Title

Artisanal and industrial biochar production and co-composting for acidic mine waste reclamation

Conference Dates

September 15-20, 2019

Abstract

Frequently, mine wastes tend to be acidic with little to no organic matter and CEC, and present phytotoxic concentrations of available metals as well as unfavorable hydraulic properties. Biochar and compost have been shown to have a great potential to improve mine waste physicochemical properties, however, most research has been done applying them independently or mixing them right before application. Biochar has potential as a soil amendment due to its capacity to, reduce soil acidity, improve hydraulic properties, retain nutrients and immobilize organic and inorganic pollutants. It also has potential as a C capture medium given its recalcitrant nature. Despite its potential, biochar, in general, provides low nutrient and labile C inputs and is expected to contain polycyclic aromatics. Here we present a strategy to address these shortcomings by co‑composting, a process in which biochar is part of the composting matrix from the start. We expected that biochar would aid in N and P enrichment of the final product and that co-composting would improve its surface functionality, providing increased CEC. Furthermore, we study the use of these biochars and co‑composts as amendments to a highly acidic mine tailing substrate in a pot experiment with the aim of reducing pollutant dispersion by favoring the development of a sustainable vegetative cover so stabilize the material and reduce its erodibility and retain soluble metals within the amended material. We expected the amendments to increase water holding capacity and available water content while decreasing bulk density, acidity and metal mobility, resulting in a higher biomass production.

Biochar was produced by gasification (Ankur Scientific PG-45 downdraft gasifier) and flame curtain pyrolysis (Kon‑Tiki pyrolizer) of woodchips from programed and weather‑related tree removal. Temperatures on both systems (∼1,000°C and ∼650°C) were higher than those of conventionally used processes, deriving in high amounts of fixed C (>70%), indicative of C sequestration potential, and high pH values (>10) and CaCO3 equivalence, indicative of pH adjustment potential. Kon‑Tiki is a semi‑artisanal production technique with reduced emissions as pyrolysis gases are mostly combusted and do not escape to the atmosphere. Gasification is a technology aimed at producing energy. Energy derived from gasification of woodchips was ∼700 kWeh per ton of sun‑dried biomass with a biochar yield of 19.97%.

Co‑composts were produced during six months using yard waste and gasification‑ or kon‑tiki-derived biochars added at a 17.6% w/w ratio within 210 Lt insulated reactors. Composts with biochar showed a slight tendency of N and P enrichment as well as higher pH and CaCO3 equivalence, indicating a higher potential to work as pH adjustment agents. Biochar co‑compost had a higher content of fixed C, indicative of its capacity to function as C capture medium. A pot experiment was set up to study the effects of pure compost, raw biochar and co‑compost on a highly acidic mine tailing (pH 2-3). Three w/w doses (2.5, 7.5, 15%) were selected with the aim of evaluating the effects of low doses which could be more economically viable to apply in reclamation schemes as well as to analyze the metal immobilization potential associated with organic matter under acidic conditions, as substrate pH was not expected to rise more than 3 pH units. Lolium perenne was selected as plant cover. Preliminary evaluations show raw biochar has the greatest potential to increase pH while also increasing electrical conductivity the most. All amendments increase water holding capacity and available water content. Biomass production in pots was favored by all amendments but was highest in compost and co‑compost.

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