Atmospheric emissions of toxic elements (As, Cd, Hg, and Pb) from brick making plants in China

Abstract

A multiple-year emission inventory of As, Cd, Hg, and Pb from brick making plants in China has been first established for the period 2008–2013 by employing the available emission factors and annual activity data. The atmospheric emissions of toxic elements were determined by a bottom-up methodology with the provincial-level statistical data on raw materials (coal, coal gangue, coal ash and clay) consumption and the reasonable emission factors of toxic elements. The provincial average concentrations of toxic elements in different raw materials were elaborately reviewed and calculated with multiple statistical mean calculation methods. Simulation experiments were performed to determine the emission factors of toxic elements from different raw materials. The results show that the total national emissions of As, Cd, Hg, and Pb from brick-making plants have been increasing to 644.05 t, 94.96 t, 9.71 t, and 3269.79 t in 2013, at an annual average growth rate of 22.8%, 25.6%, 19.2%, and 24.6% due to the lack of atmospheric pollutant control devices, respectively, which are higher than that of emissions from coal-fired plants (except for Hg) in China. Coal ash is the main source of As, Cd, and Pb emissions, accounting for 87.9%, 89.5%, and 88.4% of the respective total emissions due to the high consumption with high concentration and emission factor of TE. Shandong, Henan, Hubei, Hunan, Sichuan and Guangxi are the largest emitting provinces. Advanced technologies and integrated countermeasures to control toxic elements from brick making plants are urgently needed.

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1. Introduction

During the past decade, worldwide environmental toxic element (As, Cd, Hg, and Pb) levels derived from anthropogenic activities have increased considerably, and accordingly, the environmental and health impacts of toxic elements (TE) are of great concern.^1–5 These TE released from anthropogenic sources can be associated with both suspended particulate matter and flue gas.^6,7 A consensus is emerging that the suspended particulate matter and flue gas can remain in the atmosphere and travel long distances, as exemplified by Asian dust storms that result in the transport to America.⁸ Meanwhile, these TE can be deposited in the soil and biota thereby increasing the TE levels in the soil and biota. Poisoning incidents associated with TE have increased gradually in China, such as the arsenic poisoning in Guizhou,⁹ cadmium poisoning in Hunan,¹⁰ and high blood lead levels in children in Hunan.¹¹ In addition, according to the first National Soil Pollutant Survey during the period 2005–2013 accomplished by the Ministry of Land and Resources and the Ministry of Environmental Protection, approximately 16.1% of Chinese soil was polluted by TE and other chemicals.¹²

Source identification and quantification of TE are of extreme significant measures which are useful for control their emissions and reduce their adverse effects.¹³ Among anthropogenic activities, the mining activities (mining, transportation, utilization and waste disposal) are the main sources.¹⁴ The anthropogenic emissions of various TE (especially Hg) from coal combustion and coal-fired plant sector have been received fairly intensive studies.^1,7,15–18 However the investigation of the emission inventory of TE from brick making plant is limited.¹⁹

The production of red clay brick which produced from natural clay, has a history of more than 5000 years and the processed are consisted of crushing, screening, stirring, ageing, molding, pressing, drying in oven, firing and cooling.²⁰ The fired brick, originally produced from natural clay, is the most common construction material in China (account for 70%) and typically fired in firing kilns (950–1150 °C) that burn coal without atmospheric pollutant control devices (APCDs).²¹ With the development of brick making technology, the brick kiln types can be generally subgrouped into two categories as traditional brick kilns (also called batch kiln, including downdraft kiln, end fire kiln, quadratic kiln and circular kiln) and continuous kilns (annular kiln and tunnel kiln).²¹ The downdraft kiln and tunnel kiln are the mainly brick kilns used in China. The downdraft kiln accounts for approximately 20% of the brick production, while the amount of manufacturers is taken up about 60%. According to national statistics, the coal consumption of brick making was increasing gradually from 8 million tons in 2008 to 19.3 million tons in 2013 (ESI Table S1†).²² The TE associated with coal could be released from coal and entered into atmosphere directly during combustion without being purified by APCDs. In addition, with the urbanization and land contradiction intensified, the mining wastes (coal gangue, coal ash) were used to substitute natural clay. It has been reported that the concentration of TE in coal gangue and coal ash are higher than that of coal and natural clay.^23,24 Although the utilization of mining wastes (coal gangue, coal ash) can relieve the exhaustion of clay partly, the corresponding environmental impacts deserve more attention. Therefore, the investigation of the emission characterization of TE from brick making plant is of extreme significant.^21,25–28

