Influence of ionic strength and pH on the limitation of latex microsphere deposition sites on iron-oxide coated sand by humic acid

This study, for the first time, investigates and quantifies the influence of slight changes in solution pH and ionic strength (IS) on colloidal microsphere deposition site coverage by Suwannee River Humic Acid (SRHA) in a column matrix packed with saturated iron-oxide coated sand.Triple pulse experimental (TPE) results show adsorbed SRHA enhances microsphere mobility more at higher pH and lower IS and covers more sites than at higher IS and lower pH. Random sequential adsorption (RSA) modelling of experimental data suggests 1 μg of adsorbed SRHA occupied 9.28 ± 0.03 × 109 sites at pH7.6 and IS of 1.6 mMol but covered 2.75 ± 0.2 × 109 sites at pH6.3 and IS of 20 mMol. Experimental responses are suspected to arise from molecular conformation changes whereby SRHA extends more at higher pH and lower ionic strength but is more compact at lower pH and higher IS. Results suggest effects of pH and IS on regulating SRHA conformation were additive.

► We quantified the coupled role of pH & IS and humic acid on colloid deposition.
► Humic acid enhances microsphere mobility more at higher pH and lower IS.
► pH and IS may control the behaviour of humic acid by regulating its conformation.
► The effect of pH and IS on regulating humic acid conformation is additive.