The change of representative elementary volume of DNAPL influenced by surface active agents during long-term remediation period in heterogeneous porous media

The effectivity and applicability of a novel sugar-based anionic and nonionic Gemini surfactant synthetized for the perchloroethylene-contaminated groundwater remediation

Surfactant-enhanced aquifer remediation (SEAR) has effectively removed dense nonaqueous phase liquids (DNAPLs) from the contaminated aquifers. However, restricted by structural defects, typical monomeric surfactants undergo precipitation, high adsorption loss, and poor solubilization in aquifers, resulting in low remediation efficiency. In this study, a novel sugar-based anionic and non-ionic Gemini surfactant (SANG) was designed and synthesized for SEAR. Glucose was introduced into SANG as a non-ionic group to overcome the interference of low temperature and ions in groundwater. Sodium sulfonate was introduced as an anionic group to overcome aquifer adsorption loss. Two long-straight carbon chains were introduced as hydrophobic groups to provide high surface activity and solubilizing capacity. Even with low temperature or high salt content, its solution did not precipitate in aquifer conditions. The adsorption loss was as low as 0.54 and 0.90 mg/g in medium and fine sand, respectively. Compared with typical surfactants used for SEAR, SANG had the highest solubilization and desorption abilities for perchloroethylene (PCE) without emulsification, a crucial negative that Tween80 and other non-ionic surfactants exhibit. After flushing the contaminated aquifer using SANG, > 99 % of PCE was removed. Thus, with low potential environmental risk, SANG is effectively applicable in subsurface remediation, making it a better surfactant choice for SEAR.

Groundwater and soil contamination by LNAPL: State of the art and future challenges

Contamination by Light Non-Aqueous Phase Liquids (LNAPL) represents a challenge due to the difficulties encountered in its underground assessment and recovery. The major risks arising from subsoil LNAPL accumulation face human health and environment, gaining a social relevance also in the frame of a continuously changing climate. This paper reports on a literature review about the underground contamination by LNAPL, with the aims of providing a categorization of the aspects involved in this topic, analyzing the current state of the art, underlying potential lacks and future perspectives. The review was focused on papers published in the 2012-2022 time-interval, in journals indexed in Scopus and WoS databases, by querying "LNAPL" within article title, abstract and/or key words. 245 papers were collected and classified according to three "key approaches" -namely laboratory activity, field based-data studies and mathematical simulations- and subordinate "key themes", so to allow summarizing and commenting the main aspects based on the application setting, content and scope. Results show that there is a wide experience on plume dynamics and evolution, detection and monitoring through direct and indirect surveys, oil recovery and natural attenuation processes. Few cues of innovations were found regarding both the use of new materials and/or specific field configuration for remediation, and the application of new techniques for plume detection. Some limitations were found in the common oversimplification of the polluted media in laboratory or mathematical models, where the contamination is set within homogeneous porous environments, and in the low number of studies focused on rock masses, where the discontinuous hydraulic behavior complicates the address and modeling of the issue. This paper represents a reference for a quick update on the addressed topic, along with a starting point to develop new ideas and cues for the advance in one of the greatest environmental banes of the current century.

Enhanced reactivity and mechanisms of mesoporous carbon supported zero-valent iron composite for trichloroethylene removal in batch studies

Ordered mesoporous carbon (CMK-3) supported nanoscale zero-valent iron (nZVI) composites were synthesized and used for the removal of trichloroethylene (TCE). The nZVI/CMK-3 composites exhibited high TCE removal efficiency in a batch study, which was 2.5 times that of nZVI alone. They also displayed excellent reusability, with 65.2% removal efficiency after three treatments. Dechlorination dominated the process of TCE removal (75.3%-79.4%), whereas adsorption accounted for 20.6%-24.7%. CMK-3 enhanced the dechlorination rate and efficiency of TCE by nZVI, and the enhancement was favored with the increase in CMK-3 content. The Tafel analysis and H₂ evolution experiments indicated the mechanisms of CMK-3 action in nZVI/CMK-3 composites for TCE removal. CMK-3 serves as a direct electron transfer, whereas CO was identified as the functional group involved; the other involved the acceleration of redox reaction of atomic hydrogen owing to the superior hydrogen adsorption capacity of CMK-3. The present study provides new perspectives for seeking more efficient nZVI to reinforce the dechlorination process; however, more studies are warranted in the long-term performance of nZVI/CMK-3 in the aquifer condition.

Etched glass micromodel for laboratory simulation of NAPL recovery mechanisms by surfactant solutions in fractured rock

Fractured porous media receive less attention than classic porous media in terms of remediation processes and sui` techniques that can be applied efficiently. An etched glass micromodel was built in order to simulate a fractured bedrock. The purpose of this paper was to evaluate the feasibility of surfactant-alcohol injection to recover NAPL with this fractured glass micromodel. The influence of several parameters influencing NAPL recovery via surfactant injection were tested in the micromodel: the ratio of alcohol to surfactant, the total concentration of active matter (alcohol + surfactant), the number of pore volume injected, the direction of the injection, and the continuous or pulsed injection mode. These tests made it possible to identify the key parameters for a better recovery of NAPL in a fractured environment, which are: continuous upward injection, six pore volume of surfactant solution and a n-AmOH/n-BuOH ratio of 2.5. Micromodel experiments were compared to previous reported experiments using the same surfactant solutions injected in classical porous media. The lower capillary number being required for NAPL recovery in porous media is probably related to the better sweep and the increase in surface area available for NAPL dissolution. NAPL recovery may be improved by increasing the capillary number by increasing the injected surfactant solution viscosity with polymer or by injecting foam.