Preparation and Modification of Various Adsorbents from Rice Husk and Their Use for the Removal of Toxic Dyes and Heavy Metal Ions from Aqueous Media

Abstract

Rice Husk (RH) was collected from the local rice processing mill in district Mardan, Khyber Pakhtunkhwa, Pakistan. The RH was thermally modified in the absence of air at 400oC to prepare the Rice Husk Char (RHC). Whereas the thermal modification of RH in the presence of air at 700oC leads to the production of Rice Husk Ash (RHA). Both the RHC and RHA were modified with KOH and labeled as KOH Modified Rice Husk Char (KMRHC) and KOH Modified Rice Husk Ash (KMRHA). The thermally and Thermo-chemically modified material were subjected as adsorbents for the removal of toxic dyes (Orange G and Titan yellow) and heavy metal ions (Pb2+) from aqueous media. Variation in the experimental conditions (agitation time, dye/metal ions concentration, adsorbent dose, pH and temperature) play significant role in the adsorption process. The maximum adsorption capacity of Orange G (OG) on RHC and KMRHC was investigated as 28.8 mg/g and 38.4 mg/g respectively at pH = 4 using initial dye concentrations of 80 mg/L containing 2g/L of the adsorbent dose with agitation speed of 240 rpm at 303K. Under the same experimental conditions the uptake capacity (mg/g) of OG on RHA and KMRHA was inspected as 23.05 mg/g and 36.45 mg/g respectively. The percentage adsorption of OG on RHC and KMRHC were recorded as 65.1% and 96.0% respectively, whereas that on RHA and KMRHA was noted as 57.6% and 91.1% respectively. These results indicated that KOH modification of RHC and RHA resulted into efficient adsorbents for the removal of OG dyes from aqueous media. Almost similar results were also investigated for the removal of Titan yellow (TY) dye on these adsorbents. Slight variation the adsorption capacities (mg/g) and percent adsorption was attributed to the structural and compositional changes in the OG and TY dyes. The maximum removal of TY on these adsorbents were investigated at pH = 3.5. The optimum adsorption capacities (mg/g) of Pb2+ ions on RHC, KMRHC, RHA and KMRHA-adsorbents were investigated as 92.2mg/g, 119mg/g, 86.75mg/g and 113.7 mg/g respectively. This suggested that the modification of RHC with KOH enhanced its adsorption capacity from 92.2 to 119mg/g while that of RHA from 86.75 and 113.7 mg/L. The metal ions adsorption experiments were performed using initial Pb2+ ions concentrations of 250 mg/L containing 2g/L of the adsorbent dose at pH = 6 with stirring speed of 240 rpm at 303K. The maximum adsorption efficiencies of Pb2+ ions were noted as 74%, 95.9%, 86.75% and 90.6% on RHC, KMRHC, RHA and KMRHA-adsorbents respectively. This revealed that the percent

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adsorption of Pb2+ ions on KMRHC and KMRHA are greater than the corresponding values of RHC and RHA, which suggested that KOH modified Rice Husk Char and Rice Hush Ash (KMRHC and KMRHA) are efficient adsorbents for the removal of Pb2+ ions from aqueous media. Adsorbent dose have a significant role on the adsorption capacity of metal ions on adsorbents. For 1g/L of adsorbent dose, the maximum adsorption capacity for Pb2+ ions on RHC, KMRHC, RHA and KMRHA was inspected as 136mg/g, 172mg/g, 106mg/g and 152 mg/g respectively other experimental conditions were the same as mentioned above. Thermodynamic studies of the adsorption of OG, TY and Pb2+ on RHC, KMRHC, RHA and KMRHA indicated that the values of ΔG and ΔH were negative which revealed that the adsorption process is exothermic and spontaneous. The negative value of ΔS suggested that randomness decreases at the interface of adsorbent-adsorbate during the adsorption process. The kinetics study indicated that the experimental data of the adsorption process best fits to pseudosecond order kinetic model. The equilibrium data was tested on Langmuir, Freundlich and Temkin adsorption isotherm models. It was inspected that the data followed all the three isotherm models (R2>0.90). However, the values of correlation coefficients (R2) indicated that the data is mostly best fit to the Langmuir isotherm model (R2>0.98) which suggest for chemi-sorption process. The RHC, RHA and KOH modified RHC and RHA were characterized by XRD, SEM, FTIR and EDX. XRD pattern indicated the crystalline entities in the amorphous matrix had blocked the aggregation of silica particles and hence enhanced the surface area of the adsorbents for the maximum adsorption of dyes and heavy metal ions. SEM images indicated that the KOH modification of RHC and RHA made these materials highly porous which enable them more efficient adsorbents for the adsorption of toxic dyes and heavy metals from aqueous media. In the light of experimental results so obtained it was found that within the experimental range of various parameters used, the adsorption rate is appreciably affected but the overall order and hence the mechanism of adsorption may remain the same. However, the nature and mechanism of adsorption may vary by changing the type/nature of different adsorbents prepared. The overall results obtained in this work suggested that our new material would be applied for the removal of toxic dyes and heavy metals from waste water as an alternate and easier option. It will also motivate further research in the direction of removal of contaminants by applying modified RH.