Photocatalytic Membrane Reactors (PMRs) in Water Treatment: Configurations and Influencing Factors

Abstract

Abstract:The lack of access to clean water remains a severe issue all over the world. Coupling photocatalysis with the membrane separation process, which is known as a photocatalytic membrane reactor (PMR), is promising for water treatment. PMR has developed rapidly during the last few years, and this paper presents an overview of the progress in the configuration and operational parameters of PMRs. Two main configurations of PMRs (PMRs with immobilized photocatalyst; PMRs with suspended photocatalyst) are comprehensively described and characterized. Various influencing factors on the performance of PMRs, including photocatalyst, light source, water quality, aeration and membrane, are detailed. Moreover, a discussion on the current problems and development prospects of PMRs for practical application are presented.

Photocatalytic Membranes in Photocatalytic Membrane Reactors

“…Typical examples are esterification and de-hydrogenation reactions (thermodynamically controlled reactions), in which the removal of water or hydrogen, respectively, increases the reaction yield. The extraction of an instable intermediate through the membrane in a MR might give an improvement of the reaction selectivity; this is the case of partial oxidation reactions and hydrogenation reactions carried out in MRs; also, the selective removal of the reaction products might also improve the downstream processing.In the case of homogenous catalysts, their immobilization in or on a membrane represents an efficient way to achieve the catalyst recovery, regeneration, and reuse in successive catalytic runs [9].The membrane can also define the reaction volume providing a contacting zone for two immiscible phases (e.g., in phase transfer catalysis) [10] avoiding the use of polluting auxiliaries, like solvents, in agreement with the green chemistry principles.The design of the membrane reactor requires a multidisciplinary approach in which different disciplines, like: chemistry, chemical engineering, membrane engineering, and process engineering, give their contribution in order to achieve a synergic combination of the separation and reaction processes that allows optimal performances in terms of productivity and sustainability.When a membrane is combined with a photocatalytic process the MR is indicated as photocatalytic membrane reactor (PMR) [11] and it can be designed in two main configurations, depending from the catalyst confinement [12][13][14]: (i) PMRs with solubilized or suspended photocatalyst; and, (ii) PMRs with photocatalyst immobilized in/on a membrane.Both configurations present specific advantages and limitations depending from the specific application. In a PMR with a solubilized or suspended photocatalyst, a membrane with appropriate molecular weight cut off (MWCO) can be used for the retention of the catalyst in the reactor.…”

“…In the first one, the photocatalytic reaction and membrane separation processes take place in one apparatus, i.e., an inorganic or polymeric membrane is submerged in the slurry photocatalytic reactor. In split-type PMRs the photocatalytic reaction and membrane separation take place into two separate apparatuses, i.e., the photocatalysis module and the membrane module, which are appropriately coupled [12,16]. Besides, on the basis of a different approach, this kind of PMRs can be also classified while considering the position of the light source, which can be: (i) above/inside the feed tank; (ii) above/inside the membrane unit; and, (iii) above/inside to an additional vessel placed between the feed tank and the membrane unit [13].In PMRs with immobilized photocatalyst, the photocatalyst is not solubilized/suspended into the reacting environment, but it is immobilized in/on the membrane, giving a photocatalytic membrane (PM).…”

“…In such a system, the membrane acts both as a selective barrier for the contaminants to be degraded, thus maintaining them into the reaction environment, and as the support for the photocatalyst. PMRs with immobilized photocatalyst are intrinsically integrative-type PMRs, i.e., the photocatalytic and the membrane separation processes take place in the same unit.In general, in the state of the art literature it has been reported that the PMRs with solubilized/suspended photocatalyst permit to achieve higher efficiency when compared to that of the immobilized system, due to the larger active surface area, which guarantees a good contact between the photocatalyst and the pollutants [12,14]. As a consequence, more numerous are the studies of PMRs with solubilized/suspended photocatalyst, in view of large scale applications [17][18][19][20].…”

“…">Operating Parameters and Limits of Photocatalytic Membranes (PMs)Working under proper operating conditions is important to obtain good PMR performance with both solubilized/suspended and immobilized photocatalyst. The most important factors affecting the PMR performance, influencing the photocatalytic process and/or the membrane process, were reviewed by various authors [12][13][14]. These factors can be summarized, as follows.…”

“…When a membrane is combined with a photocatalytic process the MR is indicated as photocatalytic membrane reactor (PMR) [11] and it can be designed in two main configurations, depending from the catalyst confinement [12][13][14]: (i) PMRs with solubilized or suspended photocatalyst; and, (ii) PMRs with photocatalyst immobilized in/on a membrane.…”