**1. Introduction**

Steam injection is an important remediation technology that can effectively remove organic pollutants during site remediation [1–3]. In particular, steam injection technology can remediate contaminated sites containing multiple volatile or semi-volatile organic pollutants [4]. The remediation efficiency of steam injection technology primarily depends

**Citation:** Liu, R.; Yang, X.; Xie, J.; Li, X.; Zhao, Y. Experimental Investigation on the Effects of Ethanol-Enhanced Steam Injection Remediation in Nitrobenzene-Contaminated Heterogeneous Aquifers. *Appl. Sci.* **2021**, *11*, 12029. https://doi.org/10.3390/ app112412029

Academic Editor: Bing Bai

Received: 1 November 2021 Accepted: 24 November 2021 Published: 17 December 2021

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**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/).

on the permeability of the stratum medium [2,5,6]. The volatility of organic compounds in water is closely related to temperature [7–10]. However, the transmission of steam may be limited by the soil particle size, resulting in a reduction in remediation efficiency [11,12]. In general, the remediation efficiency is the highest in homogeneous sand because of the large particle size and non-layered nature of the soil. Many researchers have used homogeneous sand to analyze the characteristics of steam injection [4,13,14]. However, a heterogeneous stratum structure is quite common at actual remediation sites [15,16]; thus, the impact of heterogeneous strata on the remediation effect should be considered.

Some studies have shown that the heterogeneity of the stratum can lead to unsatisfactory remediation effects [2]. Even in sandy soils, the heterogeneity of soil also affects the remediation effect. At present, systematic research on the migration, distribution, and remediation effects of steam in heterogeneous aquifers is lacking. In addition, only a few studies exist on the enhanced steam remediation of heterogeneous aquifers.

In the present study, a combination of thermal technologies such as steam injection with thermal conduction heating (TCH) [17] or steam injection with electrical resistance heating (ERH) was adopted to improve the remediation efficiency of steam injection in heterogeneous layers [18], with the aim of using steam injection in high-permeability zones and TCH or ERH in low-permeability zones. However, these methods are more expensive and complicated. Therefore, a method for strengthening steam injection to repair heterogeneous aquifers is needed.

Chao et al. proposed the addition of alcohol in the repair process of air sparging (AS) to enhance remediation efficiency [19]. N-pentanol and n-heptanol were selected as strengthening reagents as both could enhance the removal efficiency of organic compounds; however, the effect of n-heptanol was stronger than that of n-pentanol. The key mechanism is that alcohol molecules can interact with organic compounds at the interface, resulting in increased organic compound volatilization. Based on the theory of enhanced volatilization, it is assumed that alcohols can enhance volatilization more than surfactants [19,20]. Volatility is also a key factor in the process of steam injection and remediation of organic pollutants, which also determines the increased volatilization of organic compounds [21]. According to some studies, alcohol can be used as a surfactant to reduce the surface tension of the solution [22,23] and thus can be used to enhance the volatilization of organic compounds. Previous studies have shown that the addition of ethanol, n-propanol, and n-butanol can reduce surface tension and enhance mass transfer [24]. This study aimed to explore the effects of these alcohols on organic compounds as strengthening reagents.

During steam injection remediation, the removal rate of organic compounds increases with an increase in temperature, and the remediation efficiency increases significantly at boiling point [8,25,26]. Some organics can form azeotropes with water at temperatures lower than their boiling point [27,28]. Therefore, a lower azeotropic temperature can quickly remove organic matter. As alcohol and water can form azeotropes, this study investigated which alcohol could better reduce the azeotropic temperature of organic compounds using the common semi-volatile organic compound nitrobenzene (NB) in contaminated sites as the target pollutant.

Alcohol-enhanced steam injection has rarely been studied, therefore, this study investigated the effects of alcohol on the remediation of heterogeneous aquifers. The effects of the four alcohols (ethanol, n-butanol, n-pentanol, and n-heptanol) on the surface tension and azeotropic temperature of NB were studied. Heterogeneous layers with different permeability ratios (R) were selected to compare the temperature distribution and removal efficiency of organic pollutants under different permeability ratios. The migration and distribution mechanisms of steam in a lens heterogeneous aquifer were also studied. In addition, the effect of alcohol-enhanced remediation was studied for a stratum with a weak remediation effect. This study aims to select an alcohol to enhance the effect of steam injection to remediate contaminated heterogeneous aquifers, thus extending the application of steam injection technology.
