Diluted acid and microwave-assisted extraction for trace element determination in biochar by ICP-OES and ICP-MS

Determination of trace elements in biochar usually involves complex and inefficient sample preparation strategies due to the high carbon content and presence of silicates in the sample matrix, as well as to the variety of raw materials used in its production. Most methods are time-consuming, employ hazardous reagents (e.g. hydrofluoric acid), and are prone to analyte contamination and loss. Another issue is the lack of validation to ensure that these methods provide accurate results. In this study, we describe a sample preparation strategy to determine Al, As, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mo, Mn, Na, Ni, P, Pb, S, Sr, Ti and Zn in biochar by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). The method includes a dry ashing step, followed by microwave-assisted extraction with diluted nitric acid, and hydrogen peroxide. Initially, it was evaluated for efficacy using a hog waste biochar and compared to a similar and commonly reported extraction with aqua regia. The method’s accuracy was assessed by addition and recovery experiments, with analyte recoveries in the 89.6%-114% range. Limits of detection were in the 0.02-3000 and 0.006-0.02 mg kg-1 ranges for ICP-OES and ICP-MS, respectively, with lower values for HNO3 + H2O2 compared to aqua regia. Relative standard deviation (RSD) values using HNO3 + H2O2 were below 10% for all analytes, except As (15%), Cr (12%), and Pb (11%), while aqua regia values were in the 15%-63% range. The method was then applied to the analysis of five commercially available biochar samples.