The importance of nano-porosity in the stalk-derived biochar to the sorption of 17β-estradiol and retention of it in the greenhouse soil

Natural estrogens in greenhouse soils with long-term manure application are becoming a potential threat to adjacent aquatic environment. Porous stalk biochar as a cost-effective adsorbent of estrogen has a strong potential to reduce their transportation from soil to waters. But the dominant adsorption mechanism of estrogen to stalk biochars and retention of estrogen by greenhouse soils amended with biochar are less well known. Element, function groups, total surface area (SAₜₒₜₐₗ), nano-pores of stalk biochars, and chemical structure of 17β-estradiol (E2, length 1.20 nm, width 0.56 nm, thickness 0.48 nm) are integrated in research on E2 sorption behavior in three stalk-derived biochars produced from wheat straw (WS), rice straw (RS), and corn straw (CS), and greenhouse soils amended with optimal biochar. The three biochars had comparable H/C and (O + N)/C, while their aromatic carbon contents and total surface areas (SAₜₒₜₐₗ) both varied as CS > WS > RS. However, WS had the highest sorption capacity (logK ₒc), sorption affinity (K f), and strongest nonlinearity (n). Additionally, the variation of Langmuir maximum adsorption capacity (Q ⁰) was consistent with the trend for SA₁.₂₋₂₀ (WS > RS > CS) but contrary to the trend for SAₜₒₜₐₗ and SA<₁.₂ (CS > WS > RS). These results indicate that pore-filling dominates the sorption of E2 by biochars and exhibits “sieving effect” and length-directionality-specific via H-bonding between –OH groups on the both ends of E2 in the length direction and polar groups on the inner surface of pores. After the addition of wheat straw biochar, the extent of increase in the sorption affinity for E2 in the soil with low OC content was higher than those in the soil with high OC content. Therefore, the effectiveness for the wheat straw biochar mitigating the risk of E2 in greenhouse soil depended on the compositions of soil, especially organic matter.