Evaluating the volatilization behavior of PAHs in oil-based residues from drilling cuttings of different thicknesses based on the C-history method

: Drilling cuttings, the primary byproduct generated from the exploration and mining of 10 shale gas, are potentially hazardous types of waste that seriously deplete land resources and pose 11 environmental safety problems. In this paper, oil-based drilling debris is used as a typical porous 12 medium phase solid waste, and the law and characteristics of unsteady diffusion and release of 13 PAHs are studied under the specific situation of oil-based drilling debris with different thicknesses 14 as the "source". The results showed that(1) Thickness has a greater influence on the diffusion and 15 release law of PAHs from oil-based drilling cuttings residues, the diffusion release of PAHs 16 increases with the increase of sample thickness. (2) The thickness of the porous media source is 17 the main influencing factor of the PAHs diffusion and release diffusion related parameters. The 18 higher the thickness, the diffusion coefficient D m increases with the increase in thickness. The 19 distribution coefficient K and the initial diffusible release concentration C m0 change in a similar 20 manner, and both increase with the thickness.


Introduction 23
This study by Xiong et al [1] has demonstrated that the proposed C-history method is 24 effective for simultaneously measuring the characteristic emission parameters: the initial emittable 25 concentration, the diffusion coefficient, and the partition coefficient of formaldehyde and VOCs in 26 building materials. By using small scale and full scale chambers, and testing at different 27 temperatures, the C-history method is shown to have excellent volume and temperature 28 adaptability. Independent experiments have made the measured parameters in a closed chamber 29 more convincing, and suggest that these parameters can be used to estimate VOC emissons under 30 real conditions. The main advantage of the C-history method is that it is rapid. The proposed 31 C-history method may also be helpful for measuring the characteristic parameters of semivolatile 32 organic compounds (SVOCs) through some improvement [1]. 33 Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants produced 34 by anthropogenic activities associated with industrialization and urbanization, as well as through 35 natural activities [2]. To date, over 400 kinds of PAHs and their ramifications have been 36 identified [3]. Chemically, PAHs are a group of organic compounds which comprising of two or 37 more benzene rings bonded in linear, cluster,or angular arrangements [4]. It has been proved that 38 PAHs have environmental persistence, bioaccumulation, long-distance migration and high 39 biotoxicity. Meanwhile, PAHs are high melting and boiling points (therefore they are solid), low 40 vapor pressure, and very low aqueous solubility [5,6]. Excessive inhaled or exposed to PAHs, it 41 can cause lung cancer and other adverse reactions [7]. The USEPA has categorized 16 of the PAHs 42 as priority contaminants [8]. 43 In this paper, oil-based drilling debris is used as a typical porous medium phase solid waste, 44 and the law and characteristics of unsteady diffusion and release of PAHs are studied under the 45 specific situation of oil-based drilling debris with different thicknesses as the "source". The 46 purpose of this study is to (1) simulate the diffusion and release characteristics of PAHs in 47 oil-based residues of different thicknesses, and (2) compare and evaluate the diffusion and release 48 characteristics of PAHs in oil-based residues of different thicknesses through the C-history method. 49 It provides scientific data for the diffusion release and characteristic parameters of PAHs in solid 50 waste sources of different thicknesses of porous media, and also provides a scientific basis for the 51 environmental effects and risk management of solid waste sources of porous media during storage, 52 disposal and transportation. 53

Materials 55
The oil-based residues of drilling cuttings used in this research were collected from drilling 56 cutting manufacturing sites located in southern Sichuan. All samples were dark black powders 57 with a strong diesel odor. After collection, the samples were manually ground with a mortar and 58 pestle and kept in brown wide mouth glass bottles before leaching tests were performed. PA H 59 standards and other relevant solvents were purchased from Chengdu Ke-Long Chemical Regent 60 Co. Some properties of the tested PAHs are listed in Tab le 1, as well as their concentrations in the 61 oil-based residues. The dichloromethane and methanol were chromatographic grade and used 62 directly without further purification. 63 experiments simulated the process of oil-based drilling debris residue PAHs entering the ambient 68 air from the inside of the porous medium source from the inside of the reactor to the surface 69 during the general stacking and storage process. The closed environment chamber used in this 70 study was a cube, and to reduce the adsorption/desorption effect of the PAHs on the inner wall of 71 the closed environment chamber, the chamber was made of glass with the inner side polished. This 72 processing allowed the chamber to meet the ASTM standard [9,10]. The length, width and height 73 of the closed environment chamber were each 800 mm; the porous medium solid waste pollution 74 source was placed at the bottom of the closed environment chamber; two air holes were located on 75 the top of the closed environment chamber, and glass plugs were used to block the air holes to 76 ensure the airtightness of the closed environment chamber. During sampling, the glass plugs were 77 removed to connect the sampling tubes. To ensure the airtightness of the entire closed environment 78 chamber, all connections were sealed with polytetrafluoroethylene (PTFE) [11]. 79