The partition behavior of TE in raw materials (coal, coal gangue, coal ash and clay) during combustion is influenced by many factors including: physic-chemical properties, the concentration and association in raw materials, and operation conditions (equipment, temperature, and pressure),^6,24 thus, the emission factor of TE among various raw materials should be studied systemically. For the flue gas released from coal, coal gangue, coal ash and clay are coming together to chimney, the emission factor of TE was difficult to obtain by field test. Therefore, the simulation experiments were carried out in this study to determine the release rates of TE during firing.

The main objectives of the present study are to (1) determine the emission factor of TE from various raw materials (coal, coal gangue, coal ash and clay) in brick making plant; (2) establish the multiple-year emission inventory of As, Cd, Hg, and Pb from brick making plants for the period 2008–2013 in China. The outcome of this study is expected to provide a useful basic information for environmental management and regularization.

2. Methodology

The atmospheric emissions of TE, including As, Cd, Hg and Pb, from brick making plants were determined by employing a bottom-up methodology with the provincial-level statistical data on raw materials (coal, coal ash, coal gangue and clay) consumption and the reasonable specific TE emission factors. The plants were classified into three major categories according to the variation of brick kiln types and air pollution control devices (APCDs). The basic concept of the TE emission calculation was described as eqn (1).²⁹where E_t is the annual atmospheric emission of TE from brick making plants in China (t per year); M represents the amount of raw materials (coal, coal ash, coal gangue and clay) for brick making processes (t); C is the provincial concentration of TE in various raw materials (μg g⁻¹); EF represents the fraction of TE released to the atmosphere (%); RE is the removal efficiency of APCDs (%); i, j, k, and t represent province, raw material, kiln type, and year, respectively.

2.1. Raw materials consumption

Natural clay, like fossil fuels, is regarded as a non-renewable resources. According to statistics, more than 1 billion cubic meters of natural clay were employed to the production of bricks, equivalent to 33 350 hectares of cropland in China.³⁰ In order to restrain the consumption of natural clay and accelerate the re-utilization of mine solid wastes (coal gangue, coal ash), the announcements of speed up the wall material innovation and energy saving building promotion (Circular of State Council 1992-66, Circular of State Council 2005-33) were published in succession by the Chinese Government. The capacity, raw materials consumption, coal consumption data for Chinese brick making plants from 2008 to 2013 was stated by China Statistical Yearbooks,³¹ China Energy Statistical Yearbooks,³² China Industry Statistical Yearbooks,³³ China Urban Construction Statistical Yearbooks³⁴ (see ESI Tables S1–S5†). For the little brick output and shortage of data from references, Xizang Autonomous Region, Hainan province, Taiwan province, Hong Kong and Macau Special Administrative are not discussed. In addition, an optimized database which contained approximately 4500 brick making plants with detailed information about geographical location, capacity, kiln type, raw material type and consumption, fuel consumption, and APCDs was established in this study. The historical trend in raw materials consumption by brick making plants were presented in ESI Fig. S1,† the proportion of mine solid wastes (coal gangue, coal ash) in brick raw materials is increasing rapidly. In contrast, the clay consumption was maintained at approximately 10 million tons and with a highest level at 2009 (12 million tons). Meanwhile, the coal consumption is increasing in consistent with the increasing of brick production (ESI Table S1†).

