Environ. Sci. Technol. 1999, 33, 3641-3644
Treatment of Combined Bleach Plant Effluents via Wet Oxidation over a Pd-Pt-Ce/Alumina Catalyst QINGLIN ZHANG AND KARL T. CHUANG* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
Supported Pd-Pt-Ce/alumina catalyst showed promising activity over traditional iron oxide, zinc oxide-type, oxidation catalysts for the treatment of pulp mill effluents. However, hot solution coupled with chlorine ions in the oxidation medium not only requires special process equipment but also often leads to catalyst deactivation due to metal leaching of catalyst. Dissolution of catalyst in the reaction medium also requires a further process step to remove the toxic metal ions from the treated stream. To address these problems, we proposed a combined treatment approach for catalytic oxidation of the bleach plant effluents from a softwood kraft pulp mill. Experimental study indicated metal leaching from catalyst can be significantly reduced even avoid with the combined stream at temperatures ranging from 403 to 443 K and at 1.5 MPa in a slurry reactor. Pd-Pt-Ce/alumina catalyst showed promising activity for wet catalytic oxidation of the combined streams. At 443 K and 1.5 MPa, 65% total organic carbon (TOC) and 99% color were removed after 3 h treatment, while only 10% TOC and 79% color reduction were achieved without catalyst at the same experimental conditions. Another advantage with combined treatment approach is that that cheap ferrous-based alloy may be used as construction material for process equipment. Based on the experimental results, implications for process development are discussed.
Introduction The majority of pulp mills use the kraft bleaching process in which chlorine and/or chlorine dioxide are used as bleaching agents to remove residual lignin from pulp. After chlorination, the residual lignin is extracted from the pulp in the subsequent alkali stage. The bleaching plant effluents from the alkali stage and acidic chlorine dioxide bleaching stage are highly colored and potentially toxic, primarily due to the presence of high molecular weight, modified, and chlorinated lignin and its degradation products (1). The pH of the effluent from the acidic chlorine dioxide stage (DO) is as low as 2.2 and that from alkali stage (EOP) is about 11. These effluents also contain sulfur compounds, pulping chemicals, organic acids, resin acids, phenolics, unsaturated fatty acids, terpenes, etc. Most of the organically bound chlorine is associated with compounds of molecular weight greater than 1000 (2). These chlorinated lignins and derivatives contribute high color level and chemical oxygen demand (COD) in the effluent. Biological-based water treatment processes are incapable of significant reduction of these pollutants due to * Corresponding author telephone: (403)492-4676; fax: (403)4922881; e-mail:
[email protected]. 10.1021/es990063l CCC: $18.00 Published on Web 09/11/1999
1999 American Chemical Society
the complexities associated with microbial contamination, requirements of co-metabolism, mycelial clogging of bioreactors, and long retention times (3-5). Hence, additional nonbiological treatment steps have to be used following biological treatment (6). Wet air oxidation (WAO) has been proposed as an alternative technology for the treatment of bleach plant effluent (7-10). Prasad and Joshi (11) reported that pyrolysis (or thermal degradation) of these effluents did not occur even at 543 K in the absence of a catalyst. The noncatalytic wet air oxidation of these effluents is slow and requires very severe temperature and pressure conditions, typically 473573 K and 10 or more MPa. Hence, it is highly desirable to develop a catalytic wet oxidation process for treatment of these effluents at milder conditions. Recently, we have shown that commonly employed metal oxides, such as MnO2, CuO, Fe2O3, etc., are not effective for oxidation of the effluents from pulp mills (12, 13). Moreover, metal oxides such as MnO2, CuO, and Fe2O3 are readily dissolved in the acidic wastewater stream, which not only results in catalyst deactivation but also requires additional process steps to remove the leached metal ions. We have also shown that Pd-Pt-Ce/alumina catalyst is highly active for the oxidation of individual effluents from the EOP or DO bleaching sequence (13, 14). However, considerable metal leaching occurred under wet oxidation conditions with individual effluents, particularly with the acidic effluent. The low pH of the wastewater has been shown to be an important factor leading to metal leaching and catalyst deactivation (14). Furthermore, the hot acidic oxidation medium coupled with the presence of chlorine requires expensive process equipment, which accounts for over 80% cost of a typical wet oxidation process. Our extensive literature survey showed no prior reports dealing with the most significant issues of metal leaching and corrosion, which are the key issues in developing an industrial catalytic wet oxidation process for the effluent treatment. To address these problems, we have proposed a novel approach of the combined treatment in this study. Experimental study was carried out with a Pd-Pt-Ce/Al2O3 catalyst at temperatures ranging from 403 to 443 K and at 1.5 MPa in a slurry reactor. Influences of reaction conditions on TOC and color removal are reported. Implications of experimental results are discussed for development of an industrial process for pulp mill effluent treatment.
Experimental Section Catalyst Preparation. Ultrafine γ-alumina (ALON, supplied by Cabot Corporation, BET surface area of 102 m2/g), palladium chloride and tetraammineplatinum(II) nitrate solution (Colonial Metals Inc., USA), and cerium acetate (Aldrich) were used as received. Alumina-supported Pd-Pt-Ce/alumina catalyst was prepared by sequential incipient wetness impregnation. The mixture of the support and the cerium-containing solution was dried for 12 h in an atmospheric rotary evaporator under infrared light. The dried mixture was then calcined in a tube reactor under air flow at 773 K for 6 h. Subsequently, Pd or Pt was added on the Ce/Al2O3 catalyst following a similar procedure described above for Ce/alumina preparation. Details for the catalyst preparation are given elsewhere (13). The palladium, platinum, and cerium loading of the catalyst was 4.05, 0.42, and 4.58 wt %, respectively, as determined by ICP-MS analysis of the dissolved catalyst solution. Experimental Procedures. The experimental setup is shown in Figure 1. The reaction vessel was a high-pressure VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic diagram of experimental apparatus.
TABLE 1. Characteristics of the Bleach Plant Effluents from Weyerhaeuser Canada Ltd. effluent
flow rate (m3)
pH
TOC (mg/L)
color Pt-Co (mg/L)
EOP DO combineda
363-409 341-454 704-863
11 2.0 8.5
∼1200 ∼700 ∼850
1571 1455 2345
a Precipitation occurred after the two effluents were combined. After filtration, TOC, color, and pH were measured.
Parr reactor (model 4653, Parr Instrument Inc., USA). It was made of SS-316 stainless steel with a Pyrex glass liner. The reactor, equipped with a glass impeller, had an effective volume of 300 mL. A thermal sensor and an external heating element were also provided to control the reactor to (1 K. The operating pressure of the oxidation reaction was controlled by a back-pressure controller in the exit line. The effluent samples were supplied by Weyerhaeuser Canada Ltd. Table 1 shows the characteristics of the individual and the combined effluents. In the experiments, typically 1 g of catalyst powder (
