Co-adsorption of phenol and calcium from an industrial wastewater stream
Van der Merwe, K.
MetadataShow full item record
The pulp and paper industries are large producers of industrial wastewater that contain both organic and inorganic pollutants that influence water quality and availability. Conventional wastewater treatment methods such as chemical precipitation and membrane filtration are not economical and environmentally friendly. In comparison, adsorption is more cost-effective and easier to implement on an industrial scale. The use of activated carbon as adsorbent of choice is no longer viable due to the high manufacturing costs compared to the low levels of contaminants that need to be removed. Biochar adsorbents produced by hydrothermal liquefaction (HTL) from biomass waste such as paper sludge is a less expensive alternative to activated carbon. The utilisation of paper sludge, destined for landfilling, as feedstock for adsorbent production offers cost-effective waste management solutions that also contribute to the preservation of the environment. However, limited information is available on the application and success of HTL produced paper sludge-based biochar adsorbents for the removal of problematic pollutants such as phenol and calcium, especially with regards to industrial wastewater streams. The aim of this study was to determine the effectiveness of paper sludge-based biochar as adsorbent for the co-removal of phenol and calcium from the wastewater stream of an industrial paper mill. The objectives of this study were to understand the behaviour of the paper sludge-based biochar when subjected to synthetic phenol and calcium environments, to understand how and why the affinity of the paper sludge-based biochar towards phenol changes in the presence of calcium and lastly, to understand the performance of paper sludge-based biochar in a real industrial wastewater stream, compared to that of commercial activated carbon. The paper sludge-based biochar was produced through batch hydrothermal liquefaction at a temperature of 300°C by using paper sludge collected from a local paper mill as feedstock. The produced biochar was characterised and subjected to various adsorption scenarios. The performance of the characterised paper sludge-based biochar was firstly evaluated in synthetic single phenol (10 ppm – 150 ppm) and calcium (600 ppm – 1000 ppm) environments whereas the adsorbent dosage (2 g.L⁻¹ – 12 g.L⁻¹) and the initial concentration of each adsorbate was varied. The performance of the biochar was then tested in a synthetic binary environment with both phenol (10 ppm) and calcium (600 ppm), containing concentrations similar to that found in the collected industrial wastewater. Lastly, the performance of the paper sludge-based biochar was tested in the collected industrial wastewater and compared to the performance of commercial activated carbon. The adsorption experiments were performed at a temperature of 25°C ± 2°C, rotary speed of 150 rpm and solution pH of 8. The phenol concentration was determined by high-performance liquid chromatography (HPLC) analysis, where inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was used to determine the calcium concentration. The chemical and structural characteristics of the biochar had a large influence on its adsorptive capabilities towards both phenol and calcium. Low phenol removal efficiencies of 12.30% ± 0.83% were obtained with a maximum adsorbent dosage of 12 g.L⁻¹ as only a limited number of active sites were available on the surface of the biochar due to the low surface area of the produced biochar. The low surface area was attributed to the blockage of pores by mineral-based species such as calcite which was majorly present in the produced biochar. The mineral-rich nature of the biochar also resulted in the biochar not being an appropriate adsorbent for the removal of calcium from both synthetic and real wastewater streams. This can be attributed to the biochar not consisting of exchangeable sites that can accommodate ion exchange mechanisms for calcium adsorption. Also, it was found that the calcium, in co-operation with the hydroxyl ions in the solution, rather attacked some of the surface functional groups present on the surface of the biochar, than adsorbing itself. In the binary adsorption experiments, the addition of calcium had a negative effect on the adsorption of phenol. The phenol removal efficiency decreased with 15.76% ± 1.06% when calcium was added to the adsorption medium. This can be attributed to the mass transfer limitations experienced in the medium due to the low concentration of phenol molecules that had to compete with the high concentration of calcium hydroxide molecules also diffusing to the surface of the biochar. The produced biochar performed better in the collected industrial wastewater than the synthetic solutions, due to the presence of other less-soluble phenolics other than phenol. The biochar achieved a COD removal efficiency of 77.83% ± 5.22%, close to the COD removal efficiency of 92.72% ± 6.22% obtained by commercial activated carbon. Therefore, biochar derived from paper sludge, a waste product produced by the pulp and paper industry, has the potential to replace expensive adsorbents such as activated carbon for the treatment of contaminated industrial wastewater streams. The impact of the pulp and paper industry will also be reduced by the usage of paper sludge for adsorbent production and applications. Firstly, the direct processing of wet biomass such as paper sludge in HTL offers cost-effective waste management solutions that can be used as an alternative to the current disposal methods employed. And secondly, pulp and paper mills can now effectively utilise their own waste products such as paper sludge to clean the wastewater produced during the paper manufacturing activities. This will then result in the production of cleaner water, with a lower COD concentration, that can be re-circulated back into the system without adversely affecting the properties of the paper products produced in the process. Also, by re-circulating the water, the demand for freshwater resources by the pulp and paper industry can be reduced. Since the characterisation results showed that the chemical and structural characteristics of the biochar had a large influence on the adsorption of phenol, it is recommended that most of the ash compounds are removed from the paper sludge before the biochar is produced. The removal of the ash compounds from the paper sludge beforehand will ensure that pore development is not limited during the HTL process, and therefore biochar can be produced with more attractive chemical and structural characteristics for adsorption applications.