Solar-drivenphotocatalyticdecompositionofmicrocystin-LR : from laboratory development to on-site demonstration

Harmful algal blooms (HABs) found in various water bodies worldwide have been a huge concern due to their adverse impacts on human health and ecosystems. In particular, HABs associated with cyanobacteria have been of great interest because of their potential to generate and release biological toxins, especially, lethal microcystins (MCs). The overall goal of this study was to develop a new sustainable approach to decompose MCs, preferably on-site and in real-time with minimal effort, fewer chemicals, and low energy inputs. To achieve the goal, a high efficiency nitrogen-doped TiO2 photocatalytic film immobilized onto a glass substrate was fabricated via integrated sol-gel synthesis employing nitrogen-containing surfactants as pore-templating agent and nitrogen-dopant. The film exhibited visible light-activated, nanoporous, and transparent properties. Effects of surfactant type, calcination temperature, coating layers, and reaction pH on the photocatalytic decomposition of microcystin-LR (MC-LR) were investigated under visible light. Eventually, the TiO2 film was able to successfully decompose MC-LR on-site in a lake under solar radiation in real-time. This study implies the high potential of the TiO2 film for on-site and real-time decomposition of many organic contaminants in water by using sustainable solar energy. doi: 10.2166/ws.2017.047 om https://iwaponline.com/ws/article-pdf/17/6/1722/204836/ws017061722.pdf er 2019 Hesam Zamankhan Malayeri Hyeok Choi (corresponding author) Department of Civil Engineering, The University of Texas at Arlington, Arlington, TX 76019-0308, USA E-mail: hchoi@uta.edu Mallikarjuna Nadagouda Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45324, USA


INTRODUCTION
Contamination of water resources with natural and anthropogenic chemicals has been a huge concern worldwide. Particularly, the increasing occurrence of harmful algal blooms (HABs) alarms water and health authorities and the general public (Nfodzo et al. ). Specifically, HABs associated with cyanobacteria (so-called cyano-HABs) produce and release lethal biological toxins such as microcystins (MCs) (Bownik ). MCs are a group of natural toxins that act as hepatotoxins and promote formation of tumors (Antoniou et al. ). Animal poisoning and fish killing have been reported in conjunction with MCs, resulting in significant economic losses (Svircev et al. ; Bownik ). Among more than 100 MC congeners, microcystin-LR (MC-LR) is the most notorious due to its high toxicity and prevalence and thus the United States Environmental Protection Agency has placed the toxin on the drinking water health advisories (USEPA ).
Particulate algae can be easily removed by conventional water treatment processes but cyanobacterial toxins dissolved in water are usually recalcitrant and hard to remove (Lawton & Robertson ). Many technologies, including activated carbon adsorption, coagulation/sedimentation, membrane separation, and chemical oxidation, have been tested for treatment of MC-LR (Campinas & Rosa ; Li et al. ; Roegner et al. ; Jasim & Saththasivam ). However, these ex-situ treatment approaches benefit only those who directly use treated water. They do not provide a systematic tool to protect the ecosystem in HAB sites. As a result, the overall goal of this study is to develop an on-site (or in-situ) treatment approach for removal of cyanobacterial toxins. Considering many limitations occurring during on-site applications, such a treatment approach should be characterized, if possible, with minimal effort, fewer chemicals, and low energy inputs. Once developed, the approach is also important with respect to source water management.
In order to achieve the goal and to fulfill the requirements, this study proposes to use a high efficiency nitrogen-doped TiO 2 photocatalytic film immobilized onto a glass substrate. TiO 2 photocatalysis is one of the most effective water treatment processes (Choi et al. ; Lazar et al. ; Pelaez et al. ). Strong hydroxyl radicals generated from TiO 2 non-selectively and readily attack and decompose organic contaminants in water including MCs.
The catalytic process does not either require addition of other chemicals or consume TiO 2 materials. However, the only requirement is to irradiate the TiO 2 surface with UV with high photon energy above the band gap of TiO 2 . This greatly inhibits the utilization of solar radiation as a sustain- Meanwhile, for on-site applications of TiO 2 photocatalysis, TiO 2 should be immobilized firmly onto a substrate such as glass. Both TiO 2 film and substrate should also be transparent to improve light penetration and utilization in particular when TiO 2 films are installed and stacked onsite in parallel (Choi et al. ). To exhibit high reactivity and thus to decompose MCs in real-time under solar radiation, the structural properties of TiO 2 films should also be controlled. In particular, a porous structure is beneficial to light absorbance of TiO 2 and accessibility of reactants to TiO 2 (Zakersalehi et al. ). Surfactants and block copolymers as pore templates have been widely used to control the porous structure during sol-gel synthesis of TiO 2 (Bosc et al.

