The effect of municipal solid waste compost on the reduction of lead and zinc toxicity and uptake of nutrients by two medicinal species Marrubium cuneatum and Verbascum speciosum in a soil contaminated with heavy metals

The toxicity of heavy metals is one of the most important environmental issues in the current century. This research aimed to investigate the effect of municipal solid waste compost on the absorption of nutrients and lead and zinc metals in M. cuneatum and V. speciosum plants.

In a greenhouse experiment, compost at four levels (0, 1, 3 and 5% w/w) was completely mixed with natural soil contaminated with heavy metals (Pb and Zn). After six months of harvesting the plants, the shoot and root biomass was determined. Also, the concentration of macronutrients and micronutrients, Pb and Zn in the roots and aerial parts of plants and available Pb and Zn in the soil were measured using the ICP-OES. In order to investigate the correlation between the measured plant and soil parameters, principal component analysis (PCA) was performed

Compost significantly improved the shoot dry weight of M. cuneatum and V. speciosum by 13 and 19%, respectively. 5% compost significantly reduced shoot lead by 64 and 34.4% in M. cuneatum and V. speciosum, respectively, compared to the control. Compost was more effective in increasing potassium, phosphorus, copper, and nickel than magnesium, manganese, and calcium, and increased shoot potassium by 22 and 32%, respectively, in M. cuneatum and V. speciosum compared to the control; this increase was 30 and 14% for copper and 19 and 21% for phosphorus, respectively. PCA analysis showed that, among the investigated elements, potassium, phosphorus and copper were most affected by composting and had the maximum role in improving plant growth and reducing lead toxicity.

Municipal solid waste compost improved the growth of M. cuneatum and V. speciosum and reduced phytotoxicity by immobilizing lead in the soil.