Use of plant waste in the treatment of heavy metal polluted wastewater

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Introduction. Anthropogenic pollution is one of the global environmental issues of the modern world. The main approach to solving this issue is the transition to a nature-like economy of a closed cycle where the waste of one production is not accumulated but used as raw material for the other production. The article considers the possibility of using three renewable large tonnage byproducts of agro-industrial complex: technical hemp hulls, wheat straw (carbonized), and buckwheat hulls (granulated) for the manufacture of heavy metal sorbents (biosorbents). Biosorbents made of ecologically pure natural raw materials can be used for treatment of surface wastewater of urban settlements and wastewater from the territory of mining enterprises from heavy metals. The purpose of this article is to estimate the efficiency of wastewater treatment of heavy metals (in particular, Al, Cu, Fe, Pb, and Zn).
 Materials and methods. The sorbents in this study were made from agro-industrial byproducts: hemp hulls (by-product of technical hemp processing); straw (dry wheat stalks from fields in Chelyabinsk Region, crushed and carbonized); granulated buckwheat hull. The sorbate was surface wastewater urbanized area from Chelyabinsk city and acid mine drainage from the Ural region mining enterprise. Laboratory studies of the interactions occurring in the sorbent-sorbate system were carried out using the limited volume method with static sorption, wherein the pollutants were in the liquid phase and brought into contact with a static sorbent and a dynamic test, wherein the pollutants in a mobile liquid phase were filtered through a layer of sorbent.
 Results. The structure and surface composition of agro-industrial wastes such as hemp hulls, carbonized straw, and granulated buckwheat hulls (biosorbents) were studied. The obtained data confirmed the ecological safety of the studied materials. The main chemical elements in their composition are carbon, oxygen, potassium. The efficiency of sorption by the biosorbents in removing heavy metals from urban surface wastewater and mining-influenced water was determined.
 The research data showed that under static conditions, buckwheat hulls best absorb pollutants from urban surface wastewater, removing copper, iron, lead, and zinc ions from surface wastewater with 100 % efficiency. With the increase of temperature from 0 °C to 10–20 °C, the efficiency of purification also increases. An exception is the process of copper ions sorption, where the maximum efficiency is achieved at 0 °C with minimum sorbent contact time.
 Under dynamic conditions, it is more effective to use hemp hulls, also showing its efficiency in removing pollutants from acid mine drainage — 100 % of aluminum, copper, and lead ions were removed.
 Conclusions. The obtained results show that the examined agro-industrial byproducts are ecologically safe and effective sorption materials and allow extracting heavy metals (aluminum, copper, iron, lead, zinc) from urban surface and mining enterprises runoff.