Evaluating the effectivity of peat as sorbent material for mining water purification in pilot scale systems

• Main Author: Campos Lopez, F. (Felipe)
• Format: Dissertation
• Language: English
• Published: University of Oulu 2018
• Subjects: Environmental Engineering
• Online Access: http://urn.fi/URN:NBN:fi:oulu-201805091619; http://nbn-resolving.de/urn:nbn:fi:oulu-201805091619

The development of the mining industry has led to an increase in the environmental impacts generated by the industry. Mining influenced waters have a severe impact over the surrounding aquatic environment as the waters may contain pollutants in dissolved and particulate form, such as heavy metals and metalloids. The severity of these impacts is dependent on, among other factors, the hydrological characteristics of the receiving water bodies as well as the mine water composition. Among the numerous methods that have been developed for mining water purification, adsorption via biosorbents has proven to be an effective and sustainable option. A number of biosorbent materials have been extensively studied for their metal adsorption capacity such as bark, seaweed, modified cotton, lignin, and peat, among others. In Finland, peat is of interest because it is widely available. Although natural peat has been found to possess high sorption capacity for metal and metalloids, a variety of treatments (physical, chemical, etc.) have also been investigated aiming to increase sorption capacity or to modify chemical and/or physical properties that can improve its application as a sorbent. It is important to note that although several studies have reported on the sorption capacity of natural and modified peat, these studies have been mostly conducted in a laboratory scale and used synthetic water samples. Only a small number of purification systems using peat as a biosorbent have been reported in pilot-scale or full-scale scenarios. There is therefore a lack of knowledge regarding the suitability of peat as a biosorbent for metal removal from real water samples containing a mix of contaminants. Furthermore, there is a need for reports describing the performance of peat in pilot systems simulating real applications. The main objective of this thesis was thus to evaluate the suitability of using peat as a sorbent for the purification of mine process and drainage waters in two pilot-scale purification systems. Another objective was to evaluate the effect of the systems design parameters over metal removal and use the obtained results to conclude on the viability for full-scale applications. For this purpose, a mix-and-settling system and a horizontal filter system were tested using natural and chemically modified peat as sorbents and real drainage water was collected from a mining site in Northern Finland. A factorial design was used in the planning of experiments to evaluate the effect of operational factors (sorbent type, dose, mixing intensity and mixing time) in a mix-and-settling system. The purification efficiencies achieved in the system showed removal efficiencies as high as 80% for Ni and 68% for As, when a high dosage of natural peat was combined with high levels of mixing time and mixing intensity. Further statistical analysis showed that sorbent dose was the most influential factor affecting purification efficiency. Two small-scale horizontal filters were built (three compartments, sand-peat-sand) to evaluate the suitability of natural and modified peat as sorbent agents in such systems. High removal rates of selected metals was achieved, e.g., Ni with 98% and 96% of removal in the modified and natural peat filters respectively and As 87% removal by the natural peat filter. Removal efficiency at the end of test period was still satisfactory although it decrease treated water volume in both pilots (20–30%). Low hydraulic conductivity of peat makes the scaling-up of the filter system to a full- scale application non-viable, as the required retention times would be excessively high for this purpose. Overall peat proved to maintain its adsorption properties when applied to pilot-scale systems, with the mix-and-settling system showing to be a promising technology for the purification of mine influenced waters. Nevertheless, concerns such as the improper mixing of peat in the system and poor settling of particles need to be solved before full-scale application can become a reality.