This study proposes a method to convert non-structural calcium-rich construction and demolition waste fines into adsorbents of heavy metal ions by mixing waste fines with diammonium hydrogen phosphate solution to produce hydroxyapatite, which has high surface areas and excellent ion-exchange capacity with heavy metal ions. As a result, environmental polluting waste is converted into environmentally cleaning material. Waste putty powders was chosen as the representative waste to investigate the detailed formation process of hydroxyapatite and the key reaction parameters of the reaction. Results showed that hydroxyapatite can be produced on waste putty particles. Higher ageing temperatures or longer ageing duration are beneficial to the yield and crystallinity of the produced hydroxyapatite. Adsorption testing confirmed that Ni2+ can replace Ca2+ in the hydroxyapatite lattice, leading to the formation of a new crystal, arupite (Ni3(PO4)2•8H2O), and contributing to a modest adsorption capacity for Ni2+ (15 mg/g) for the hydroxyapatite-containing waste putty.
An estimated 2 billion tons of construction and demolition (C&D) wastes are produced each year globally. These wastes create enormous negative effects on the environment in terms of land occupation and environmental pollutions of land, water, and air (1–4). Recycling the C&D wastes for new applications is the most promising way to combat the negative impacts of C&D wastes as well as resources recycling. Generally, C&D wastes are classified into structural and non-structural wastes depending on their qualities and mechanical properties. For instance, waste and demolished concrete is such kind of structural waste with high mechanical properties. It can be crushed into particles and can be used as recycled concrete aggregates (RCAs) in new concrete, which is well known as recycling concrete (RC) (1, 5–10). Every year, abundant studies on manufacturing methods and properties of RCAs as well as the fresh and hardened properties of RCA-incorporated RC are carried out worldwide, contributing towards the progress of recycling such waste. Nevertheless, decorative construction waste, a characteristic kind of non-structural waste, receives very little attention in the last few decades towards to its reuse.
Construction putty is one of the widely applied decorative materials on the surface of building components to fill cracks, scratches, and other irregularities such that a smooth surface is formed for subsequent paint and varnish. Normally, a thickness of 20 mm and 50 mm of construction putty (or about 1 kg/m2) is required for coating building components. Thus, one can expect that a huge amount of waste construction putty (WP) is correspondingly produced globally along with the two billion tons of C&D wastes each year. By now, in addition to landfill or as partial filler for subgrade, effective reuse methods of WP are rarely reported, possibly due to issues such as cost efficiency and reuse technology. In particular, reuse of WP with inferior mechanical properties cannot be focused without value added technology. This paper intends to fill this gap to develop a value added reuse technology for WP based on a wet chemical method to convert WP into an adsorbent of heavy metals. Hydroxyapatite (HAP), with the ideal formula Ca10(PO4)6(OH)2, is chosen as the target adsorbent in this method. HAP belongs to the apatite family and commonly exists in natural bones and teeth (11–14). It has salient features in terms of high stability, good biocompatibility, high bioactivity, and high sorption capacity (12, 15–17). Particularly, HAP possesses the salient capacity of adsorbing heavy metal ions through a so-called ion-exchange process (
where
Equation [1] suggests that heavy metal ions can replace the calcium in the lattice of HAP. A maximum of 10 heavy metal units can be strongly stabilized by one HAP unit. The adsorption capability of HAP on a variety of toxic metal ions has been successfully confirmed, such as Co2+, Cu2+, Cd2+, Zn2+, Ni2+, and Pb2+ (
In the case that HAP contains calcium and PO42- as its substantial constituents, and that calcium-rich minerals, i.e., calcium carbonate, lime, and calcium-magnesium fine particles are commonly used in construction putty as the filler (
To demonstrate the feasibility of the proposed approach, a preliminary study was carried out in this paper to investigate the detailed formation process of HAP and the key reaction parameters of the reaction between WP and DAP solution. The produced WP-HAP particles were comprehensively characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and Brunauer, Emmett, and Teller (BET) methods. Moreover, the ion-exchange of the produced WP-HAP powder with Ni2+ (that is commonly used in refining, steel, welding, and electroplating industries and is one of the most harmful toxic metal to human beings even in small quantities that can cause lung fibrosis, kidney disease, and allergic dermatitis (
WP was collected from interior walls of a discarded building in the old campus of Anhui University of Science and Technology, China. Its chemical components and mineral composition were examined by X-ray fluorescence (XRF) and XRD, respectively, as shown in
Chemical composition of the waste construction putty by XRF
Oxide | CO2 | CaO | MgO | SO3 | SiO2 | Al2O3 | Fe2O3 | Na2O | K2O |
---|---|---|---|---|---|---|---|---|---|
Content (%) | 43.6 | 28.0 | 13.7 | 7.9 | 5.6 | 0.6 | 0.2 | 0.2 | 0.1 |
XRD pattern of the waste construction putty.
WP was first grounded into fine powder passing through a 200-mesh sieve (less than 75 μm) and then dried in an oven at 150 °C for 24 hours until mass equilibrium was reached. (NH4)2HPO4 (DAP) and NiSO4·6H2O of analysis purity without further purification were purchased from the Sinopharm Chemical Reagent Co., Ltd. Tap water was used throughout this study with an aim to maintain the cost of the proposed process to the minimum.
WP-HAP powder was prepared based on the chemical reaction between DAP solution and the WP. This reaction is determined by two major factors: aging temperature and duration. Three aging temperatures (23°C, 50°C and 80°C ) and four aging durations (3, 9, 15 and 21 days) were used in the study to evaluate their effects on the morphological and mineralogical properties of the WP-HAP.
