Chromium and CBE

Almost everyone who reads up on current affairs is aware of the circular economy. In sustainability circles it is one step beyond the recycling economy and two steps in front of the traditional linear economic model.

In a linear manufacturing system, the sequence is: resource gathering; raw material modification/purification; product manufacture; retail; consumer use; end-of-life (disposal). Leather has followed a model like this for 1000s of years – like most other materials. It is extremely hard to sustain on a planet that has finite resource and it is not the most efficient model of resource use.

The circular economy (CE) has been shown to be more efficient in the use of planetary resources when compared to a linear model and it is highly likely that standard business practice will focus on CE as the best available technique when modelling future manufacturing.

However, recent development in sustainability research have shed light on a new concept known as the circular bioeconomy (CBE). Not to be confused with the natural biogenic cycles (carbon, sulfur, phosphorus, and nitrogen cycles), the CBE is the overlap between the biosphere (natural world) and the technosphere (the human built environment). A good example of products in the circular bioeconomy will be something like wood. A tree grows from elements found in the biosphere to produce a starting material that is harvested. 

The harvested tree is converted into wood and the wood is then used for articles in the technosphere, e.g., a window frame. The wood is used in-life for however many generations it can survive. Its care and repair, or re-use in another product, e.g., a picture frame, with re-purposing and remanufacture allows it to be technospere recycled and if that is done indefinitely then its life cycle will be a circular model.

 

If that window frame, after many years of use, accidently or on purpose, begins to rot away then it will leave the circular technosphere and will be broken down into its elements using the biogenic cycles. Those elements will then be recycled into a new tree and the circular bioeconomy of wood has been perfectly described. 

A piece of plastic cannot do this. A piece of plastic must be made susceptible to bio- or other forms of degradation – for it to be able to enter the biogenic cycles. The very fact that plastic is chemically inert means that it can only be kept in the technosphere circular economy – if it is the type that can be recycled.

 

 

So, the fundamental question is: “Can a chromium tanned leather be biodegraded in a biosphere – after it has had a long life in the technosphere CE?” The simple answer is, yes. However, there are certain conditions where it cannot break down. If the leather does not have the ideal moisture content, then the biodegradation will take longer than we would normally expect. Moisture levels that are too high, or too low, will mean the micro-organisms will struggle. Waterlogged leathers, like wood, will take time. Likewise, bone-dry organic matter will take time.

Recent experiments have shown that dry wet-blue does not break down in the ISO 20200 test as can be expected. Another major factor is how much is the chromium shielding the amino acid’s peptide bond (a major target for collagenases). If the chromium is hydrolysed and begins to leach out the leather, then the degradation begins at pace.

It is also well known that a fungicide interferes with the ISO 20200 test and it will take time for the compost fungi to overwhelm the fungicide. A fungicide is vital for in-use leather functionality, but they can be restrictive in the end-of-life phase

 

Chromium (III) is a common element found in soil. It exists at particularly high levels if the soil contains traces of naturally occurring chromite ores. The Rustenberg area of South Africa, where many of the chromium mines are found, has high levels of Cr III. It is very unusual to find high levels of chromium (VI), Cr VI, unless the soil has been recently contaminated or if the chemical conditions favour Cr VI formation.

 

Cr III is normally found in soil at concentrations that range from 0.9 to 1500 mg/kg (Williams, 1988). Naturally occurring levels as high as 3200 mg/kg have been reported. The median level of Cr III in soil is typically 62-84 mg/kg. Plants will take up chromium through their roots and the element will generally accumulate in the roots. Most Cr III occurs as insoluble compounds in the soil that are not readily mobile. These chromium compounds will be constantly in equilibrium with Cr VI particularly at high soil pH values. Cr VI rapidly gets reduced to Cr III by the organic materials present in the soil. Regardless, soluble chromium species that are typically in the Cr VI form, will be able to move into the plant where they will be rapidly converted back into the Cr III species. 

A crucial question is how much can the plants tolerate before a decrease in yield is seen – stunting or other toxic effects? The answer is it depends on the plant. Concentrations in the plant can show a 10% yield reduction when they reach 2.5 mg/kg to 4 mg/kg. This is commonly caused by soil concentrations that are at the 500 mg/kg level. This means that a chromium tanned leather that breaks down in the ISO 20200 test will leave its tanning agent in the resulting biomass. The chromium will then enter farmland and will begin to accumulate in the soil. The chromium can leach out of the soil and into underground water system – which occurs in any natural system anyway. The responsible farmer will then add chromium sludges or chromium-containing composts at a rate that does not cause the soil concentration to reach the 500-ppm level for fear of causing plant growth issues.

 

Williams, J.H. Chromium in sewage sludge applied to agricultural land. Commission of the European Community. Brussels, Belgium. p. 4-9. 1988.

 

In the next issue: Fungicides – which ones do we use? What are the biocidal products regulations? Can they affect the breakdown of leather in a composting environment? Why do we need them, and which ones are checked for in leather testing?