Study On The Removal Of Sulphate And Cr (Ⅵ) From Water By Modified Rice Straw: Characteristics And Mechanism

Anions pollutants such as sulphate (SO₄^2-), hexavalent chromium (Cr (VI)) are commoncomposition in water, and can cause various environmental problems. Electrostaticforce-based adsorption technique may be effective to eliminate negative charged anionpollutants from aqueous solution. Absorption is rapid, and has the potential to recover someelements. Many kinds of materials can be used as adsorbents in wastewater treatment.Especially, low-cost adsorbents originated from agricultural/industrial wastes has becameattractive in recent years because of its significance both in environmental remediation and inreuse of wastes resource. In this study, rice straw that is a common agricultural residues insouthern China was modified into anionic adsorbent (RS-AE) to remove SO₄^2-and Cr (VI)from water. The surface morphology and chemical structure of raw straw and RS-AE werecharacterized and analyzed. Then, the removal characteristics and mechanism of sulphate andCr (VI) by RS-AE were investigated. The RS-AE adsorption column was made and used toseparate sulphate from water. In addition, removal of SO₄^2-by the method of RS-AEcombined with microorganism (sulphate reducing bacteria) also was carried out.To prepare RS-AE, raw rice straw was firstly treated by NaOH alkaline treatment, thenreacted with epichlorohydrin and finally quaternized with trimethylamine solution. The effectof NaOH concentration, dehydration of alkaline treated products and the temperature ofquaternization on the yield and adsorption efficiency of RS-AE was discussed. The resultsshowed that the yield was enhanced with increase of NaOH concentration and quaternizationtemperature, but the adsorption efficiency was decreased. Dehydration of alkaline treatedstraw was important for preparation of RS-AE. Compared with non-dehydration, filtrationand ethanol treatment, compression was more suitable for preparation of RS-AE. The yield ofRS-AE could reach77%under the condition of10%(W/V) NaOH solution, compressiondehydration and80℃quaternization temperature, and RS-AE exhibited a higher adsorptionefficiency75%for sulphate. The modification mechanism included three steps: the–OHcontained in straw was activited by NaOH; the activated–OH was etherized and crosslinked by epichlorohydrin; the etherized product was quaternized with trimethylamine.Characterization methods, such as scanning electron microscope (SEM), Fouriertransform infrared spectrum (FT-IR), solid state¹³C nuclear magnetic resonance (¹³C-NMR),X-ray diffraction (XRD) and pore autoanalyzer (Hg compression method) and so on, wereemployed to determine the element content, surface morphology, functional groups, crystaland pore structure of raw rice straw and prepared RS-AE. The results showed that manycellulosic fibers were exposed on the surface of straw after modification, and the content ofnitrogen and chlorine increased. The total nitrogen content of RS-AE is2.75%, from whichthe total exchange capacity was calculated as1.96mEq·g^-1. The main functional groups ofRS-AE were quaternary amino and hydroxyl groups. The crystalinity degreee and crystalinityindex of straw was decreased after modification, especially,101-crystal plane of cellulose wasdisappeared and diffraction intensity at002-crystal plane was impaired. The pore area of rawstraw was4.51m²Â·g^-1and pores maily were mesopores (2⁵0nm); while pore area of RS-AEwas1.19m²Â·g^-1and macropores (>50nm) account for about94%.Batch adsorption experiments showed that adsorption of sulphate on RS-AE was rapid,and the equilibrium time was about20min. The best sulphate removal was observed in pHrange of3⁸. The adsorption followed Langmuir adsorption model, and the maximumsulphate adsorption capacity of RS-AE was evaluated as74.76mg·g^-1, which was higher thanthat of raw rice straw (11.68mg·g^-1). Gibbs free energy ΔG of the adsorption were negative(-2.35^-3.46kJ·mol^-1), and ΔH, ΔS and adsorption activated energy Eawere8.49kJ·mol^-1,0.0376kJ·K^-1·mol^-1and19.3kJ·mol^-1, respectively. These adsorption thermodynamicparameters suggested that the adsorption of sulphate by RS-AE was a spontaneous,endothermic chemical adsorption. Pseudo-second order rate equation can be used to describethe adsorption process. The