Sorption Of Lead (Pb~2+) In Aqueous Solution By Chemically Modified Biochar Derived From Coconut Fiber And Its Mechanism

Biochar and chemically modified biochar have received widespread attention as an environmentally friendly material to remediate contaminated sites with its physico-chemical characteristics. Technologies such as Fourier transformation infra-red spectrophotometry (FTIR), scanning electron microscope (SEM), BET specific surface area analysis and C, H, N, and O elemental analysis were used to research the surface chemical composition and changes of the physico-chemical properties of coconut fiber-derived biochars through chemical modifications. Kinetics and isothermal adsorption experiments were carried out to examine the adsorption characteristics and capacity for Pb2+of modified coconut fiber-derived biochars (MCFBs). Energy-dispersive X-ray Spectroscopy (SEM-EDS) technology was applied to investigate the distribution of lead on Pb-loaded biochars. Then we explored the lead species of Pb-loaded CFBs and MCFBs and investigated the combination manner and mechanism of Pb2+adsorbed on biochars, by using X-ray absorption fine structure (XAFS) and FTIR technologies. The results were presented below.i) We investigated the effects of three types of coconut fiber-derived biochars (CFBs) pyrolysed at different temperatures300,500and700癈(as control), and nine types of these biochars modified with ammonia, hydrogen peroxide and nitric acid, respectively, referred to as modified coconut fiber-derived biochars (MCFBs). Results showed that the specific surface area was almost120times higher for the unmodified biochar pyrolysed at700癈compared to that pyrolysed at300癈. The cation exchange capacity of biochar pyrolysed at300癈and modified with nitric acid increased nearly fourfold compared to the control. Loosely corrugated carbon surface and uneven carbon surface of the biochar pyrolysed at300癈were produced during ammonia and nitric acid modifications. Number of peaks and the peak intensity of functional groups on biochars including the peak intensity of hydroxyl O-H stretching, alkene C=C stretching vibration and anhydride C-O-C stretching vibration decreased with increasing pyrolysis temperature. In the FTIR spectrum of modified biochars pyrolysed at300癈, the peak intensity of C=O stretching of carboxyl increased while the peak intensity of C=O stretching of quinones or ketonic acids decreased both on biochars modified with nitric acid and hydrogen peroxide; the peak intensity of C=O stretching of carboxyl of biochar produced at300癈and modified with ammonia is decreased. Both the modified biochars produced at500and700癈had similar changes in functional groups, but these changes were not as obvious as those observed for the biochars produced at300癈. The dissolving of soluble mineral oxides and the chemical reactions of functional groups on the biochars were main reasons for the changes of the basic properties of the biochars during the modification treatments. These modification treatments changed the contents of elements, functional groups, cation exchange capacity and the specific surface areas of the CFBs. Further research should be done to improve the understanding of how different physico-chemical characteristics of biochar affect the removal of pollutants in natural environments.ii) Chemical modification treatments of coconut-fiber derived biochars (CFBs) affected their physico-chemical properties of MCFBs. Next with that, we investigated their sorption characteristics and capacities of these biochars for lead (Pb2+). The pseudo first order model did adequately describe the adsorption results of Pb2+onto CFBs, hydrogen peroxide and ammonia modified CFBs, however, nitric acid modified biochars were befittingly described by the pseudo second order model. The Langmuir model was a better fit for Pb2+on all CFBs and MCFBs in isothermal adsorption. The Langmuir-qm values of biochars for Pb2+showed that, the adsorption capacity of the biochars for Pb2+increased with increasing pyrolysis temperature. The Langmuir-qm values of biochars pyrolysed at 300℃and modified with ammonia and nitric acid increased from49.4to101.4and86.8mg g-1, compared to the control; Typically, the qm values for Pb2+of the biochars pyrolysed at500and700℃and modified by nitric acid decreased from88.3and140.9mg g-1to26.5and29.1mg g-1compared to the unmodified biochars. The SEM-EDS results presented that the order of the Pb contents on representative Pb-loaded biochars was700Contral>300NH3·H2O>500Control>300HNO3>300Control.The percent of residual Pb2+cannot released by1M HC1aqueous solution on Pb-loaded biochar pyrolysed at700℃(700Control), MCFBs pyrolyzied at300℃and modified with ammonia and nitric acid aqueous solution (300NH3·H2O and300HNO3) are significantly higher compared to that of unmodified CFB pyrolyzied at300℃(300Control).iii) The LCF (linear combination fitting) analysis results of k2-weighted (extended X-Ray absorption fine structure) EXAFS spectra of all Pb-loaded biochars indicated that Pb-montmorillonite, Pb(C2H3O2)2, PbSO4, Pb-Al2O3and Pb3(PO4)2were the five most important Pb species for the Pb-loaded biochars. The percent sum of PbSO4, Pb3(PO4)2and Pb-Al2O3in the Pb-loaded biochars pyrolysed at500and700℃and modified with nitric acid decreased from59.0and64.7%to8.4and3.4%, while the percentage of Pb-montmorillonite raised from27.4and23.8%to41.4and48.0%, and the percentage of Pb(C2H3O2)2increased from7.9and0%to43.7and35.9%, respectively, compared to the control. The FTIR results proved that the modification of the biochar with different reagents enhanced its surface complexation with free carboxyl functional groups and free hydroxyl functional groups of lead on the biochars. We divide mechanisms into the following four parts:(1) equivalent Pb2+exchange with K+and Na+;(2) equivalent Pb2+exchange with Ca2+and Mg2+;(3) exchangeable Pb2+except equivalent of alkaline earth metals (Ca2+, Mg2+, K+and Na+) and (4) stable Pb2+on biochars.Overall, our results indicated that the chemically modified coconut fiber-derived biochar with ammonia or nitric acid pyrolysed at300℃and unmodified biochar pyrolysed at700℃were the most promising biochars for remediation of water and soils contaminated with lead. Further research on the specific molecular or atomic combination, valence bond and bond length of various adsorded forms of lead should be carried out to understand the stability of Pb sorbed on biochar and the mechanisms of Pb bioavailability and toxicity in Pb contaminated environment.