Basically, the humic acids (HAs) from different soils were isolated and further purified according to an alkaline extraction method from the International Humic Substance Society (IHSS) (Xu et al., 2011). Briefly, the dried soil was pre-treated with a 1 M HCl solution to separate the supernatant (i.e., fulvic acid) from the soil, followed by the addition of 0.1 M KOH under N2 purging and 0.3 M K+, as KCl. Then, the supernatant was acidified to pH of 1.0 using 6 M HCl to precipitate HAs, which were pelleted by centrifugation and then suspended in a 0.1 M HCl/0.3 M HF solution overnight for five times to minimize ash content. After HF digestion, Milli-Q water was used to wash HA with the purpose of minimizing ions. HA characterization data were previously reported elsewhere (Fujitake et al., 2012) and on IHSS website (http://humic-substances.org/13c-nmr-estimates-of-carbon-distribution-in-ihss-samples/). 

The HA-groundwater suspension batch experiments were conducted basically similar to a previously reported procedure (Xu et al., 2014). In brief, 5-6 mg of the purified HAs were pre-equilibrated in artificial groundwater in the centrifuge tubes for 48 h at room temperature (20 ºC) to reach the dissolution equilibrium, since the HAs were extraceted under pH < 1 but the pH of artificial groundwater is 5.5. Then, ~50 Bq of each gamma emitting radionuclide, including 234Th, 237Np-233Pa, 210Pb, 7Be and 59Fe was added to the HA-groundwater slurry to a final volume of 4 mL. The radiolabeled HA-groundwater slurry was then mixed continuously for 7 days in the dark with an end-over-end mixer to ensure that quasi-equilibrium was attained15. After one-week period, particulate (>0.45 µm), colloidal (3 kDa to 0.45 µm), and truly dissolved (<3 kDa) phases were size fractionationated by using 0.45 µm centrifugal filter tubes followed by ultrafiltration with 3 kDa Microsep centrifugal filter tubes (Millipore). Each fraction was collected for the measurement of radionuclide activity and organic matter concentrations. All the size fractions, including the particulate, colloidal and truly dissolved phases were corrected to the same volume and geometry for the counting of 234Th, 233Pa, 237Np, 210Pb, 7Be and 59Fe activity concentrations by a Canberra ultrahigh purity germanium well gamma detector.

Concentrations of organic carbon and nitrogen in the colloidal and truly dissolved fractions were determined using a Shimadzu TOC-L analyzer. The organic carbon and nitrogen concentrations in the particulate phase of the HA-groundwater suspension were calculated as the difference between the total carbon/nitrogen contents of the added HAs and the sum of colloidal and truly dissolved phases.