2.2. Provincial averaged concentration of TE in raw materials

The coal reserves in China are unbalanced among provinces due to various geographical conditions, depositional environment, coal-forming plant. Coal reserves are abundant in the northern and western areas while coal reserves in the eastern and southern are general.³⁵ According to the statistic data reported by China Coal Industry Yearbook (2013),³⁶ 90% of the production of coal was came from Inner Mongolia, Shanxi, Shaanxi, Guizhou, Henan, Anhui, Shandong, Xinjiang and Yunnan province in 2012. Nevertheless, the coal is mainly consumed by the coastal energy-extensive consumption provinces such as Jiangsu, Zhejiang, Shanghai, Guangdong, Fujian, etc. Therefore, the large amount of coal was transported long-distances to meet the demand of energy. Extreme studies have reported that the concentration, association of TE in coal varied substantially among provinces, even the same coal seam.^37–39 As a result, the concentration of TE in coal is remarkable different between the produced and consumed processed in a single province. Combined provincial concentration of TE and the multiple statistical mean calculation methods, the provincial weighted-average concentration of TE in consumed coal was determined by Tian et al. (2013).³⁹ In this study, the provincial concentration of TE reported by Tian et al. (2013)³⁹ was applied and listed in ESI Table S6.†

Coal ash is the combustion residues (including bottom ash and fly ash) during coal combustion. According to mass balance methods, the concentration of TE in coal ash (C_a) could be calculated by following equation:²⁴

where C_c is the concentration of TE in consumed coal (μg g⁻¹). EF is the emission factor of TE during combustion (%), there have been numerous studies focus on the emission factor of TE during coal combustion in coal-fired plants and industrial boilers. The field tests emission factor data of As, Cd, Hg and Pb for previous studies were summarized and listed in ESI Table S7.† A is the ash yield of the consumed coal (%) and presented in ESI Table S6.† Once the C_c, EF, and A are obtained, the concentration of TE in coal ash can be calculated. The provincial concentration of TE in coal ash was shown in ESI Table S8,† the content of TE in coal ash are various among provinces. When compared the concentration of TE in coal ash with corresponding coal, the concentration of TE in coal ash are much higher than that of coal, which may be attributed to the loss of organic matter and the vaporization–condensation mechanism.⁴⁰

For the concentrations of TE in coal gangue, which vary among provinces due to the different sedimentary environment and geological setting. In the study, field test concentration data of As (748 samples), Cd (662 samples), Hg (353 samples) and Pb (612 samples) for different Chinese coal gangue are concluded from previous published literature and presented in ESI Tables S9–S12.† The provincial average concentration of As, Cd, Hg and Pb in coal gangue were determined by bootstrap simulation method and presented in Table 1. Owing to the lack of available domestic field tests results of TE in clay, the average concentration of As, Cd, Hg, and Pb are assumed as 11.2, 0.09, 0.065 and 26.0 μg g⁻¹ according to the background value of Chinese soil (Table 1),⁴¹ respectively.

Table 1 Provincial distribution of average concentration of toxic elements in coal gangue (μg g⁻¹)