In a typical procedure, 200 ml 2 M DAP solution was initially prepared in a beaker. 133.6 g WP powder was then added into it to achieve a Ca/P ratio of 1.67, at which the produced HAP crystal may have a similar structure as those existing in bones and teeth (
A simple adsorption experiment was conducted under ambient temperature and pressure to confirm that the produced WP-HAP powder can adsorb Ni2+ through the ion-change process. The waste solution with heavy metal ions was simulated by mixing NiSO4·6H2O with deionized water with the concentration of Ni2+ as 100 mg/L. 100 ml waste solution was mixed with 0.2 g 15d50 WP-HAP in a beaker, which was then sealed and shaken at the speed of 100 r/min by a rotating shaker for 72 hours. After that, the waste solution was filtered and the solid substance on the filter paper was dried at 105°C for 24 hours for SEM, EDS, XRD analysis to study the adsorption process of the produced WP-HAP powders. The amount of Ni2+ adsorbed by the WP-HAP was determined by a PerkinElmer optima 7000 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES).
Morphological examination and elemental analysis were performed using a S-3400N scanning electron microscopy equipped with X-ray energy-dispersive spectroscopy. The operating acceleration voltage was 15 kV. Samples prepared for SEM observation were coated by a layer of gold particles to improve their electrical conductivity before SEM operation. Mineralogical composition was investigated by a SmartLab 9KW multi-function X-ray powder diffractometer using Cu_Kα radiation at 45 kV and 200 mA at a scan speed of 8°/min between 2θ of 10° to 60°. Chemical composition of WP was determined by a XRF-1800 X-ray fluorescence spectroscopy. Brunauer, Emmett, and Teller surface area of WP and WP-HAP samples was tested by an ASAP2020 specific surface area and aperture analyzer.
XRD patterns of WP-HAP samples aged for 3 d.
The contents of these three new minerals are clearly dependent on the aging temperature. For example, the characteristic peaks of HAP were only found in 3d50 and 3d80. Intermediate phosphate phases such as monetite and farringtonite were formed at the ambient temperature (23°C). No HAP was identified in this sample, suggesting that the ambient condition was insufficient to produce HAP only with 3 d of aging by the proposed method. At a higher aging temperature (50°C), HAP appeared in the XRD pattern, and the content of intermediate phosphate phases was greatly reduced. The presence of intermediate phosphate phases suggests one of the drawbacks of wet methods of HAP synthesis that the production of crystallographically pure HAP is challenging when other phases of phosphates are present (
The effect of aging temperature on the mineralogical composition of the produced WP-HAP made with a longer aging duration (15 d) is displayed in
XRD patterns of WP-HAP samples aged for 15 d.
Aging duration is another important factor influencing the growth of the produced HAP. Samples 3d50, 9d50, 15d50, and 21d50 were chosen to study the mineralogical evolution of HAP with aging duration at a fixed aging temperature. The chosen aging temperature was 50°C, since HAP can be synthesized at this temperature as shown in
XRD patterns of WP-HAP samples aged at 50°C.
Morphology of the WP-HAP samples varied with the aging duration and temperature, as revealed by the SEM images shown in
Typical SEM images of WP and WP-HAP samples: (A)/(a) WP; (B)/(b) 3d23; (C)/(c) 3d50; (D)/(d) 3d80; (E)/(e) 15d23; (F)/(f) 15d50; (G)/(g) 15d80.
The features of the produced WP-HAP samples revealed by SEM imaging can be further confirmed by their BET areas, as shown in
BET areas of WP and WP-HAP samples.
Larger surface area is desirable for adsorbing heavy metals (
XRD patterns of WP-HAP samples (15d50) after adsorption of Ni2+. (A: arupite; D: dolomite; C: calcite; M: monetite; F: farringtonite; H: hydroxylapatite)
Ion-exchange with Ni2+ also modified the morphology of the produced WP-HAP sample, as revealed by the SEM images shown in
SEM images and EDS result of WP-HAP (15d50) samples after adsorption testing: (a) and (b) SEM; (c) EDS result.
Ni content in the 15d50 WP-HAP sample after the adsorption test was measured by ICP-OES. The adsorption capacity of this sample for Ni2+ is 15 mg/g. This adsorption capacity is not high but is still comparable to other reported available adsorbents for Ni2+, i.e., 35.8 mg/g of barley straw (
A method to upcycle non-structural C&D wastes as a heavy metal adsorbent is demonstrated in this study. Experimental study confirmed that HAP can be produced on the surface of waste putty particles by soaking the WP particles in a DAP solution at temperatures higher than 23°C. Higher aging temperature or longer aging duration is beneficial to the yield and crystallinity of the produced HAP crystals. Adsorption testing confirmed that ion-exchange of Ca2+ with Ni2+ in the produced HAP does take place, leading to the formation of arupite. Although a modest adsorption capacity for Ni2+ (15 mg/g) was achieved by the produced adsorbent due to the relatively large particle size of the WP, much higher adsorption capacity for heavy metal can be achieved by using finer WP powder.
The proposed wet chemical method for WP reuse is simple and cost-effective. More research will be carried out to optimize the manufacturing process and enhance the adsorption capacity of the new waste construction putty-based heavy metal adsorbent.
This work was supported by China Postdoctoral Science Foundation(NO. 2018M632518), Anhui Postdoctoral Science Foundation (NO.2017B150 and 2018B248), Natural Science Foundation of Anhui University (KJ2018A0074), Key Research and Development Program Project of Anhui Province (201904a07020081), Nature Science Foundation of Anhui (1908085QE213) and Huainan Science and Technology Planning Project (2018A363).
The authors declare no conflict of interest.