Hydroxyapatite And Hydroxyapatite-Facilitated Copper Transport In Quartz Sand Column

Featuring unique electronic, medical, mechanical, and chemical properties,engineered nanomaterials are ideal candidates for a multitude and potential application.Because of their strong ability to fix metal ions, hydroxyapatite nanoparticles （nHAP）,have been successfully applied for the remediation of contaminated soil and purification ofwastewater polluted by metal ions such as Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, and Co²⁺. However,little attention has been paid to their potential environmental risks as they are applied forremediating contaminated soils or waters. Specially, nHAP can alter the transport and fateof these contaminants, such as Cu, by dramatically affecting their distribution amongmobile and immobile phases.Firstly, quartz sand was selected as collector and saturated packed columns wereconducted to investigate the effects of solution chemistry on the transport behavior ofnHAP in water-saturated sand, such as varying humic acid concentration, pH, ionicstrength, ionic composition, and Cu concentration of bulk solution through measuring zetapotential （ζ-potential） and representative Ci/C0of nHAP （Ciand C0are effluent andinfluent nHAP concentration, respectively）. The transport behavior of nHAP was found todepend on the solution chemistry of bulk solution. It was suggested that ζ potentials ofnHAP became more negative with increasing humic acid concentration and the change insolution composition from0to10mg/L humic acid yielded an increase in the ζ potentialsof nHAP colloids from-15mV to-55mV and a sharp decrease in attachment efficiencyfrom1.0to0.012. This is due to the existence of both electrostatic repulsion and sterichindrance effects due to adsorption of humic acid on nHAP. An increase in bulk solutionpH resulted in a slightly increasing nHAP concentration in the effluent. This is due tohigher ζ-potentials of nHAP at higher bulk solution pHs. The ζ-potentials of nHAP becameless negative with increasing bulk solution ionic strength, ionic composition (monovalentK⁺, and divalent Ca²⁺and Cu²⁺cations), and Cu concentration conditions due to thecompression of the electrostatic double layer of nHAP. That is, the value of ζ-potential ofnHAP decreased from-51mV to-13mV and from-50mV to-22mV when the ionicstrength was increased from1to100mmol/L of NaCl and from0.1to5mmol/L of CaCl2,respectively. Divalent cations (Ca²⁺and Cu²⁺) were significantly more effective atscreening the surface charge of nHAP than monovalent cations, therefore, the value ofCi/C0decreased dramatically with increasing divalent cation concentrations of bulk solution. Ca²⁺bridged the humic acid-modified nHAP colloidal particles, which causesgreater deposition. Moreover, Cu²⁺had a greater effect on the transport behavior than Ca²⁺due to their strong exchange with Ca²⁺of nHAP and their surface complexation withnHAP. The order of effectiveness of the cations in enhancing the deposition of nHAP wasCu²⁺> Ca²⁺> K⁺.Secondly, column experiments were conducted to investigate the effects of velocity,collector size, and iron oxide grain coating on the transport behavior of nHAP in watersaturated conditions. Increasing pore water velocity increased the concentration of nHAPin the effluent. The result is consistent with classic filtration theory. That is, increasingpore water velocity in a porous media decreases the number of collisions occurringbetween a passive colloid and particles or substrates, and should decrease colloiddeposition/retention in the media. Relative effluent concentration of nHAP decreasedwhen the sand median grain size decreased. The result was attributed to increasedstraining of the nHAP colloids; i.e., blocked pores act as dead ends for colloids. When thecolloid size is small relative to the sand pore sizes, straining becomes a less significantmechanism of colloid removal. The iron oxide grain coating was found to have a stronginfluence on the transport behavior of nHAP due to their involvement in the deposition onpatchwise surface with chemical heterogeneity arising from iron oxide grain coating. Therelative effluent concentration of nHAP decreased significantly with increasing fraction ofiron oxide grain coating.In addition, column experiments were performed to investigate the facilitatedtransport of Cu in association with nHAP in packed column at different Cu concentrationand iron oxide grain coating conditions. The effluent concentration of Cu that wasassociated with nHAP decreased with increasing Cu concentration in the bulk solution.The facilitated transport behavior was due to the differences in the capacity of nHAP tobind Cu and the retention of nHAP with changes in bulk solution Cu concentration. Inparticular, iron oxide coated sand, which causes surface chemistry heterogeneity, wasfound to have a dramatic effect on the facilitated transport of Cu with nHAP. That is, thepercentage of Cu associated with nHAP significantly decreased from31.8%to6.3%wheniron oxide coating fraction increased from0to0.36.The results presented above can be used for better predicting the transport and fate ofnanomaterials and contaminants in association with nanomaterials through the subsurfaceand groundwater environments.