Utilization of biochar produced from agricultural residues for the removal of pesticide and pharmaceutical micropollutants in surface water biofiltration

Abstract

In recent years, there has been a growing interest in exploring possible agro-based biochar for use in different remediation applications. This thesis therefore investigated the feasibility for biochar produced from rice straw (RSBC), corn cob (CCBC), coconut husk (CHBC) and coconut shell (CHBC) to immobilize micropollutants in water, with the aim of finding a better alternative to activated carbon (GAC). First, 3% (w/w) amendment applications of the selected adsorbents were set up to evaluate their influence on leachate properties. Results showed significant increase in pH (>8) in biochar leachates which decreased leachability of metals, while GAC with lower pH (<6) showed greater metal leaching. Also, all amendments increased hydraulic conductivity (K) by 11.6%, except for CHBC and CCBC that decreased K by 54.7% and 36.9%, respectively. The characteristics of the biochars in batch adsorption studies were compared to evaluate their adsorption of four pharmaceuticals and two herbicides micropollutants. Although, removal efficiencies were feedstock type dependent, RSBC exhibited significantly higher sorption capacity of 12.81±0.13 mg/g (at 0.5 g/L and 10 days contact time) for oxytetracycline (OTC), compared to that of GAC of 19.11±0.72 mg/g for the same compound. Partition coefficient (Kd) values were used to compare how effective the different adsorbents are in the reduction of micropollutants availability and transfer in aqueous solution. Comparatively, the kinetic study indicated that RSBC and CHBC showed better adsorption for most micropollutants than other biochars. CHBC amendment of sand biofilters showed a reduction in hydraulic flow from 24.48 m/day in fresh fine sand (FISA) to 1.87 m/day in CHBC amended fine sand, which facilitated the micropollutant biodegradation process. All measured compounds were attenuated by a combination of sorption and biodegradation processes, however, pharmaceuticals were removed more significantly (p<0.05) than herbicides in both amendment types. Overall, biodegradation accounted for >90% removal in CHBC and <60% for FISA columns. Microbial analysis confirmed a shift in bacterial community composition for CHBC versus FISA columns, but depth as the most critical community structuring factor. In this work, RSBC and CHBC were shown to have potential for cheap, and environmentally friendly amendments to enhance removal of micropollutants in surface water biofiltration.