Natural organic matter and ionic strength (CaCl2) affect transport, retention and remobilization of silica encapsulated DNA colloids (DNAcol) in saturated sand columns

In the terrestrial environment, interactions between natural organic matter (NOM) and colloids can lead to the formation of an environmental corona around colloids, influencing their transport behaviour and, ultimately, their ecotoxicity. We used a synthetically designed colloid tagged with DNA (DNAcol) as a surrogate for natural colloids and investigated its transport in saturated sand columns. We varied the concentrations of NOM and ionic strength (CaCl2), to better understand the transport and release of DNAcol in porous media under both steady and transient porewater chemistry conditions. In addition, we aimed to understand the main factors that control deposition and release of DNAcol under tested conditions. To induce transient chemistry, we replaced the in-jection solution containing NOM and/or CaCl2 with Milli-Q water. The results showed that the deposition rate of DNAcol was inversely proportional to the concentration of NOM. The deposition rate increased significantly even under low ionic strength (CaCl2) conditions of tested conditions. Notably, the influence of NOM on the transport of DNAcol was most pronounced at the lowest range of [Ca2+]/DOC ratios, and the attachment of DNAcol to the sand grains was negligible. Moreover, the results showed while the DLVO theory captured the general trend of experimental results, it significantly underestimated the deposition of DNAcol in the presence of CaCl2. Under transient porewater chemistry conditions, colloid remobilization was observed upon flushing the column with Milli-Q water, leading to a secondary peak in the breakthrough curves. We observed that under transient porewater chemistry conditions, when the ionic strength of the solution was 10 mM, the magnitude of the remobilization peak was more significant compared to conditions with 1 mM ionic strength. Our work emphasized the complex interplay between water quality on the one hand and deposition and release of colloidal matter in saturated porous media on the other hand.

Automated summary

Interactions between natural organic matter (NOM) and colloids in terrestrial environments have a significant impact on the transport behavior and ecotoxicity of the colloids. This study investigated the transport and release of a DNA-tagged synthetic colloid as a surrogate for natural colloids in sand columns. The results showed that the deposition rate of the colloid was inversely proportional to the NOM concentration and significantly increased under low ionic strength conditions. Moreover, the DLVO theory underestimated the colloid deposition in the presence of CaCl2. Flushing the columns with Milli-Q water induced colloid remobilization, leading to a secondary peak in the breakthrough curves. Our work highlights the complex interplay between water quality and the deposition and release of colloidal matter in saturated porous media.