Location: Contaminant Fate and Transport ResearchTitle: Humic acid facilitates the transport of ARS-labeled hydroxyapatite nanoparticles in iron oxyhydroxide-coated sand Author
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2012
Publication Date: 2/9/2012
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2384.pdf
Citation: Wang, D., Bradford, S.A., Harvey, R.W., Gao, B., Cang, L., Zhou, D. 2012. Humic acid facilitates the transport of ARS-labeled hydroxyapatite Nanoparticles in iron oxyhydroxide-coated sand. Environmental Science and Technology. 46:2738-2745. Interpretive Summary: An understanding of factors that influence the transport and fate of colloids, including microorganisms, in aquifers is needed to assess risk of contamination and to develop remediation strategies. The objective of this research was to better quantify colloid retention on variable fractions of iron oxide grain coatings in the presence of humic acid (HA) at several different solution pH. In the absence of HA, colloids had only limited transport capacity due to their tendency to aggregate and to attach. Conversely, the mobility of colloids was greatly enhanced in the presence of HA which stabilized the colloid suspensions and diminished attachment by partially masking positive charges on the solid surfaces. Attachment was further decreased at higher pH conditions due to charge reversal on iron oxide surfaces. This information will be of interest to scientists and engineers concerned with predicting the fate of colloids in soils and aquifers.
Technical Abstract: Hydroxyapatite nanoparticles (nHAP) have been widely used to remediate soil and wastewater contaminated with metals and radionuclides. However, our understanding of nHAP transport and fate is limited in natural environments that exhibit significant variability in solid and solution chemistry. The transport and retention kinetics of Alizarin red S (ARS)-labeled nHAP were investigated in water-saturated packed columns that encompassed a range of humic acid concentrations (HA, 0-10 mg L-1), fractional surface coverage of iron oxyhydroxide coatings on sand grains (', 0-0.75), and pH (6.0-10.5). HA was found to have a marked effect on the electrokinetic properties of ARS-nHAP, and on the transport and retention of ARS-nHAP in granular media. The transport of ARS-nHAP was found to increase with increasing HA concentration because of enhanced colloidal stability and the reduced aggregate size. When HA=10 mg L-1, greater ARS-nHAP attachment occurred with increasing ' because of increased electrostatic attraction between negatively charged nanoparticles and positively charged iron oxyhydroxides, although alkaline conditions (pH 8.0 and 10.5) reversed the surface charge of the iron oxyhydroxides and therefore decreased deposition. The retention profiles of ARS-nHAP exhibited a hyperexponential shape for all test conditions,suggesting some unfavorable attachment conditions. Retarded breakthrough curves occurred in sands with iron oxyhydroxide coatings because of time-dependent occupation of favorable deposition sites. Consideration of the above effects is necessary to improve remediation efficiency of nHAP for metals and actinides in soils and subsurface environments.