Page 418 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 7 ENVIRONMENTAL AND SAFETY ISSUES WITH NANOPARTICLES
References nanoscale particles from the CMP wastewater to
reclaim the water [1, 2].
[1] H. Makino: Denchuukenhoukoku, 281040 (1982). Wastewater of nanosized metal colloid, which is
[2] H. Makino: J. Aerosol Res., Jpn., 4(3) (1989). hard to be removed by coagulation–sedimentation
[3] K. Saito: J. Soc. Powder Technol., Jpn., 38(7), 493–502 process, is discharged in such diverse fields as met-
(2001). alworking factory, electronic components factory,
and pigment-manufacturing factory [3]. It is reported
7.2.4 Nanoparticles in wastewater that various trace elements of heavy metal are con-
tained in wastewater discharged from a pulp produc-
The volume of industrial and domestic wastewater is tion plant [4]. A spent emulsion, which contains
increasing significantly year by year with the change nanosized copper colloid, is discharged from plants
in the lifestyle based on mass consumption and mass manufacturing copper cables for electrical industry,
disposal brought about by the dramatic development and the treatment for purification of effluents is
of the economy and industry. Effective advanced examined by the integrated membrane system based
wastewater treatment is required because wastewater on ultrafiltration (UF) and nanofiltration (NF) [5]. A
contains a variety of constituents such as particles, glass company generates the wastewater containing
organic materials, and emulsion depending on the fine clay and glass particles from the grinding
resource. process of glass surfaces during production of CRT
Inorganic nanoparticles are not generally stabilized glass used for TVs and monitors. Separation of fine
in the liquid because they form aggregates of some clay and glass particles by microfiltration (MF)/UF is
sort more or less. For wastewater reclamation and examined in order to treat glass industry wastewater
reuse, these nanoparticles can be removed from the for reuse in the manufacturing process [6].
liquid by the advanced treatment processes such as
membrane filtration following biological treatment 7.2.4.2 Organic wastewater
processes. Organic materials such as macromolecules The colored substances are free from regulatory con-
are regarded as soft nanoparticles judging from their straint of water quality so far because they are not
sizes, in contrast with hard inorganic particles. considered hazardous substances. However, water
color is being recently used as a standard for the judg-
7.2.4.1 Inorganic wastewater ment of the purity in water because the removal of
Chemical mechanical polishing (CMP) is one of the color becomes important for wastewater reclamation
fastest growing processes in semiconductor industry, and reuse.
and it has become an integral part of the state-of-the-art Dye works are scattered across the country as the
fabrication line for the multilayer wiring board of industry with local tradition. The dyehouse effluent is
large-scale integrated circuit (LSI). Besides the semi- discharged in large quantity, and it has extremely com-
conductor devices, CMP is widely applied to the plex composition because it contains not only dye but
magnetic head, the magnetic memory, and the imag- also dyeing aid and finishing agent. In general, dye
ing devices. The process is primarily used for polish- cannot be removed by standard biological treatment
ing the device side of a semiconductor wafer through because of its low environmental biodegradability.
the mechanical downforce of slurry abrasive in com- Dye wastewater is treated by coagulation–sedimentation
bination with chemical oxidation of wafer surface. In and activated sludge processes, and nanoparticles pro-
general, colloidal silica is used as abrasive slurry to duced in the course of the treatment are released into
planarize the oxide wafer surface. Particles in slurry the environment [7].
are highly charged to avoid aggregations between The color is often imparted by organic substances,
particles or between particles and wafer surfaces. predominantly humic substances. Aquatic humic
During the process, large volumes of ultrapure water substances including humic and fulvic acids are a
are consumed to clean the surface of the wafer, which term referring to a broad class of naturally occurring
generates large quantity of CMP wastewater typically mixture of organic compounds, ubiquitous in sur-
having high solid content resulting from slurry face waters, ground waters, and soil pore waters.
abrasive particles of SiO , Al O , or CeO , depending They are a complex mixture of heterogeneous
3
2
2
2
on the nature of the CMP application. The quantity of organic materials in terms of elemental composition,
CMP wastewater generated is expected to increase chemical functionality, and molecular size distribu-
proportionally with the growing needs of the CMP tion since humic substances can be derived from any
processes. As a result, the treatment and reuse of organic materials, including plant and animal debris,
CMP wastewater has become increasingly necessary. microfauna, biowaste, pesticide, and others. The
The CMP wastewater has been generally treated with molecular weights of humic acids range from several
the conventional chemical coagulation–sedimentation thousands to several tens of thousands daltons, and
process, producing large quantity of sludge. those of fulvic acids range from several tens of thou-
Currently, a membrane filtration process coupled sands to several hundred thousands daltons. Because
with chemical pretreatment is used to separate the of this versatility, humic substances are known to
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