pseudo-second order rate constant kp2increased with the rise oftemperature, and fluctuated with sulphate initial concentration. In addition, the adsorptionkinetics was possibly controlled by liquid membrane diffusion. Sulphate anions were mainlyremoved through the way of ion exchange with chlorine ions on RS-AE. The measuredexchange coefficient was1.8that was close to the theoretical value2. RS-AE exhibited an selectivity for common anions in the order SO₄^2-> CrO₄^2-> NO3-> PO43-. Regenerationexperiments suggested that RS-AE has the potential for repeatedly use in removal of sulphate.Batch adsorption experiments for Cr (VI) removal by RS-AE indicated that theadsorption was apparently affected by solution pH conditions. For instance, best removal wasobtained in pH range of2⁶, then with the increase of pH, adsorption efficiency wasdecreased, and when the pH>9, the adsorption was inhibited. The adsorption equilibriumtime for Cr (VI) was short. Langmuir adsorption model fitted the adsorption equilibrium databetter, and the calculated maximum adsorption capacity was18.7mg·g^-1at298K. Theadsorption thermodynamic parameters (ΔG, ΔH, ΔS, and Ea) were evaluated. It found that ΔGwas negative, and ΔH, ΔS and Eawere11.5kJ·mol^-1,0.0664kJ·K^-1·mol^-1and24.5kJ·mol^-1,repectively. Tthe results clearly demonstrated that the adsorption of Cr (VI) on RS-AE was aspontaneous, endothermic chemical adsorption. Adsorption kinetics studies showed that thecomplete process of adsorption could be described by pseudo-second order adsorption kineticmodel, while the Elovich's equation only can be used to describe the initial adsorption stage.Diffusion model analysis suggested that Cr (VI) removal rate was controlled by liquidmemerbrane diffusion process, however, in diluted Cr (VI) solution, intraparticle diffusionmight be the determined step. The removal mechanism of Cr (VI) was ion exchange coupledwith Cr (VI) reduction to Cr (III). X-Ray photoelectron spectrum (XPS) characterizationshowed that Cr (III) was formed in the surface of RS-AE, moreover, adsorption of Cr (VI)under acidic conditions can favor the reduction of Cr (VI) to Cr (III).RS-AE adsorption columns were prepared to investigated the dynamic adsorption ofsulphate under fixed-and expanded-bed operation conditions. The breakthrough curves wereanalyzed with Thomas dynamic adsorption model. The experimental results showed that, forRS-AE fixed-bed columns, sulphate breakthrough time increased with the rise of H/D(height/diameter) ratio; while with the increase of sulphate initial concentration, thebreakthrough time was shorten. And RS-AE fixed-bed columns were easy to be regeneratedby0.1M NaOH and0.1M HCl solution. For RS-AE expanded-bed columns, a positiverelationship between influent flow rate and the expansion ratio was found. With the increase of expansion ratio, the breakthrough time reduced, however, the exhaustion time wasextended. At expansion ratio7%, the maximum column adsorption capacity from Thomasmodel was16.69mg·g^-1. From the comparison between fixed-and expanded-bed operations,it found RS-AE expanded-bed column with low expansion ratio was more suitable to removesulphate because of its larger column adsorption capacity and longer exhaustion time.The method of RS-AE combined with sulphate reducing bacteria (SRB) was tested forsulphate removal by static cultivation experiments. A better removal of sulphate can beobtained with the combined method in contrast to separately use of RS-AE or SRB. FromSEM images, it could be seen that many spherical bacteria were attached on the surface ofRS-AE forming adsorption-degradation system of sulphate. The experiments results showedthat sulphate concentration of culture medium can be reduced from1000mg·L^-1to100mg·L^-1in60hours by RS-AE combined with SRB. Furthermore, a anaerobic bioreactor was designedto evaluate the continuously removal of sulphate from synthesized wastewater by RS-AEcombined with microorganism. The best removal percentage of sulphate and total organiccarbon (TOC) were observed as76%and65%in two months running experiments. It isnecessary to point out that RS-AE started to be degraded by bacteria in45days, whichreduced the TOC removal percentage of effluent. However, this phenomenon suggested thatRS-AE was a biodegradable adsorbent that deserved further research.