Mine land rehabilitation with C4 grasses, biochar and BioCNG

Picture a slope of old tailings with nothing growing on it. When the wind picks up, it takes the fine material with it. When it rains, the water cuts channels and carries the rest downhill. On bare ground like this, the metals left behind by mining go wherever the dust and the water go. Covering that ground with living plants is the oldest fix there is. The hard part is getting anything to grow, and paying for the work once the mine has stopped earning.

At GoSí BioTech we work on one answer to both problems: a fast-growing grass that covers the ground, a harvest that feeds a biogas digester, and the leftovers, biochar and digestate, going back into the soil.

Why plants, and why this grass

Using plants to hold tailings and metals in place has a name: phytostabilization. A review in Environmental Health Perspectives calls it a feasible alternative to costly clean-up for mine tailings. The idea is not to pull the metals out. It is to keep them in the ground, bound up in and around the roots, while a plant cover stops wind and water from moving the soil. The plants that work best keep the metals in their roots and out of their leaves and stems.

Our current candidate is Pakchong 1, a hybrid of Napier grass and pearl millet. It is a C4 perennial grass. C4 plants concentrate carbon dioxide inside the leaf, which keeps photosynthesis efficient in conditions where many other plants struggle. Once planted, it grows back after every cut. Napier grass has been tested on contaminated ground. In a six-month pot trial in Thailand, cadmium stayed mostly in the roots, not the shoots, and the authors named Napier grass a candidate for cadmium phytostabilization. In another Thai study, on gold-mine tailings, a Napier variety grown with biochar or cow manure took up far less arsenic and manganese than without them.

Where biochar fits

Biochar is plant material heated with very little oxygen until it turns into a porous, carbon-rich solid. Two major reviews describe what it does in polluted soil. Its large surface binds contaminants, including metals, so less of them moves through the soil or into plants. The James Hutton Institute review adds a practical point: on degraded soils, biochar works best mixed with compost or manure, not on its own. That matches the gold-tailings study above.

In our loop, biochar is made from part of the grass harvest and goes back onto the same ground.

Where BioCNG fits

The rest of the harvest goes to an anaerobic digester, a closed tank where bacteria break the grass down into biogas. Cleaned and compressed, that gas is BioCNG, a renewable gas that can replace fossil gas or diesel in vehicles and engines. Napier grass, including Pakchong 1, has been digested into biogas in more than one study. What comes out of the digester besides the gas, the digestate, is a wet, nutrient-rich material that is spread back on the land.

So the cycle runs: degraded soil, Pakchong 1, biomass, then BioCNG, biochar and digestate, and back into the soil.

The 10-hectare pilot

We have designed a 10-hectare pilot to test this on a real abandoned mine site. It is planned to run for 18 months and is built to work in different climates. The candidate regions are Tarkwa in Ghana, the Pilbara in Australia, the Witwatersrand in South Africa, and Spain.

The plan, in order: clear debris, add compost and test the soil for heavy metals; plant Pakchong 1 stem cuttings at 50 cm spacing; install rainwater harvesting for irrigation; monitor growth, erosion and metal levels; then harvest and process the biomass in a digester and a mobile biochar unit. Metal testing is built in, with limits of cadmium below 1 mg/kg and lead below 300 mg/kg.

The sizing case for the pilot is 57,600 m³ of BioCNG a year, 171 tonnes of biochar and 228 tonnes of digestate. These are design figures for the pilot, not measured results.

Alongside the pilot we run research on the grass itself: sequencing first, edits later. It is research, not a released variety.

Who this is for

Mine owners with closed or closing sites, governments responsible for abandoned mine land, and investors who want restoration that produces something saleable. We are looking for partners and funding to run the pilot. If you hold a site, or want to back one, we would like to hear from you.