Goethite effects on transport and activity of lysozyme with humic acid in quartz sand

Abstract

Proteins play important roles in environmental processes and are essential to soil and ecosystem health. However, little is known about the transport of proteins, especially enzymes, in natural environments with significant heterogeneity. Here, we investigated the transport and retention of a model enzyme lysozyme (LSZ) with/without the presence of purified Aldrich humic acid (PAHA) by saturated transport experiments in heterogeneous sand columns containing mixtures of uncoated (QS) and goethite-coated quartz sand (GQS) with (GQS/(QS + GQS)) mass fractions w = 0, 0.1, 0.3 and 0.5. The impact of different PAHA/LSZ mass ratios (f = 0, 0.2 and 0.4) on LSZ mobility was also examined in a GQS/QS column (w = 0.3). The results revealed that the transport of pure LSZ and PAHA decreased with increasing w from 0 to 0.5, largely because of increased microscopic roughness and enhanced patch controlled electrostatic attraction between sand grains and LSZ or PAHA. Furthermore, an overall decline of the breakthrough curve plateau was observed when f was fixed as 0.2 at different w due to progressive PAHA-LSZ aggregation and physical straining. In these GQS/QS columns, positively charged LSZ broke through earlier because goethite of GQS tended to first capture negatively charged PAHA and PAHA-LSZ complexes/aggregations. At w = 0.3, LSZ retention was increased with increasing f and the retention profiles exhibited a hyper-exponential shape. Due to the preferential retention of overall negatively charged and larger-sized PAHA-LSZ aggregates in the GQS/QS column, the enzyme activity of retained LSZ was lower than that in the effluent and decreased with increasing w. The synergistic effect of humic acid and iron (hydr)oxide coating decreased LSZ mobility. These findings may facilitate a better understanding of the transport and retention of proteins in heterogeneous natural environment, and help to deal with the potential environmental risk posed by hazardous proteins.