Development of Biochar and Cement-Based Materials for Urban Runoff Quality Improvement

Heavy metals are one of the major chemical pollutant groups in urban runoff. Porous concrete is a potential alternative to conventional runoff management systems with the ability to remove heavy metals and it is the main application of cement-based products in urban runoff treatment. The cement paste is the main contributor to the removal capacity of the porous concrete. The major drawbacks of porous concrete are the high pH (>8.5) of the effluent water, decalcification of the porous concrete and leaching of adsorbed pollutants. Overall, the addition of adsorbent materials to the porous concrete increase’s removal efficiencies (7% - 65% increase) without neutralizing the effluent pH. Meanwhile, the addition of reduced graphene oxide is successful in reducing the leachability of the removed heavy metals. The addition of pozzolanic materials can lower the effluent pH while maintaining similar removal efficiencies to unmodified porous concrete. Nevertheless, understanding of the impact of adding biochar to cement-based treatment systems remains limited. Hence, this study was focused on evaluating the feasibility of biochar and cement mixed adsorbents for heavy metal removal from urban runoff. Most biochar used has been synthesized under controlled laboratory conditions using furnaces purged with inert gases. Therefore, in this first phase of this study, the removal of Cu, Pb and Zn using biochar synthesized with paddy husk and sawdust feedstocks in an industrial scale double chamber downdraft pyrolysis reactor was carried out. The effect of pyrolysis temperature and the effect of feedstock in the removal of Cu, Pb and Zn was evaluated by conducting batch adsorption experiments. Synthesized adsorbent materials were characterized using proximate analysis, zero-point charge, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The biochar yield was in a lower range compared with the literature attributed to the higher heating rate (50 °C/min) in the pyrolizer. The paddy husk biochar and sawdust biochar synthesized in the temperature range 350-450 °C and 450-550 °C performed best in the removal of the three heavy metals. Surface complexation, co-precipitation, ℼ-electron interactions, physical adsorption and surface precipitation were the main mechanisms of removal of the three heavy metals. In the second phase of this study, the effects of the type of biochar, the percentage of biochar addition, and the particle size of the biochar on the removal efficiency of Cu, Pb, and Zn, as well as the effect of contact time on the removal efficiency of Cu, Pb, and Zn, along with the compressive strength. The peak intensities of OH-, CO32- and calcium silicate hydrate (CSH) peaks increased with increasing biochar addition levels, reflecting increased hydration product formation. The reduction of particle size of biochar causes the polymerization of the CSH gel. However, no significant changes were observed in heavy metal removal, irrespective of the percentage of biochar addition, the particle size of biochar, or the type of biochar added to the cement paste. Heavy metals bonded with OH-, CO32- and CSH functional groups. The results demonstrate that biochar can be used as a cement replacement without negatively impacting heavy metal removal. However, neutralization of the high pH is needed before safe discharge. In the third phase of this study, the use of biochar modified with ordinary Portland cement (OPC) as an adsorbent for heavy metal removal was tested along with an evaluation of the impact of competing ions and nutrients on the adsorptive removal process. The cement-modified biochar composites, featuring functional groups from biochar and hydrated OPC, exhibited significantly enhanced adsorption capacities for Cu, Pb, and Zn, approximately 3.3, 2.4, and 6 times higher, respectively, compared to unmodified biochar upon introducing 1.5% (v/v) OPC. The Langmuir monolayer adsorption model accurately described the adsorption data, with heavy metals binding to functional groups like C=C and C=O, CO3-2, silanols (Si-OH), siloxanes and calcium silicate hydrate. There were negligible effects from NO3- on the removal of Cu, Pb and Zn, PO43- ions enhanced Pb removal, and NH4+ ions slightly reduced the removal of all three metals, while humic acid increased removal. The findings indicate that using OPC boosts heavy metal adsorption, and insights into the influence of competing ions and nutrients apprise strategies for using OPC-modified biochar in practical applications. The final phase of this research was focused on evaluating the environment and economics of the process of absorption of Cu, Pb and Zn from synthetic stormwater utilizing industrially synthesized paddy husk biochar (PHBC), cement-modified biochar adsorbent and comparing it with commonly available adsorbent, activated carbon and zeolite. The results showed that adding 1.5% (v/v) of cement to biochar decreased the overall negative impact on the environment compared to plain biochar. The total global warming potential (kg CO2 eq) was lowest for the cement-modified biochar and was approximately 2 times lower than that of PHBC. For cement-modified biochar, zeolite and PHBC between 50-90% of the impact was from the raw material collection. The SDBC had the highest sensitivity towards transportation distances among the adsorbents considered. The cement-modified biochar adsorbent took precedence in multi-metal systems due to its heightened adsorption capacity. About 50% of the cost saving was obtained by using cement-modified biochar compared with plain PHBC due to increased adsorption capacity. Also, the potential application of the cement-modified biochar was evaluated in the fourth phase. Column testing revealed a loss of permeability likely due to heavy metal precipitates, making the cement-modified biochar ineffective as a filter. Furthermore, pH adjustment was necessary for safe discharge due to elevated effluent pH levels. The cement-modified biochar can be used as a secondary filter. When using a dosage of 0.25 g/L, an acceptable effluent pH and maximizing adsorption capacity for the heavy metals can be achieved. Contact time studies indicated that optimal removal requires a detention time of 3.5 hours. Overall, this study provided insight into the behavior of cement and biochar composites in removing heavy metals, identified the main removal mechanisms and stability of the removed heavy metals along with evaluating possible applications. Future research work should prioritize reducing the required contact time for removal, thereby enhancing the material's applicability in pollutant remediation efforts.<p></p>