Membrane distillation is a process that utilizes differences in vapor pressure to permeate water through a macro-porous membrane and reject other non-volatile constituents present in the influent water. This review considers the fundamental heat and mass transfer processes in membrane distillation, recent advances in membrane technology, module configurations, and the applications and economics of membrane distillation, and identifies areas that may lead to technological improvements in membrane distillation as well as the application characteristics required for commercial deployment.

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Advances in Membrane Distillation for Water Desalination and Purification Applications

Widespread pollution of terrestrial ecosystems with petroleum hydrocarbons (PHCs) has generated a need for remediation and, given that many PHCs are biodegradable, bio- and phyto-remediation are often viable approaches for active and passive remediation. This review focuses on phytoremediation with particular interest on the interactions between and use of plant-associated bacteria to restore PHC polluted sites. Plant-associated bacteria include endophytic, phyllospheric, and rhizospheric bacteria, and cooperation between these bacteria and their host plants allows for greater plant survivability and treatment outcomes in contaminated sites. Bacterially driven PHC bioremediation is attributed to the presence of diverse suites of metabolic genes for aliphatic and aromatic hydrocarbons, along with a broader suite of physiological properties including biosurfactant production, biofilm formation, chemotaxis to hydrocarbons, and flexibility in cell-surface hydrophobicity. In soils impacted by PHC contamination, microbial bioremediation generally relies on the addition of high-energy electron acceptors (e.g., oxygen) and fertilization to supply limiting nutrients (e.g., nitrogen, phosphorous, potassium) in the face of excess PHC carbon. As an alternative, the addition of plants can greatly improve bioremediation rates and outcomes as plants provide microbial habitats, improve soil porosity (thereby increasing mass transfer of substrates and electron acceptors), and exchange limiting nutrients with their microbial counterparts. In return, plant-associated microorganisms improve plant growth by reducing soil toxicity through contaminant removal, producing plant growth promoting metabolites, liberating sequestered plant nutrients from soil, fixing nitrogen, and more generally establishing the foundations of soil nutrient cycling. In a practical and applied sense, the collective action of plants and their associated microorganisms is advantageous for remediation of PHC contaminated soil in terms of overall cost and success rates for in situ implementation in a diversity of environments. Mechanistically, there remain biological unknowns that present challenges for applying bio- and phyto-remediation technologies without having a deep prior understanding of individual target sites. In this review, evidence from traditional and modern omics technologies is discussed to provide a framework for plant-microbe interactions during PHC remediation. The potential for integrating multiple molecular and computational techniques to evaluate linkages between microbial communities, plant communities and ecosystem processes is explored with an eye on improving phytoremediation of PHC contaminated sites.

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The Interaction between Plants and Bacteria in the Remediation of Petroleum Hydrocarbons: An Environmental Perspective

The Ag/AgCl/TiO2 nanocomposite thin films are prepared on the pre-coated SiO2 soda-lime glass substrates by a sol–gel method for depositing TiO2 films, and then loaded with Ag/AgCl nanoparticles (NPs) by an impregnating precipitation photoreduction method. The as-prepared composite thin film exhibits a highly visible-light photocatalytic activity for degradation of 4-chlorophenol (4-CP) in water. The photocatalytic mechanism is proposed on the basis of the fact that the Ag NPs are photoexcited due to plasmon resonance, and then charge separation is accomplished by the transfer of photoexcited electrons from the Ag NPs to the TiO2 conduction band and the simultaneous formation of OH radical and Cl0, which cause the photocatalytic degradation of organic pollutants. The proposed mechanism is further confirmed by the detection of hydroxyl radicals. On the other hand, 4-CP can also be oxidized directly by plasmon-induced h+ (or Ag+) on Ag NPs, thereby accelerating the photooxidized Ag NPs back to their initial state. Therefore, the Ag NPs can be rapidly regenerated and the Ag/AgCl/TiO2 system remains self-stability.

Tom Frauenrath

Papers

Identification and chemical characterization of specific organic constituents of petrochemical effluents.