The Mechanism And Influence Factors Of Carbon Nanotube Stabilization In And Removal From Water

Carbon nanotubes(CNTs) will be inevitably released into the aqueous environment during manufacturing,use and/or disposal.Increasing studies show the toxicity of CNTs,which raises concerns over their fate and transport in the water.Stabilization and removal are two key processes determining the fate and transport of CNTs in the aqueous environment.Therefore,this paper was aimed to study:(1) stabilization of multiwalled carbon nanotubes(MWNTs) in eight types of fresh surface waters;(2) interaction of CNTs with tannic acid(TA,as a surrogate of natural organic matter) and its role for CNT stabilization in water;(3) removal efficiencies of dispersants-stabilized CNTs from water by.two commonly used coagulants-poly aluminum chloride(PAC) and KAl(SO₄)₂·12H₂O.The main results are as follows:1.MWNTs in particulate aggregates could not be stabilized in any of the water samples by shaking,but could be stabilized in one type of water sample with a relatively high content of total organic carbon by sonication.The stability of surfactant-facilitated MWNT suspensions in the water samples depended on the types of surfactant and water.Sodium-dodecyl-benzenesulfonate(SDBS) and polyethylene-glycol-octylphenyl-ether(TX100) stabilized MWNTs could partly suspended in the water samples.For cetyltrimethyl-ammonium-bromide(CTAB) facilitated MWNT suspension, all the water samples used could destabilize the suspended MWNTs.Negatively-charged anions and/or colloids in the waters were speculated to precipitate the CTAB-stabilized MWNTs.2.TA could strongly adsorb onto the surface of CNTs.At pH＞5,the adsorbed TA could stabilize CNTs by electrostatic repulsion and steric repulsion;whereas at pH＜5,adsorbed TA molecules could bridge and precipitate suspended CNTs.Cationic ions(Na⁺,Mg²⁺,Ca²⁺,or La³⁺) could aggregate the stabilized CNTs by electric double-layer compression,with critical coagulation concentrations exponentially correlated to ionic valence,consistent with the Schulze-Hardy rule.3.PAC had a better removal rate for the TA-,humic acid- and SDBS- stabilized CNTs than KAl(SO₄)₂·12H₂O,but the removal rate for TX100-stabilized CNTs was poor than KAl(SO₄)₂·12H₂O.Both coagulants could not effectively remove the CTAB-stabilized CNTs.The removal of CNTs by PAC was a process of cohesion and flocculation involving charge neutralization,electrical double-layer compression and bridge effect.However,the removal of CNTs by KAl(SO₄)₂·12H₂O was a process of cohesion only in which electrical double-layer compression played the main role.