	As	Cd	Hg	Pb
Anhui	18.8	0.07	0.630	28.7
Beijing	1.62	0.66	0.100	17.2
Chongqing	3.08	2.21	0.553	24.4
Fujian	12.0	0.19	0.070	32.3
Gansu	3.47	0.08	1.350	8.35
Guangxi	3.84	0.25	0.330	47.1
Guizhou	3.02	0.26	0.137	9.64
Hebei	2.38	0.21	0.163	18.7
Heilongjiang	6.00	0.17	0.035	23.5
Henan	6.45	0.69	0.120	36.1
Hubei	17.3	0.38	0.230	38.3
Hunan	4.14	0.95	0.071	35.2
Inner Mongolia	13.5	3.52	0.326	12.8
Jiangsu	2.04	0.03	0.177	17.8
Jiangxi	17.1	0.53	0.200	12.4
Jilin	7.45	0.18	0.430	30.7
Liaoning	9.26	0.17	0.150	26.4
Ningxia	0.97	0.15	0.154	8.54
Qinghai	2.70	0.03	0.308	10.7
Shaanxi	1.90	0.75	0.300	24.5
Shandong	5.34	0.76	0.740	38.6
Shanxi	3.62	0.23	0.320	25.3
Sichuan	10.7	2.13	0.412	29.7
Xinjiang	1.85	0.12	0.092	5.28
Yunnan	6.20	0.59	0.380	15.2
Zhejiang	13.0	0.51	0.750	17.1
Chinese soil	11.2	0.09	0.065	26.0

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2.3. Emission factors of TE

TE in coal/coal gangue may be associated with both organic matter and inorganic minerals (clay minerals, carbonate minerals, sulfides), and most of them could occur in both forms. Many studies focus on the modes of occurrence of TE and hypothesized that As, Cd, Hg, and Pb are predominately associated with sulfide minerals.^37,42–45 With the decomposition of sulfides minerals and organic matter during high temperature combustion, these TE (As, Cd, Hg, and Pb) will released from coal/coal gangue and entered into atmosphere. Coal ash is the combustion residues during coal combustion which includes both bottom ash collected from the bottom of combustion boiler and the fly ash obtained by the APCDs (electrostatic precipitators, fabric filters, cyclones, desulfurization systems). After being releasing from the coal-fired furnace, As, Cd, Hg, Pb and their compounds accompanied with fine fly ash (include vaporization-condensation) in the flue gas can be removed by APCDs and enriched in fly ash.⁴⁰ The TE in the fly ash may be re-volatized during high temperature combustion. The release rates of As, Cd, Hg, and Pb are attributed to the combustion/firing technology and operating conditions (temperature, pressure).^24,45 Currently, the firing facilities in China brick making plants are downdraft kiln and tunnel kiln, there into, tunnel kiln account for approximately 80% of brick production in China. The firing temperature of both downdraft kiln and tunnel kiln is maintained at 950–1150 °C, and the flue gas released from coal, coal gangue, coal ash and clay are coming together to chimney. Therefore, the specific release rate of TE from coal, coal gangue, coal ash and clay is difficult to acquire by field test. In addition, the previous studies on the emission factor of TE during brick making processed is limited.

In order to evaluate the emission factor of TE from various raw materials, a simulation experiment in a fixed bed reactor was carried out in this study. The fixed bed reactor consists of a cylindrical stainless steel reactor tube and an electric heater of 4 kW to ensure the reactor being heated to desired temperature and pressure.⁴⁶ 50 kg of the coal, coal gangue, coal ash and clay samples which sampled from three tunnel kiln brick making plants in Anhui and Henan provinces were fed into the reactor and heated to the firing temperature (1150 °C) under constant air flow and maintained 1 h. The flue gas samples were sampled isokinetically from the air outlet, following the Ontario Hydro Method (ASTM, 2002),⁴⁷ the particle associated with flue gas were collected in a filter placed in a heated area before impingers. All TE were collected in 10% H₂O₂ and 6% HNO₃. The recovered samples were analyzed for As, Cd, Pb and Hg by inductively coupled plasma mass spectrometry (ICP-MS) and cold vapour atomic absorption spectrometry (CV-AAS), respectively. According to the brick plant historical data, the generation ratios of raw materials in brick and fly ash form during firing process are 99% and 1%, respectively. Therefore, the generation amount of fly ash during firing process may be negligible. After combustion, the combustion residues were collected and crush to pass through a 150-mesh sieve in order to homogenize for chemical analysis. The powdered unfired samples and their combustion residues were acid digested using an acid solution (HCl : HNO₃ : HF) with ratio of 3 : 3 : 2 in a microwave oven. The concentration of As, Cd, Pb and Hg were determined by ICP-MS and CV-AAS, respectively. The quality control was determined by standard reference materials NIST 1632b (coal), the recovery (92.4–103%) for all the TE in standard reference materials were with the range of the certified values. The recovery is 93.7–104% when solutions with known TE concentrations are applied. The LOD and LOQ of the selected toxic elements (As, Cd, Pb and Hg) were 0.01 μg L⁻¹, 0.001 μg L⁻¹, 0.001 μg L⁻¹ and 0.001 μg L⁻¹, and 0.03 μg L⁻¹, 0.004 μg L⁻¹, 0.005 μg L⁻¹ and 0.004 μg L⁻¹, respectively. The analysis of all the samples was conducted in triplicates, with the relation standard deviations ranging from −2.36% to 1.73% and below the control level of ±5 wt%.

