Environmental Engineering Reference
In-Depth Information
ConsiderableresearchworkisunderwaytobetterdescribewhatthefateisofENPs
in the atmosphere. The current emphasis is on defining how ENPs interact, and are
retained, adsorbed or absorbed by other suspended particles or by living organisms.
4.2
Water
Information on transport, distribution and fate of ENPs in aquatic ecosystems is
limited, and derives mainly from insights given by colloid chemistry and colloid
movementinaqueoussystems.Hydrophobiccolloidsareinsolubleinwater.Such
colloids are stabilized by their electro-kinetic properties, which depend on their
electrical charge. The magnitude of charge is responsible for the stability and is
referredtoasthezetapotentialasdeinedbytheequation:
= 4 q
D
Where ʶ iszetapotential,qischargeontheparticle, ʴ isthicknessofthezoneof
inluenceofthechargeontheparticle,andDisdielectriccharge.Thezetapotential
is a repelling force that protects cells from coalesces due to intermolecular or inter-
particle forces (i.e.,Van der Waal's forces). This happens when attractive forces
overcome repulsive ones (Sawyer and Mc Carty 1967 ). Guzman et al. ( 2006 )
describedhowENPsaretransportedinanaqueoussystembyusingTiO 2 as model
nanoparticle.TheyconcludedthatwhenmediumpHapproacheszeropointcharge
(pHzpc),therepulsion/zetapotentialbetweennanoparticleshavingasimilarsurface
potentialdecreasesandtheytendtoagglomerate(Guzmanetal. 2006 )(Table 4 ).
Agglomerated nanoparticles have less mobility and induce sedimentation.
ThepresenceofNaturalOrganicMatter(NOM)inluencesthetransportofENPs
in the environment. Humic and fulvic acids and polysaccharides contribute to the
NOM content of aqueous systems. NOM provides a surface for adsorption of
nanoparticles (Table 4 ). Such adsorption changes the surface charge and charge
densityofENPs,andcanaffecttheirwatertransport(Ghoshetal. 2008 ;Guzman
et al. 2006 ; Hyung et al. 2007 ). Keller et al. ( 2010 ) studied the electrophoretic
mobilityofnanoparticles(viz.,TiO 2 , ZnO and CeO 2 )inwaterbodieslikeground-
water,lakes,riversandseawater.Thetransportofthesenanoparticlesdependedon
particlesizeandwasdominatedbythepresenceofNOMandionicstrengthofthe
transportmedium,whereasitwasshowntobeindependentofpH(Table 4 ).
ENPsintheaquaticenvironmentcanaggregate,dissolve,adsorb,orinteractwith
NOM, which may impart a colloidal-like stabilization (Batley and McLaughlin
2010 ). However, the fate of nanoparticles in aqueous systems is not well under-
stood, and will be better elucidated only after much additional and intense investiga-
tion (Moore 2006 ; Wiesner et al. 2006 ). How ENPs behave in the environment
dependsonthefollowingfactors:type,characteristics(sizeandsurfaceproperties)
andprocessusedtomakethenanoparticle,andthephysico-chemicalpropertiesof
thewater(pH,ionicstrengthanddissolvedorganiccarboncontent),inwhichthe
πδ
ζ
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