Feedstock selection influences performance and mechanism of DNA adsorption onto biochar

Antibiotic-resistance genes (ARGs) in wastewater may promote antimicrobial resistance in consumers of crops irrigated with wastewater. Removal of DNA from wastewater may thus mitigate potential environmental risks associated with irrigation and environmental release of recycled wastewater. Although biochar adsorbents are a potentially cost-effective strategy for removing DNA from water, biochar feedstock influence on performance has not been studied across a range of feedstock classes. Our objective was to produce biochar from 5 distinct feedstocks (manure (MN), black mustard (Brassica nigra) (MU), orange peel (OP), pine pellet (PP) and macadamia nutshell (MNS)) at a fixed pyrolysis temperature (500 °C), characterize biochars and relate characteristics to DNA adsorption. Adsorption reached equilibrium within two hours and kinetics fit the pseudo-second order model. Adsorption rates increased from MNS, PP, OP, MN to MU, with rates of 3.06 × 10⁻², 5.65 × 10⁻², 1.78 × 10⁻¹, 4.00 × 10⁻¹ and 5.05 × 10⁻¹ mg g⁻¹ min⁻¹, respectively. Adsorption isotherms fit the Freundlich model, with affinities increasing from PP, MNS, OP, MN to MU (K_d = 1.30 × 10⁻², 1.35 × 10⁻², 1.27 × 10⁻¹, 1.96 × 10⁻¹ and 1.42, respectively). DNA adsorption on biochars increased with ionic strength from I = 0 – 0.10 M except for MN. Ca²⁺ addition increased adsorption for biochars at I = 0.01 and 0.10 M, except for MN, which increased only with I = 0.10 M. Lower sensitivity of adsorption on MN biochar to ionic conditions indicates a different mechanism may control adsorption. The high ash content of MN biochar may favor direct bonding of DNA to ash minerals compared to π-π interactions likely driving DNA adsorption to structural carbon. These findings help understand how feedstock-driven variability in biochars translates to DNA immobilization and will assist researchers and stakeholders in determining the most suitable feedstocks for this purpose.