The use of chromium salt is an extremely popular and widespread tanning method because the leather it produces is stable at high temperatures and is more resistant to cracking than vegetable tanned leather. However, during processing, chrome tanned leather can contaminate surface water due to chemical substances leaking from the material. This is an edited version of a paper published by W Long, L Peng, B Shi, R Wang, and W Zhang of the Key Laboratory of Leather Chemistry and Engineering of Sichuan University, discussing the leaching behavior of leather in water and the conditions that affect this phenomenon.

Tanning is the process of converting animal skins or hides into leather. This is a way to use resources and recycle organic waste from the meat industry to produce valuable items, such as footwear, to meet consumer comfort and fashion requirements. There are many tanning methods, including metal salts, aldehyde derivatives, synthetic tanning agents, vegetable tanning agents and their combined tanning agents. Due to the unique properties of chrome tanning, it accounts for more than 80% of the leather produced in the past 20 years.

In the production process of chrome tanned leather, chromium is incorporated into collagen fibers through at least three interactions-including chromium compounded with collagen, non-productive combination of chromium and collagen (chromium combines with collagen, but does not cross-connect ) And the adsorption of chromium to the collagen matrix. Different interactions lead to different binding abilities. The total amount of chromium absorbed by the collagen matrix does not exceed 1.5%, which can be easily removed. Nearly 58.5% of chromium is compounded with collagen, which has a significant influence on the thermal stability of the chromium-collagen complex and is difficult to remove. Approximately 40% of chromium is tightly bound to collagen, but there is no cross-linking, because this has no effect on thermal stability and it is more difficult to remove.

When leather products become waste at the end of their useful life, the chemicals in the material—especially chromium—can leak and contaminate surface water, soil, and even groundwater. The migration law may be related to the interaction between chromium and collagen.

In this study, deionized water was used as an leaching agent to simulate the leaching behavior of leather in surface water. The liquid-solid ratio, pH value, contact time, temperature and rotation speed and other parameters and their effects on the chromium leaching behavior of the chrome tanned leather product, as well as the leaching kinetics of chrome tanned leather, are studied.

In order to determine the chromium content in the finished chrome tanned leather, a microwave method was used to digest 1 g of chrome tanning powder. The microwave digestion procedure includes a 10 minute increase to 1,400W, a 20 minute digestion step at 1,400W, and then a cooling phase. Adjust the digestion solution to a constant total volume and mix well. The inductively coupled plasma emission spectrometer was used to directly analyze the total chromium in the leather.

The leaching experiment was carried out with an air constant temperature shaker, and the temperature accuracy was 0.1oC. The leachate that has passed through the 0.45μm membrane is digested by wet digestion with concentrated nitric acid and hydrogen peroxide (3:1, v/v). The digestion solution was filtered and adjusted to a constant total volume with deionized water, and the concentration of chromium was analyzed using an inductively coupled plasma emission spectrometer.

The calibration standard (0.20–5.00mg L-1) is prepared by diluting the chromium standard stock solution (100mg L-1). All tests are performed in duplicate. Then, the kinetics of chromium leaching of chrome tanned leather powder in aqueous solution was studied in detail. This part of the leather is thinner than the leather used in the previous experiment. Kinetic studies were conducted at four different temperatures (20°C, 25°C, 30°C and 35°C). First put 4g of chrome tanning powder into a 250ml Erlenmeyer flask and fully soak it with 80ml of ultrapure water to simulate surface water. The 18 flasks were then kept in a fixed temperature shaker at 60 rpm. Take two flasks at 1 hour, 2 hours, 4 hours, 8 hours, 24 hours, 48 ​​hours, 96 hours, 168 hours, and 240 hours to evaluate the total chromium content of the extract.

After the leather was digested by a microwave digester, the total chromium content of the two chrome tanned leathers measured by ICP-OES was 21445.83mg/kg and 20672.50mg/kg, respectively.

The leaching experiment of finished chrome-tanned leather shavings (FCTLS) was carried out using various experimental parameters, including liquid/solid ratio, pH, contact time, temperature and rotation speed.

