Environmental risk assessment of arsenic in Zn-Pb mine tailings

Zn-Pb mine tailings of Zanjan (province of Iran) contain high concentrations of toxic metals, especially arsenic that can potentially release into the surrounding soils and water pathways. These tailings create leachate due to their connection with water, which is very dangerous to the environment and to the masses of animals and human beings. Unfortunately the geochemical and mineralogical processes that control arsenic release from tailings are not fully studded. Therefore this research aimed to assess the leaching characteristics of arsenic under different environmental conditions to recognize the parameters and factors controlling its concentration in the leachate.

Leaching of arsenic from Zn-Pb mine tailing was investigated in various environmental scenarios. In order to determine the amount of leached arsenic in different conditions, four leaching protocols, including toxicity leaching procedure test (TCLP), synthetic precipitation leaching procedure test (SPLP), field leach test (FLT) and leachate extraction procedure (LEP) were used. Also, to study the effect of various factors such as pH, contact time, particle size and solid to liquid ratio on leaching of arsenic from Zn-Pb mine tailings batch leaching experiments were carried out. Risk assessment code (RAC) based on fractionation method was also applied to evaluate the environmental risk of arsenic mobility.

The results indicated that in both samples of tailings ,the highest amount of arsenic was measured in the residual fraction that is relatively stable under natural environmental situations. According to RAC index, the tailings in term of arsenic placed in the low-risk tailings group. In TCLP, LEP, FLT and LEP test the concentrations of leached arsenic were low and the highest leaching concentration of arsenic occurred in the LEP test and sample S1. According to SPLP analysis, Zn-Pb tailings will have a high potential for contamination of surface and underground water. The mobility of arsenic in the tailings was entirely influenced by pH, contact time, particle size and solid to liquid ratio. The leaching of arsenic enhanced in acidic and alkaline condition and the amount of arsenic leached in acidic condition was much higher than that leached in alkaline condition. In fact, pH-dependent leaching behavior of arsenic from Zn-Pb mine tailing was amphoteric. Actually pH-dependent leaching behavior of arsenic from Zn-Pb mine tailings was amphoteric. In sample S1, the highest amount of arsenic was released from particles with the size of 500-600 µm, while in the sample S2 particles with the size of 150-250µm released the highest amount of arsenic.

By considering the results of leaching protocols, and arsenic fractionation and also with regards to factors affecting the concentration of arsenic in leaching water, reducing the pore water and contact time, and controlling the pH and preventing the spreading of tailings in the environment are the most important factors in preventing and managing the spread of pollution in the environment.