Biochar Selection for Reducing Mine Soil Metal Availability.

Approximately 500,000 abandoned mines across the U.S. pose a considerable, pervasive risk to human health and the environment; world-wide the problem is even greater.  Lime, organic materials, and other amendments have been used to decrease metal bioavailability in contaminated mine wastes and to promote the development of a mine waste stabilizing plant cover.  The demonstrated properties of biochar (e.g., alkaline pH, plant-available nutrients, metal sorption and improvements in soil water relations) make it a potential amendment for remediating metal contaminated mine soils.  However, methods are needed for matching beneficial biochar properties with mine waste metal toxicities.  A three-step procedure was developed to identify biochars that are most effective at reducing heavy metal availability.  Step 1: a slightly acidic extract of the mine spoil soil was produced, representing the potentially available metals, and used to identify metal removal properties of a library of 38 different biochars (e.g., made from a variety of feedstocks and pyrolysis or gasification conditions).  Step 2:  evaluation of how well these biochars retained (i.e., did not desorb) previously sorbed metals.  Step 3: laboratory evaluation of the performance of 1%, 2.5% and 5% biochar additions (by weight) for reduction of mine soil metal bioavailability.  Investigation results were used to identify specific biochar(s) and application rate(s) to reduce metal availability and aid in remediation of a specific metal contaminated soil.  Results from this study are being used to develop directed methods for matching biochars to specific metal contaminated soils to reduce metal availability.