bioengineering Review Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application Marta M. Rossi * , Edoardo Dell’Armi , Laura Lorini, Neda Amanat , Marco Zeppilli , Marianna Villano and Marco Petrangeli Papini Department of Chemistry, Sapienza, University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy; [email protected] (E.D.); [email protected] (L.L.); [email protected] (N.A.); [email protected] (M.Z.); [email protected] (M.V.); [email protected] (M.P.P.) * Correspondence: [email protected] Abstract: Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents partic- ularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be Citation: Rossi, M.M.; Dell’Armi, E.; converted into non-toxic compounds by indigenous microbial activity. Despite several advantages Lorini, L.; Amanat, N.; Zeppilli, M.; offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and Villano, M.; Petrangeli Papini, M. the difficulty in the management and control of the microbial activity, can affect the effectiveness Combined Strategies to Prompt the of a bioremediation approach. However, those issues can be addressed through coupling differ- Biological Reduction of Chlorinated ent strategies to increase the efficiency of the bioremediation strategy. This mini review describes Aliphatic Hydrocarbons: New different strategies to induce the reduction dechlorination reaction by the utilization of innovative Sustainable Options for strategies, which include the increase or the reduction of contaminant mobility as well as the use Bioremediation Application. of innovative strategies of the reductive power supply. Subsequently, three future approaches for Bioengineering 2021, 8, 109. https:// doi.org/10.3390/bioengineering a greener and more sustainable intervention are proposed. In particular, two bio-based materials 8080109 from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., poly- hydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from Academic Editors: Sabine bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current Kleinsteuber and Bruna Matturro to stimulate biological reactions. Received: 1 July 2021 Keywords: bioremediation; biological reductive dechlorination; chlorinated aliphatic hydrocarbons; Accepted: 29 July 2021 sustainable materials; biochar; polyhydroxyalkanoates; bioelectrochemical systems Published: 3 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Background published maps and institutional affil- Chlorinated aliphatic hydrocarbons (CAHs) are ubiquitous contaminants whose iations. presence in groundwater has persisted for many decades, mainly due to the physical- chemical characteristics of these compounds [1,2]. In particular, CAHs belong to Dense Non-Aqueous Phase Liquids (DNAPLs), for which contamination scenarios differ signifi- cantly from Light NAPLs scenarios, where the separate phase floats at the top of the water Copyright: © 2021 by the authors. table due to its lower density than water [3,4]. Licensee MDPI, Basel, Switzerland. The two major problems associated with aged CAHs-contaminated sites are the This article is an open access article secondary source management and the containment and remediation of the contamination distributed under the terms and plume. Secondary source refers to the presence of the solvent in a separate phase, primarily conditions of the Creative Commons Attribution (CC BY) license (https:// accumulated in the low-permeability layers of the aquifer (clay lens), which work as a creativecommons.org/licenses/by/ source of long-term contaminants [5]. On the other hand, contamination plume refers to 4.0/). prolonged contamination in the same direction of water flow due to the slow and steady Bioengineering 2021, 8, 109. https://doi.org/10.3390/bioengineering8080109 https://www.mdpi.com/journal/bioengineering Bioengineering 2021, 8, x FOR PEER REVIEW 2 of 14 plume refers to prolonged contamination in the same direction of water flow due to the slow and steady release of secondary sources. The distribution of contaminants is highly dependent on the hydrogeological characteristics of the site and concentration levels differing greatly from source to source. Today, in-situ technologies can operate and significantly reduce costs, although detailed site characterization is required. In this context, many improvements have been made in modeling and interpreting the data to support the design and adoption of the right strategy [6]. For the remediation of chlorinated solvents plume, the In-Situ BioengineeringBioremediation2021, 8, 109 (ISB) technologies are growing attention. The reasons for the success2 of 14 are the public support, success on the dissolved contaminants, and a comparatively low cost when it is effective [release7]. If the of secondary presence sources. of degradation The distribution intermediates of contaminants is is highly detected dependent during on the site characterization, thehydrogeological biological characteristicsNatural A ofttenuation the site and concentrationphenomenon levels differingoccurs greatlydue fromto the source to source. presence of specific microorganismsToday, in-situ technologies [8]. canFor operate the andreductive significantly dechlorination reduce costs, although (RD) de- of widespread contaminants,tailed site characterizationsuch as tetrachlor is required.oethane In this (TeCA), context, many perchloroethylene improvements have been(PCE), and trichloroethylenemade (TCE), in modeling in an and anaerobic interpreting theenvironment, data to support theone design of the and adoptionmost widely of the right used strategy [6]. For the remediation of chlorinated solvents plume, the In-Situ Bioremediation approaches is enhanced(ISB) technologies natural attenuation are growing attention. (ENA) The [9,10]. reasons forTh thee intervention success are the public provides support, the addition of an organicsuccess fermentable on the dissolved substrate contaminants, to andproduce a comparatively short-chain low cost whenfatty it acids is effective (acetate [7]. and other volatile Iffatty the presence acids, of degradationVFAs) and intermediates the direct is detected electron during donor, site characterization, the molecular the biological Natural Attenuation phenomenon occurs due to the presence of specific mi- hydrogen [11,12]. Therefore,croorganisms almost [8]. For the any reductive ferm dechlorinationentable substrate (RD) of widespread can be a contaminants, potential suchsource of carbon and hydrogenas tetrachloroethane to stimulate (TeCA), RD, incl perchloroethyleneuding carbohydrates (PCE), and trichloroethylene (sugars), alcohols, (TCE), in oils, an anaerobic environment, one of the most widely used approaches is enhanced natural solids (e.g., bark mulch,attenuation chitin), (ENA) and [9,10 complex]. The intervention compounds provides the(e.g., addition whey of an and organic cellulose) fermentable [13]. The RD is a well-known,substrate step-by-step to produce short-chain reaction fatty. Focusing acids (acetate on and the other sequential volatile fatty RD acids, pathway VFAs) of PCE through TCE, andcis-dichloroethyene the direct electron donor, (cis-D the molecularCE), and hydrogen vinil [11chloride,12]. Therefore, (VC) almost to ethene, any fermentable substrate can be a potential source of carbon and hydrogen to stimulate RD, shown in Figure 1, theincluding efficiency carbohydrates of each (sugars), step alcohols,can be oils,dramatically solids (e.g., bark different mulch, chitin), depending and com- on the environmental conditionsplex compounds and (e.g., the whey microbia and cellulose)l populations [13]. The RDresponsible is a well-known, for the step-by-step reactions [14,15]. If the specificreaction. population Focusing onhas the developed sequential RD pathwayin the aquifer of PCE through (Dehalococcoides TCE, cis-dichloroethyene spp.), the (cis-DCE), and vinil chloride (VC) to ethene, shown in Figure1, the efficiency of each step reaction may proceedcan until be dramatically ethene different production. depending As on with the environmental every technology, conditions and serious the microbial issues may be associated withpopulations ISB, and responsible the following for the reactions situation [14,15]. Ifmay the specific seriously population affect has meeting developed the in the aquifer (Dehalococcoides spp.), the reaction may proceed until ethene production. remediation goals: As with every technology, serious issues may be associated with ISB, and the following • Low efficacy onsituation separate may phase seriously or affect highly meeting adsorbed the remediation fraction goals: (at source); • Substrate availability• Low limitations; efficacy on separate and phase or highly adsorbed fraction (at source); • • Substrate availability limitations; and Incomplete degradation• Incomplete pathways degradation (stall pathways at an (stall