Preparation of calcium oxalate--bromopyrogaliol red inclusion sorbent and application to treatment of cationic dye and heavy metal wastewaters

摘要

Dye pollutants are a major class of environmental contaminants. Over 100,000 dyes have been synthesized worldwide and more than 700,000 tons are produced annually and over 5 percent are discharged into aquatic environments. The adsorption or sorption is one of the most efficient methods to remove dye and heavy metal pollutants from wastewater. However, most of the present sorbents often bear some disadvantages, e.g. low sorption capacity, difficult separation of spoil, complex reproduction, or secondary pollution. Development of novel sorbents that can overcome these limitations is desirable. On the basis of the chemical coprecipitation of calcium oxalate (CaC_2O_4), bromopyro-gallol red (BPR) was embedded during the growing of CaC_2O_4 particles. The ternary C_2O_4~(2-)-BPR-Ca~(2+) sorbent was yielded by the centrifugation. Its composition was determined by spectrophotometry and AAS, and its structure and morphology were characterized by powder X-ray diffraction (XRD), laser particle-size analysis, and scanning electron microscopy (SEM). The adsorption of ethyl violet (EV) and heavy metals, e.g. Cu(II), Cd(II), Ni (II), Zn(II), and Pb(II) were carried out and their removal rate determined by spectrophotometry and ICP-OES. The adsorption performance of the sorbent was compared with powder activated carbon. The Langmuir isothermal model was applied to fit the embedment of BPR and adsorption of EV. The saturation number of BPR binding to CaC_2O_4 reached 0.0105 mol/mol and the adsorption constant of the complex was 4.70 X 10~5 M~(-1). Over 80 percent of the sorbent particles are between 0.7 and 1.02 mu m, formed by the aggregation of the global CaC_2O_4/BPR inclusion grains of 30-50 nm size. Such a material was found to adsorb cationic dyes selectively and sensitively. Ethyl violet (EV) was used to investigate the adsorption mechanism of the material. One BPR molecule may just bind with one EV molecule. The CaC_2O_4/BPR inclusion material adsorbed EV over two times more efficiently than the activated carbon. The adsorption of EV on the CaC_2O_4/BPR inclusion sorbent was complete in only 5 min and the sedimentation complete in 1 h. However, those of EV onto activated carbon took more than 1.5 and 5 h, respectively. The treatment of methylene blue and malachite green dye wastewaters indicated that only 0.4 percent of the sorbent adsorbed over 80 percent of color substances. Besides, the material can also adsorb heavy metals by complexation with BPR. Over 90 percent of Pb~(2+), and approximately 50 percent of Cd~(2+) and Cu~(2+), were removed in a high Zn~(2+)-electroplating wastewater when 3 percent of the material was added. Eighty-six percent of Cu~(2+), and 60 percent of Ni~(2+) and Cd~(2+), were removed in a high Cd~(2+)-electroplating wastewater. The embedment of BPR into CaC_2O_4 particles responded to the Langmuir isothermal adsorption. As the affinity ligand of Ca~(2+), BPR with sulfonic groups may be adsorbed into the temporary electric double layer during the growing of CaC_2O_4 particles. Immediately, C_2O_4~(2-) captured the Ca~(2+) to form the CaC_2O_4 outer enclosed sphere. Thus, BPR may be released and embedded as a sandwich between CaC_2O_4 layers. The adsorption of EV on the sorbent obeyed the Langmuir isothermal equation and adsorption is mainly due to the ion-pair attraction between EV and BPR. Different from the inclusion sorbent, the activated carbon depended on the specific surface area to adsorb organic substances. Therefore, the adsorption ca-pacity, equilibrium, and sedimentation time of the sorbent are much better than activated carbon. The interaction of heavy metals with the inclusion sorbent responded to their coordination. By characterizing the C_2O_4~(2-) - BPR-Ca~(2+) inclusion material using various modern instruments, the ternary in situ embedment particle, [(CaC_2O_4)_(95) (BPR)]_n~(2n-), an electronegative, micron-sized adsorbent was synthesized. It is selective, rapid, and highly effective for