Retention capacity of biochar-amended New Zealand dairy farm soil for an estrogenic steroid hormone and its primary metabolite

We examined the retention ability of a New Zealand dairy farm soil amended with 3 types of biochar produced from a variety of feedstocks for a steroid hormone (oestradiol, E2) and its primary transformation product (estrone, E1). Biochars produced from corn cob (CC), pine sawdust (PSD) and green waste (GW) were characterised by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and solid-state ¹³C nuclear magnetic resonance spectroscopy. Batch sorption studies were performed on soil amended with each biochar (0.5% and 1% by weight) using a complex solvent extraction scheme, and isotherms were fitted to the Freundlich model. All isotherms were highly non-linear, with N values in the range 0.46-0.83 (E2) and 0.66-0.88 (E1) in soil amended with different percentages of biochars. Overall, addition of all 3 biochars was found to increase the soil sorption affinity for the hormones, with E2 sorption being the highest in the soil amended with 1% PSD biochar. There was no marked difference in hormone sorption ability in the other 2 treatments (soil treated with 1% CC biochar and 1% GW biochar). Overall, the effective distribution coefficient (Kdeff) values for E2 at the lowest equilibrium concentration (Cw 0.5mg/L) ranged from 35 to 311L/kg in soil amended with the 3 types of biochar. Addition of 0.5% of PSD biochar resulted in ~560% increase in the Kdeff value for E2, while at 1% addition of PSD biochar, uptake of E2 was nearly 1400% higher than the control. For E1, the percentage increase in Kdeff was comparatively smaller than E2; however, it still ranged from 40 to 280%, and 60 to >320% at addition of 0.5% and 1% PSD biochar, respectively, compared with the control soil. Highest treatment temperature and associated greater surface area, low ash content, higher carbon content, and the abundance of polar functional groups (e.g. -OH, C=O) may explain why the soil amended with PSD biochar exhibited high sorptive capacity for the hormones.