Divalent metal ions facilitate environmental DNA adsorption on biochar by inducing new hydrogen bonds

The ubiquitous compound pollution of antibiotic resistance genes and heavy metals makes the interactions between them and biochar inevitable. However, the interaction mechanism between heavy metal-induced DNA and biochar remains unclear. In this study, eDNA adsorption on biochar with and without the intervention of five divalent metal ions was investigated. The increased eDNA adsorption by metal ions followed the order of Cu²⁺ (180.53 % ~ 666.34 %) > Cd²⁺ (74.39 % ~ 164.04 %) > Zn²⁺ (41.63 % ~ 132.25 %) > Mn²⁺ (39.60 % ~ 102.47 %), while Mg²⁺ was statistically ineffective. The ability of the above metals to enhance adsorption was significantly consistent with their ability to unwind eDNA double strands, and we conclude that the newly exposed hydrogen bonds played a dominant role in the Mt²⁺-induced DNA adsorption process. When more hydrogen bond sites were released on the biochar surface by replacing bound water with methanol molecules, the ability of metals to increase adsorption was further enhanced (13.42 % ~ 68.97 %). Finally, FTIR was used to verify the divalent metal ions-induced increase in the number of sites where DNA binds to biochar via hydrogen bonding. An understanding of the effects of divalent metals on eDNA adsorption on biochars was developed in this study.