). Use of nitrogen-containing surfactants is interesting.
They can play dual roles as a pore template to make porous TiO 2 and as a nitrogen source to dope the porous TiO 2 with nitrogen (Choi et al. ).
In this study, such a high efficiency nitrogen-doped mesoporous transparent TiO 2 photocatalytic thin film (N-TiO 2 ) immobilized onto a glass substrate was fabricated via an integrated materials synthesis process, employing surfactant template-based sol-gel, dip coating, and calcination. Effects A borosilicate glass with an effective surface area of 10 cm 2 (20 cm 2 for both sides) was dip-coated with the TiO 2 sol by using PTL-MM01dip-coater (MTI Corporation) at a coating rate of 15 cm/min. After coating, the TiO 2 film was dried at room temperature for 1 hr and calcined in a programmable furnace (Paragon HT-22-D, Thermcraft).
Temperature was increased at a ramp rate of 60 W C/hr to 100 W C and maintained for 1 hr, then further increased to different target temperatures at 350, 400, 450, and 500 W C for 2 hr, and then cooled down naturally, forming N-TiO 2 .
To increase the number of coating layers up to seven, the dip-coating and calcination process was repeated. Control

On-site decomposition of MC-LR
To evaluate the field application potential of the TiO 2 film, an on-site test was briefly conducted in Lake Arlington (Arlington, TX), in which HABS have often occurred. The

MC-LR concentration was periodically monitored from
July to October in 2016. However, the level of MC-LR during this period was too low to implement a field test and thus some lake water was removed, placed into a confined area, and spiked with MC-LR to a target of 0.1 mg/L. The volume of the reaction solution was 10 ml.
After installing one TiO 2 film, the whole system was directly exposed to solar radiation for 4 hr without a UV filter, as  Nitrogen content decreased from 6.3 to 3.1% over the temperature increase from 350 to 500 W C because nitrogen is subject to thermal decomposition during the calcination process. The PZC of N-TiO 2 was around 6.2-6.7. When comparing N-TiO 2 (made with DEA) and control TiO 2 prepared at 500 W C, N-TiO 2 had a slightly higher surface area (61.1 m 2 /g) than control TiO 2 (50.1 m 2 /g) and superior porosity (22%) to control TiO 2 (4.0%). In addition, N-TiO 2 contained 3.1% nitrogen while control TiO 2 showed negligible nitrogen (0.5%, most probably from the impurities of the ingredients used as well as from gaseous nitrogen in the air). The properties proved the dual role of DEA as a nitrogen-doping source and a pore-directing agent.

Photocatalytic decomposition of MB under visible light
To     charged due to dissociation of its free carboxylic group while N-TiO 2 with PZC of 6.7 is positively charged, which leads to increased adsorption and decomposition of MC-LR onto N-TiO 2 due to electrical attraction.
The effect of the number of coating layers was also examined, as shown in Figure 5. When the number of coating layers increased from 1 to 3, MC-LR decomposition almost doubled. Above 3, the number of coating layers did not affect the reactivity significantly. In general, multiple coatings bring more TiO 2 loading to the film, resulting in high reactivity. However, the disproportional increase in the reactivity could be explained by the porous structure of inner TiO 2 layers becoming more collapsed during the repeated calcination processes and thus less available for subsequent chemical reaction (Choi et al. ). In addition, a reaction rate-limiting factor might be the low intensity of the visible light used in this study (3.52 mW/cm 2 ), rather than TiO 2 loading.

On-site decomposition of MC-LR under solar radiation
The previous experiments were conducted under laboratory conditions employing MC-LR in pure water and artificial visible light. Subsequently, a field test was undertaken to evaluate whether the TiO 2 film can be activated under solar radiation and thus MC-LR in a real water matrix can be decomposed, as shown in Figure 6. TOC in the lake water, representing NOM as a competing component for

CONCLUSIONS
The overall goal of this study was to develop a new sustainable approach to decompose biological toxins, if feasible, on-site and in real-time with minimal effort, fewer chemicals, and low energy inputs. A high efficiency N-TiO 2 photocatalytic film immobilized onto a glass substrate was fabricated via integrated materials synthesis processing.
The N-TiO 2 film was characterized with a highly porous,  anatase crystal, transparent, and nitrogen-doped nature. All the results proved the dual role of DEA surfactant as a nitrogen-doping source and a pore-directing agent. N-TiO 2 prepared with DEA at 450-500 W C exhibited fast decomposition of MC-LR under visible light due to its compromised properties with respect to surface area, crystal phase, and nitrogen content. N-TiO 2 film was able to successfully decompose MC-LR in a lake containing high NOM under solar radiation. This study implies the high potential of the N-TiO 2 film for on-site and real-time decomposition of organic contaminants in water by using sustainable solar energy.