The extracted chromium content is calculated by the following formula: C=C0×A where C represents the extracted chromium content, C0 represents the concentration of the leaching solution, and A represents the liquid-to-solid ratio. The effect of these leaching parameters on the chromium leaching efficiency was studied (Figure 1).

As the liquid-to-solid ratio decreases, the amount of leaching decreases slightly. However, considering the water absorption and swelling properties of the skin powder, a liquid-to-solid ratio of 10:1 is not appropriate. When the rotating speed of the vibrating screen is changed, the released chromium concentration is between 45.32mg/kg and 54.14mg/kg, and the influence of the rotating speed is small.

The leaching solution of FCTLS contains a higher concentration of chromium, which indicates that an excessively acidic environment may increase the release of compounds in the leather. At the same time, in the pH range of 4-11, the released chromium concentration is not significantly affected by the increase in pH.

Compared with the liquid-to-solid ratio, speed and pH value of the fluid, the contact time and temperature have a greater influence on the extracted chromium content. At 35°C, the extraction amount of chromium reaches the highest concentration. This indicates that the release of chromium is very sensitive to the leaching temperature, because as the contact time increases from zero to 24 hours, the extract content increases steadily. In the first eight hours, the chromium content increased rapidly with time. In the second stage (between 8 and 24 hours), the change in chromium content is relatively small. Then, the effects of temperature and contact time were further studied. The liquid-to-solid ratio and rotation speed were maintained at 20:1 and 60rpm, respectively, and deionized water was used as the fluid.

The shrinking core model is often used to describe the leaching reaction of solid reactants and spherical particles. According to this model, the leaching reaction first occurs in the outer skin (core) of the particle, and then moves into the solid, leaving the fully converted material and the inert solid (shell). The model assumes that the solid-liquid leaching reaction usually involves a chemical reaction on the surface of the unreacted reactant core, accompanied by a fluid and solid phase (pore fluid) diffusion step. Under the assumption of the rate control step, the shrinking core model can be simplified to the following equation:

In these equations, ki is the apparent constant of different control steps, x is the ratio of the liquid chromium content to the total chromium content in the dry skin powder (dimensionless), and t is the leaching time in hours. Then apply these three equations to test the control steps of water extraction of chromium. The plot is shown in Figure 1, and the correlation coefficients are listed in Table 1.

The correlation coefficient (R2) value of these fitted curves is lower than 0.96. Considering that waste leather powder is a strip-like sheet material with a loose porous surface structure, these three equations are not suitable for explaining the leaching process.

The Weber-Morris model assumes that mass transfer is a fast process, and only intra-particle diffusion is considered as the rate-determining step. The intraparticle diffusion model graph is linear at all temperatures. The high value of R2 indicates the suitability of chromium diffusion from the interstitial spaces and pores of the leather matrix into the particles of the fluid.

Most models based on reaction kinetics assume that the mass transfer rate is negligibly fast. When the solid reactant is porous, the fluid reactant can diffuse freely into the solid. Then leaching can be considered as a homogeneous reaction of the entire solid, and there will be a gradual change in the concentration of solid reactants in the particles during the leaching process. Therefore, the experimental data is also applicable to pseudo-first-order kinetics and pseudo-second-order kinetics.

The Arrhenius diagram is based on the Weber-Morris model, and the correlation coefficient is 0.9703. Then, the apparent activation energy is estimated to be 31.05kJ/mol, indicating that the energy barrier for chromium diffusion in the leather matrix is ​​higher.

In this study, the release behavior of chromium in leather was tested to simulate surface water. The results show that the main factors affecting the release of chromium from leather are temperature and contact time. The best leaching conditions are a liquid-to-solid ratio of 20:1, a rotation speed of 60rpm, and deionized water as the leaching solution. However, the amount of chromium that can be leached in the water is very low-about 0.18-0.43% of the total chromium found on leather.

The kinetics indicate that the release rate of chromium involves two types of process control. The Weber-Morris model and the quasi-second-order kinetic model are applicable to the release of chromium from leather by water. Although the release of this compound is long-lasting, more than 85% of the leachable chromium in the leather can be released within 24 hours.