Topics in Current Chemistry (2020) 378:7
1 3
Keywords Energy recovery · Hydrogen production · Industrial wastewater ·
Photocatalysis · Technology readiness level · Urban wastewater
1 Introduction
New challenges in wastewater treatment [e.g., the removal of contaminants of
emerging concern (CECs) from municipal wastewater treatment plant effluents],
the increasing interest/demand for sustainable wastewater treatment methods, and a
circular economic approach (e.g., energy saving and recovery, mass recovery from
waste, and water reuse) are leading to the investigation and development of increasingly effective wastewater treatment processes and technologies. The main problem
is related to the presence of non-biodegradable substances (organic and inorganic)
including metals, pharmaceutical compounds, and personal care products, which
persist in water and in the environment, causing serious damage to the ecosystem
and to human health. For this reason, the study of advanced oxidation processes
(AOPs) has increased significantly in recent years. AOPs are characterized by the
presence of highly reactive species able to remove and mineralize refractory organic
compounds, water pathogens, and disinfection by-products [1]. Among these processes, heterogeneous photocatalysis (HPC) has been proven to be effective in the
degradation of a wide range of refractory organic compounds. However, its application in water and wastewater treatment at full scale is far from being successfully
implemented due to the complexity of the various real aqueous matrices as well as
limitations in the process (e.g., low photoconversion efficiency) and technological
limitations (i.e., catalyst preparation method, slurry vs. supported system, reactor
design, energy consumption, etc.). In an effort to improve process efficiency while
minimizing energy cost, the new trend in HPC research deals with the formulation
of semiconductors active in the presence of visible light. In this review these issues
were addressed with regard to the application of HPC in urban and industrial wastewater treatment as well as energy recovery through hydrogen production during
treatment, by explaining and critically discussing possible advantages and disadvantages of the process. The scope was to provide the reader with some information and
tools to understand the current knowledge gaps as well as to evaluate prospective
applications of HPC in the treatment of wastewater.
2 Heterogeneous Photocatalysis: An Overview of Consolidated
and New Photocatalysts as well as Preparation Methods
Heterogeneous photocatalysis relies on the interaction between a light source and
a solid semiconductor in an aqueous matrix. Depending on the emission spectrum
of the light source and the characteristics of the semiconductor (photocatalyst),
electrons are promoted from the valence band to the conduction band of the photocatalyst, thus initiating a surface reaction will ultimately result in the formation of
highly oxidizing agents, such as hydroxyl radicals (
·
OH). The efficiency of the HPC
process in water/wastewater treatment will depend on the capacity of the system to
226
Reprinted from the journal
1 3
Keywords Energy recovery · Hydrogen production · Industrial wastewater ·
Photocatalysis · Technology readiness level · Urban wastewater
1 Introduction
New challenges in wastewater treatment [e.g., the removal of contaminants of
emerging concern (CECs) from municipal wastewater treatment plant effluents],
the increasing interest/demand for sustainable wastewater treatment methods, and a
circular economic approach (e.g., energy saving and recovery, mass recovery from
waste, and water reuse) are leading to the investigation and development of increasingly effective wastewater treatment processes and technologies. The main problem
is related to the presence of non-biodegradable substances (organic and inorganic)
including metals, pharmaceutical compounds, and personal care products, which
persist in water and in the environment, causing serious damage to the ecosystem
and to human health. For this reason, the study of advanced oxidation processes
(AOPs) has increased significantly in recent years. AOPs are characterized by the
presence of highly reactive species able to remove and mineralize refractory organic
compounds, water pathogens, and disinfection by-products [1]. Among these processes, heterogeneous photocatalysis (HPC) has been proven to be effective in the
degradation of a wide range of refractory organic compounds. However, its application in water and wastewater treatment at full scale is far from being successfully
implemented due to the complexity of the various real aqueous matrices as well as
limitations in the process (e.g., low photoconversion efficiency) and technological
limitations (i.e., catalyst preparation method, slurry vs. supported system, reactor
design, energy consumption, etc.). In an effort to improve process efficiency while
minimizing energy cost, the new trend in HPC research deals with the formulation
of semiconductors active in the presence of visible light. In this review these issues
were addressed with regard to the application of HPC in urban and industrial wastewater treatment as well as energy recovery through hydrogen production during
treatment, by explaining and critically discussing possible advantages and disadvantages of the process. The scope was to provide the reader with some information and
tools to understand the current knowledge gaps as well as to evaluate prospective
applications of HPC in the treatment of wastewater.
2 Heterogeneous Photocatalysis: An Overview of Consolidated
and New Photocatalysts as well as Preparation Methods
Heterogeneous photocatalysis relies on the interaction between a light source and
a solid semiconductor in an aqueous matrix. Depending on the emission spectrum
of the light source and the characteristics of the semiconductor (photocatalyst),
electrons are promoted from the valence band to the conduction band of the photocatalyst, thus initiating a surface reaction will ultimately result in the formation of
highly oxidizing agents, such as hydroxyl radicals (
·
OH). The efficiency of the HPC
process in water/wastewater treatment will depend on the capacity of the system to
226
Reprinted from the journal