Closed environment volatilization experiment 80
The oil-based residues of shale gas drilling cuttings were tested in this study. The XAD-2 resin was removed from the adsorption tube, and 10 µL of recovery indicator 93 (2-fluorobiphenyl, p-terphenyl-D14) was added. A fast solvent extraction instrument (BUCHI 94 E-196) was used for extraction with the following parameters: a temperature of 100 ℃, pressure of 95 120 bar, static extraction time of 5 min, and methylene chloride and n-hexane mixture (1:1, V/V). 96 A 20 mL sample rinse was performed 3 times. After the extract was blow dried with high-purity 97 nitrogen and concentrated to approximately 1 mL, it was passed through a magnesium silicate 98 purification cartridge eluted with dichloromethane and n-hexane in advance with 5 mL 99 dichloromethane and n-hexane. After the eluent was soaked in the purification column, the control 100 valve was closed and soaked for 2 min. Then, we continued to add 5 mL of the mixture of 101 methylene chloride and n-hexane and collected all the eluent. The collected eluate was blown with 102 high-purity nitrogen (purity>99.99%) to approximately 0.5 mL, and internal standards 103 (naphthalene-D8, acenaphthene-D10, phenanthrene-D10, chrysene-D12, and perylene-D12) were 104 added for determination. The total volume was 1 mL, and the sample was stored at low 105 temperature until testing on the machine. 106 The 16 organic PAH pollutants produced by oil-based residues of shale gas drilling cuttings 107 were identified and quantified by gas chromatography/mass spectrometry (GC/MS) 108 (GCMS-QP2010 Ultra; Shimadzu, Japan). The GC/MS operational conditions were as follows. 109 The inlet temperature was set at 250°C, the inlet volume was 1 µL, and the sample was under 110 splitless mode. The capillary column was an HP-5ms (30 m×0.25 mm×0.25 µm). The temperature 111 elevation program employed an initial temperature of 70℃ for 2 min before increasing at 112 10℃/min to 320℃ and holding for 5.5 min. The carrier gas was ultrapure helium (99.99%), and 113 the flow rate was 1 mL/min. The multiple reaction monitoring (MRM) mode was selected for ion 114 monitoring, and the ionization method was electron impact [12][13][14]. 115

Quality control 116
Different quality assurance and control means were applied during sample preparation and 117 chemical analysis, including the use of certified reference materials for instrumental calibration, 118 replicates, reagent blanks, and detection limit verification. 119 The standards of the 16 organic PAH compounds with 5 internal standards were analyzed in 120 behavior of the PAHs of the samples in a closed environment chamber. We assumed that the 137 oil-based residues were uniform, that the PAH diffusion process inside the oil-based residues was 138 one-dimensional and that the PAHs in the closed environment chamber were well mixed [15,16]. 139 These assumptions have been widely adopted in previous studies. Based on these assumptions, the 140 analytical solution describing PAH emissions are as follows [1,17]. oil-based drilling debris will increase the original content of PAHs, which will cause the diffusion 198 coefficient of oil-based drilling debris to increase with the increase in thickness. In addition, the 199 PAHs diffusion coefficient Dm of oil-based drilling cuttings residues of different thicknesses has 200 the same trend, that is, the diffusion coefficient of the lower molecular weight PAHs is higher than 201 that of the higher molecular weight PAHs. The most abundant PAHs released by diffusion have 202 lower molecular weight and higher molecular weight. The lighter molecular weight has a lower 203 vapor pressure and a higher air diffusion coefficient resulting in a larger diffusion coefficient. 204

The influence of thickness on the distribution coefficient K of PAHs 206
The results of the PAHs distribution coefficient K of the oil-based drilling cuttings residues of 207 different thicknesses were shown in Table 3. It can be seen from the table that the distribution 208

The influence of thickness on the initial diffusible release concentration Cm0 of PAHs 216
The results of the initial diffusible release concentration Cm0 of PAHs in oil-based drilling 217 cuttings residues of different thicknesses were shown in Table 4. The initial diffusible release 218 concentration Cm0 of the same PAHs in oil-based drilling cuttings residues at different thicknesses 219 has similar laws. The law is similar to the law of the distribution coefficient K but opposite to the 220 law of the diffusion coefficient Dm, that is, the initial diffusible release concentration of PAHs 221 decreases as the thickness increases. The larger the diffusion coefficient, the smaller the 222 distribution coefficient, which leads to a decrease in the initial releasable concentration. 223

Model theoretical data & experimental data under different thicknesses 225
Thickness is an important factor affecting the diffusion and release of PAHs from oil-based 226 drilling debris. The higher the thickness of oil-based drilling debris in a closed environment cell,

Conclusions 245
The oil-based residue of drill cuttings was investigated and discussed through experiments to 246 assess the potential for the release of dangerous components (PAHs) under specific conditions. 247 Thickness has a greater influence on the diffusion and release law of PAHs from oil-based drilling 248 cuttings residues. For the oil-based drilling cuttings residues of the same particle size, the 249 diffusion release of PAHs increases with the increase of sample thickness. The thickness of the 250 porous media source affects the time for PAHs to reach the equilibrium concentration. The 251 thickness of the oil-based drilling cuttings residues becomes smaller. The sooner PAHs reach the 252