Alkaline extraction: An optimal approach for extracting methylmercury from paddy soils.

Accurately measuring the concentration of methylmercury (MeHg) is a critical part of Hg research. While analytical methods of MeHg have not been validated for paddy soils, which are one of the most important and active sites of MeHg production. Here we compared two methods most widely used to extract MeHg from paddy soils, i.e., CuSO4/KBr/H2SO4-CH2Cl2 (referred to as acid extraction) and KOH-CH3OH (referred to as alkaline extraction). By evaluating the formation of MeHg artifact using Hg isotope amendments and quantifying the extraction efficiency using the standard spike in 14 paddy soils, we propose that alkaline extraction is an optimal choice for paddy soils, with negligible MeHg artifact (accounting for 0.62-8.11 % of the background MeHg) and consistently high extraction efficiency (81.4-114.6 % for alkaline extraction compared with 21.3-70.8 % for acid extraction). Our finding highlights the importance of suitable pretreatment and appropriate quality controls during the measurement of MeHg concentrations.