Short-term leachability of salts from Atriplex-derived biochars

Disposal of concentrate from brackish water desalination is limited by concerns of salt leaching into fresh water sources. Prevention of salt from leaching can greatly increase desalination concentrate management costs, as there are few options to recover those costs. An option for concentrate disposal is to grow halophyte crops using the concentrate as irrigation water. The purpose of this study was to determine how much of the Ca, Mg, Na, and K taken up by Atriplex species during growth can be stabilized by pyrolysis, thus preventing those cations from returning into the soil solution after harvest. Results show that salts in the biomass are concentrated into the biochars and that pyrolysis conditions affect the amount of salt that can be leached with water. Ca was the most retained of the cations: 100% and 94% were retained in A. canescens biochars and A. lentiformis 600 °C biochar, respectively. Mg retention was similar: 100% and 93% retained for A. canescens and A. lentiformis 600 °C biochars, respectively. Increasing pyrolysis temperature increased the retention of Ca and Mg; trends for K and Na were inconsistent. Soil application of these biochars will still be problematic due to low retention of Na against short-term leaching, thus limiting the potential for on-site sequestration to enable land utilization for crop production. With more understanding of the salt formation and retention mechanisms within biomass, higher amounts of salt might be sequestered longer, while taking advantage of the carbon sequestration and soil quality benefits of biochars.