Co-transport of ferrihydrite–organic matter colloids with Sb(v) in saturated porous media: implications for antimony mobility†

Both iron (Fe) and organic matter (OM) occur ubiquitously in the subsurface environment that draws tremendous attention for its impact on the fate and transport of antimony (Sb). The importance of Fe–OM colloids on the transport of Sb(V) is severely unappreciated. This study provides new insight regarding the mechanisms of FH–OM colloids (ferrihydrite–humic acid (FH–HA)) on the transport of Sb(V) in a water-saturated sand column. Batch experiments in conjunction with characterization show that Sb(V) can bind with FH–HA colloids over a range of pH (3.0–7.5), ionic strength (IS, 1–5 mM NaCl), and HA and FH concentrations. Results show that the transport of FH–HA colloids loaded with Sb(V) is highly dependent on pH and IS. The presence of 1 mg C per L HA or low pH (3.0 and 4.5) significantly hindered FH–HA–Sb(V) transport with most particles being retained on the quartz sand surface. Increasing pH and HA concentrations enhanced the transport of FH–HA colloids and thus promoted Sb(V) mobility because of the increasing electrostatic repulsion. Colloid filtration theory (CFT) calculations show the maximum transport distance (L_0.01 >59.6 m) of colloids under favorable conditions (e.g., HA ≥5 mg C per L at pH ≥6.0), as also reflected by the low attachment efficiency (α <1.2 × 10⁻⁸) and low deposition rate coefficient (k_d <1.3 × 10⁻⁶). Additionally, Derjaguin–Landau–Verwey–Overbeek (DLVO) theory calculations elucidate the interaction energy between colloids and quartz sand, showing higher repulsive energy barriers (54.6 kT for ≥5.0 mg C per L HA) under unfavorable retention conditions. Further, a non-equilibrium two-site model and a two-site kinetic attachment/detachment model successfully captured the breakthrough curves of FH–HA colloids with Sb(V). Collectively, our findings update crucial perception into the importance of FH–OM colloids on the mobility of Sb(V), which is valuable for the management and remediation of Sb-contaminated sites.