Comparison of Wet-Digestion and Dry-Ashing Methods for Total Elemental Analysis of Biochar

Abstract

Total elemental analyses of biochars presents challenges during digestion because of biochars' high chemical recalcitrance and widely varied composition. Three biochars were chosen with contrasting properties: corn stover pyrolyzed at 300 °C, oak wood at 600 °C, and poultry manure with sawdust at 600 °C. Recovery of phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), and zinc (Zn) from poultry manure biochar was 10 to 100 times less with published wet- or dry-digestion methods than when using improved methods. The published dry-digestion method returned significantly (P < 0.05) more sodium (Na), suggesting contamination from borosilicate glassware. A modified dry-ashing (MDA) method was the most precise method, demonstrating a relative standard deviation within 3.7% of the most precise method for recovery of P, K, S, Ca, Mg, and Zn from both corn and oak biochars. The MDA method is comparatively safe and effective for preparing biochar for elemental analysis.

Keywords:

• Agricultural waste
• carbon
• testing methodology

Acknowledgments

We are grateful for the financial support by NYSERDA and the USDA Hatch program as well as the technical assistance and detailed analyses provided by Michael Rutzke and the Cornell Nutrient Analysis Laboratory for the standard material and accompanying microwave-digestion data.