DNA adsorption by nanocrystalline allophane spherules and nanoaggregates, and implications for carbon sequestration in Andisols

Abstract

This study provides fundamental knowledge about the interaction of allophane, deoxyribonucleic acid (DNA), and organic matter in soils, and how allophane sequesters DNA. The adsorption capacities of salmon-sperm DNA on pure synthetic allophane (characterised morphologically and chemically) and on humic-acid-rich synthetic allophane were determined, and the resultant DNA–allophane complexes were characterised using synchrotron-radiation-derived P X-ray absorption near-edge fine structure (XANES) spectroscopy and infrared (IR) spectroscopy. The synthetic allophane adsorbed up to 34 μg mg⁻¹ of salmon-sperm DNA. However, the presence of humic acid significantly lowered the DNA uptake on the synthetic allophane to 3.5 μg mg⁻¹ by occupying the active sites on allophane so that DNA was repulsed. Both allophane and humic acid adsorbed DNA chemically through its phosphate groups. IR spectra for the allophane–DNA complex showed a chemical change of the Si–O–Al stretching of allophane after DNA adsorption, possibly because of the alteration of the steric distance of the allophane outer wall, or because of the precipitation of aluminium phosphate on allophane after DNA adsorption on it, or both. The aluminol groups of synthetic allophane almost completely reacted with additions of small amounts of DNA (~ 2–6 μg mg⁻¹ ), but the chemical adsorption of DNA on allophane simultaneously led to the formation of very porous allophane aggregates up to ~ 500 μm in diameter. The formation of the allophane nano- and microaggregates enabled up to 28 μg mg⁻¹ of DNA to be adsorbed (~ 80% of total) within spaces (pores) between allophane spherules and allophane nanoaggregates (as “physical adsorption”), giving a total of 34 μg mg⁻¹ of DNA adsorbed by the allophane. The stability of the allophane–DNA nano- and microaggregates likely prevents encapsulated DNA from exposure to oxidants, and DNA within small pores between allophane spherules and nanoaggregates may not be accessible to enzymes or microbes, hence enabling DNA protection and preservation in such materials. By implication, substantial organic carbon is therefore likely to be sequestered and protected in allophanic soils (Andisols) in the same way as demonstrated here for DNA, that is, predominantly by encapsulation within a tortuous network of nanopores and submicropores amidst stable nanoaggregates and microaggregates, rather than by chemisorption alone.