Article pubs.acs.org/IECR
Boron Removal Performance of a Solid Sorbent Derived from Waste Concrete Atsushi Iizuka,*,† Miyuki Takahashi,† Takashi Nakamura,† and Akihiro Yamasaki‡ †
Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai, Miyagi, 980-8577, Japan ‡ Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan ABSTRACT: Low-cost materials for boron removal need to be developed to reduce the overall cost of treatment of boroncontaining wastewater. In this paper, the preparation of a boron removal material using waste concrete particles is described. With high initial boron concentrations (100 and 300 mg/L boron), the removal performance was insufficient and the dominant boron removal mechanism was estimated to be precipitation of calcium borate. With a low initial boron concentration (10 mg/L boron), more boron was removed by increasing the amount of the sorbent, and the boron removal can be attributed to ion exchange. After heat treatment of the material at 175 °C, an initial decrease in the boron removal rates was observed, but after 1440 min the residual boron concentrations for the heat-treated material were much lower than those for the untreated material. This improvement is thought to be due to the generation of metaettringite, the dehydration product of ettringite.
1. INTRODUCTION Boron is an essential micronutrient for plants and humans. Furthermore, boron is an important material in many industries, such as in glass, ceramics, semiconductor, and detergent manufacturing, as well as in metal smelting and refining processes. However, a high concentration of boron in irrigation water sources has a toxic effect on most plants.1 The long-term intake of excess amounts of boron through drinking water is potentially toxic to humans and animals.2 To prevent the undesirable effects caused by boron in water, the anthropogenic discharge of boron into the environment must be controlled. Table 1 summarizes the water quality criteria for boron. For these reasons, boron removal from industrial wastewaters has received much attention in recent years. A number of physicochemical methods have been proposed to remove boron from industrial wastewaters,12 such as coagulation, sedimentation, adsorption and ion exchange, and
membrane separation. Adsorption and ion exchange with solid adsorbents are advantageous over other methods in terms of the overall simplicity and the cost of the process. Some adsorbents and ion-exchange resins show excellent boron removal performance; however, most adsorbents and resins are expensive for the treatment of a large amount of effluent. Therefore, lower-cost sorbents for the removal of boron from solution need to be developed to reduce the overall cost of the treatment of boron-containing wastewater. It is known that ettringite (Ca6Al2(SO4)3(OH)12·26H2O) has anion exchange ability. The SO42− ions in ettringite can be exchanged by other anions such as B(OH)4−, CrO42−, SeO32−,13−15 or F−.16 Hiraga and Shigemoto17 demonstrated that this ion-exchange ability can be used to remove boron from water. Ettringite is formed during the hydration of portland cement.18 Here, we focus on ettringite contained in concrete waste as an inexpensive sorbent for boron removal. In previous studies, we prepared boron removal materials that contained ettringite from concrete sludge19,20 and confirmed that the prepared materials can be used for boron removal from effluents. The boron removal performance of the materials was improved by heat treatment, which is considered to be due to the generation of the meta-ettringite (dehydrated ettringite) phase and simultaneous uptake of boron with rehydration water.19−21 In the current study, to prepare simple and inexpensive boron removal materials, we examined the boron uptake performance of crushed waste concrete particles. Concrete, which is a mixture of cement, water, and aggregates, is the predominant construction material worldwide. After the
Table 1. Water Quality Criteria for Boron standard (mg/L boron) WHO Japan
Australia Canada
EU Korea United States
drinking water guideline environmental criteria effluent standard (uniform standard) effluent standard (interim standard) drinking water guideline interim maximum acceptable concentration irrigation water guideline drinking water directive drinking water quality standard drinking water equivalent level
ref.
0.5 1.0 10
3 4 5
50−500
5
4.0 5.0
6 7
0.5−6.0 1.0 1.0 3.0
8 9 10 11
© 2014 American Chemical Society
Received: Revised: Accepted: Published: 4046
July 9, 2013 February 3, 2014 February 11, 2014 February 11, 2014 dx.doi.org/10.1021/ie402176t | Ind. Eng. Chem. Res. 2014, 53, 4046−4051
Industrial & Engineering Chemistry Research
Article
were sampled periodically through a 0.45 μm cellulose filter, and the concentrations of B and 18 other elements (Ca, Al, As, Cd, Cr, Fe, Hg, K, Mg, Mn, Na, Ni, P, Pb, S, Si, Ti, and Zn) in the samples were determined with the ICP-AES. At the end of the boron removal experiment, the solution was filtered through filter paper and the filtered solid was dried under atmospheric conditions. The dried samples were analyzed using X-ray diffraction (XRD; RINT2000, Rigaku, Tokyo, Japan).
lifetime of the concrete, crushed waste concrete is generated by demolition. In Japan, crushed waste concrete has been used for road bed materials, although demand is decreasing. Development of a new recycling method for waste concrete is an important issue.22−31 If crushed waste concrete could be used as an inexpensive boron removal material, it would be a practical use for waste concrete. We also examined the effect of heat treatment of waste concrete particles on the boron removal performance.
3. RESULTS AND DISCUSSION 3.1. Boron Removal Performance of Crushed Waste Concrete. Figures 1−3 show the variation of the boron
2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of Sorbent. The waste concrete sample used in this study was kindly supplied by Tateishi Construction Co. Ltd. (Tokyo, Japan). The sample was fine particles obtained as a byproduct in a recycling plant for waste concrete. The plant recovers aggregates contained in crushed waste concrete by the mechanical grinding method. Thus, the waste concrete sample can be considered as crushed hydrated cement particles. The main chemical compounds generated in the hydration reaction of portland cement are calcium silicate hydrates (C−S−H, e.g., 3CaO·2SiO2·4H2O), calcium hydroxide (Ca(OH)2), and ettringite (Ca6Al2(SO4)3(OH)12·26H2O). The waste concrete sample was classified with a sieve of 212 μm, and the smallest particles were used as the boron adsorbent without further treatment. The influence of heating on the boron removal performance was also examined. In this case, classified waste concrete sample was heated in an oven overnight at 175 °C and then used for the boron removal experiments. It should be noted that the sorbent surface becomes hydrophilic after heat treatment and is easy to disperse in solution. Table 2 shows the elemental
Figure 1. Variation of boron concentration with time after addition of the sorbent. Initial boron concentration: 100 or 300 mg/L B.
Table 2. Typical Elemental Composition of the Waste Concrete element
composition (wt %)
element
composition (wt %)
Ca Si Al Fe Mg S Na K Ti
16.3 11.4 4.5 2.5 1.5 0.6 0.6 0.4 0.2
P Mn Zn As Cd Hg Ni Cr Pb
0.04 0.09 0.02 0.0025 0.0005