The blended samples (blended samples were prepared between coal gangue/coal ash/(50% coal gangue and 50% coal ash) and coal with ratios of 80 : 20, 60 : 40 and 40 : 60) were also analyzed in the reactor to assess the synergy effect among raw materials. For the tiny amount of fly ash generation during brick firing process, the dust removal devices were not installed at both downdraft kilns and tunnel kilns, what is more, some downdraft kilns were not installed chimney. Therefore, the As, Cd, Hg, Pb and their compounds accompanied with fine particles in the flue gas could be released from raw materials and entered into atmosphere directly. In some large-scale tunnel kilns which used coal gangue as raw materials, a simple wet flue gas desulfurization systems were installed. The Ontario Hydro Method was performed onsite tests for TE removal efficiency at inlet/outlet of the wet flue gas desulfurization systems in a large-scale tunnel kiln in Anhui province.

The concentration of TE in the selected coal, coal gangue, coal ash, clay samples and their combustion products are presented in ESI Table S13.† TE concentrations in coal, coal gangue, coal ash and clay are in agreement with the ranges of concentrations mentioned above. In order to evaluate efficiency of the Ontario Hydro Method for TE sampling in the flue gas, mass balances were employed and a comparison was established as follows:^48,49

C_m × F_m = C_r × F_r + C_f × F_f + C_g × F_g (3)

where C_m, C_r, C_f and C_g are the concentration of TE in raw materials (coal, coal gangue, coal ash, and clay), combustion residues, fly ash, and flue gas, expressed in μg g⁻¹ or μg m⁻³, respectively. F_m, F_r, F_f and F_g represent the input/output ratios for TE in raw materials (coal, coal gangue, coal ash, and clay), combustion residues, fly ash, and flue gas, respectively. Particles associated with the fuel gas were included in As, Cd, Hg, and Pb flue gas balances. As the aforementioned, the generation ratios of raw materials in brick and fly ash form during firing process are 99% and 1%, respectively, thus, the generation amount of fly ash during firing process may be negligible. In this study, F_r is equal to the ash yield of raw materials, F_g is the total gas emission value, which could be obtained at field test. Once all the factors of eqn (3) were confirmed, As, Cd, Hg, and Pb mass balances were calculated for raw materials (coal, coal gangue, coal ash, and clay), combustion residues, and flue gas. The results of mass balances are shown in ESI Table S14,† the values of As, Cd, Hg, and Pb were 74%, 56%, 116%, and 74%, respectively (n = 12). For the large number of factors used for the calculation of mass balances, the summation of the deviations should be taken into consideration. Therefore, the results obtained by Ontario Hydro Method were acceptable.

Once all the factors (the input amount of raw materials, the provincial concentration of TE, the emission factor of TE and the removal efficiency of APCDs) of eqn (1) were confirmed, the atmospheric emissions of As, Cd, Hg and Pb could be calculated.

