ) generates an electron hole (h 1 ) with a positive charge in the valence band [6]. The promoted electrons and newly formed electron holes can effectively move to the surface of the cat- alyst. Provided that these entities remain separate, these components can initiate oxidation-reduction reactions. The electrons reduce molecular oxygen that is trapped at the sur- face of the semiconductor forming superoxide anions (O – 2 ). However, the electron holes oxidize hydroxyl anions to form hydroxyl radicals (OH – ). The presence of O – 2 and OH – is suf- ficient for degrading constituents within wastewater [1,6]. One must also note that the spacing between energy levels (also known as band gap) is dependent on the semiconductor. This is important to note because the photoenergy necessary to move an electron from one band to the next will vary. For example, titanium dioxide has a band gap of 3.2 eV, which means that ultraviolet light will be required to initiate this pro- cess [1,6]. However, using ultraviolet light can be dangerous for the operator so many have suggested modifying the band gap by integrating metals and nonmetals into a semiconductor. The integration of these metals provides new energy levels which effectively reduce the energy required for movement of elec- tron and the formation of electron holes. In many cases, visible light can be substituted for the use of ultraviolet light [7]. In the past authors have studied the impact of heteroge- nous photocatalysis on the treatment of dye wastewater [8]. This is because dye wastewater provides a medium for an effective mechanism to study within a laboratory. It is also a great way to identify the effectiveness of a semiconductor or any other substance to be an effective photocatalyst. But photoctalysis is not limited to dye wastewater treatment. A quick search of the literature has shown a study of degrading 2-chlorophenol [9], pesiticides [10], phenols [11], and herbi- cides [12] to name a few. Additional Supporting Information may be found in the online ver- sion of this article. V C 2018 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2018 1 Published online 25 June 2018 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12929 and sulfur from Congo red. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 246–253, 2019 246 January/February 2019 Environmental Progress & Sustainable Energy (Vol.38, No.1) DOI 10.1002/ep