3. Results and discussion

3.1. Emission behavior of TE in brick making processes

During firing, the TE in raw materials are distributed into combustion residues and flue gas. The partition behaviors of As, Cd, Hg, and Pb were shown in ESI Table S14,† the distribution characterization of TE are various among raw materials (coal, coal gangue, coal ash and clay). Hg is the most volatile element in coal (83.5%), coal gangue (72.6%) and clay (50.5%), while is mainly enriched in combustion residues in coal ash (5.96%). The various partition behaviors among raw materials may be determined by the physic-chemical properties and mode of occurrence of TE.^37,43

The synergy effect is an extreme significant factor which should be taken into concern during blends firing processed. In order to investigate the possible interaction among the raw materials (coal gangue, coal ash and ash), the released rates of TE versus the percentage of clay in the blends are plotted and shown in Fig. 1. The linear correlation between the released rate of TE and the ratio of clay reveals that there are lack of interaction between coal gangue, coal ash and clay during co-firing. Therefore, the emission factor of TE can be confirmed and shown in Table 2. The emission factor of As, Cd, Hg, and Pb are 28.9, 36.8, 83.5, and 36.4% for coal; 24.3, 42.2, 72.6, and 29.5% for coal gangue; 20.5, 27.6, 5.96 and 21.3% for coal ash; and 9.07, 12.9, 50.5, 11.0% for clay, respectively, which are consistent with the previous reported by Zhou et al. (2014).¹⁹ Zhou et al. (2014) analyzed the partition behavior of TE in a full coal gangue internal burned brick making plant and reported that the emission factors of As, Cd, Hg and Pb are 16.7, 43.5, 76.9, and 30.8% for coal gangue, respectively.¹⁹ In this study, the emission factors of TE obtained from the simulation experiments were employed to represent the emission factors of TE from brick making plants.

Fig. 1 Emission rates of toxic elements at different percentage of clay in raw material blends. C-clay, CG-coal gangue, CA-coal ash.

Table 2 The emission factor of toxic element and desulphurization devices' removal efficiency (RE_DSDs) in brick making processes (%)

	As	Cd	Hg	Pb
Coal (n = 3)	28.9 ± 3.9	36.8 ± 2.2	83.5 ± 7.6	36.4 ± 1.4
Coal gangue (n = 3)	24.3 ± 1.5	42.2 ± 3.4	72.6 ± 3.6	29.5 ± 3.1
Coal ash (n = 3)	20.5 ± 2.1	27.6 ± 1.9	5.96 ± 0.75	21.3 ± 1.7
Clay (n = 3)	9.07 ± 0.61	12.9 ± 0.7	50.5 ± 2.1	11.0 ± 1.5
REDSDs (n = 5)	14.9 ± 1.7	9.80 ± 1.56	5.48 ± 0.89	10.3 ± 1.75

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The concentration of TE in flue gas at the inlet/outlet of wet flue gas desulfurization systems were analyzed and shown in ESI Table S15.† It can be found that the sulfurization systems have useful impacts on the removal of TE emissions from the exhaust. The removal efficiencies for As, Cd, Hg and Pb are 14.9, 9.80, 5.48, 10.3%, respectively (Table 2). Once the factors of input amount of raw materials, concentration of TE in raw materials, emission factors of TE from raw materials, and removal efficiency of TE by APCDs were obtained, the gaseous emissions of TE from brick making plants could be calculated.

3.2. Temporal and spatial characterization of TE emissions from brick making processes

According to the annual provincial volume of raw materials consumed and the obtained average emission factors for TE, the temporal behaviors of national total gaseous emissions of TE from brick making plants in China during the period 2008–2013 are counted and presented in Table 3. According to Table 3, the total national emissions of As, Cd, Hg, and Pb have been increasing gradually from 187.99 t, 24.22 t, 3.38 t, and 872.04 t in 2008 to 644.05 t, 94.96 t, 9.71 t, and 3269.79 t in 2013, at an annual average growth rate of 22.8%, 25.6%, 19.2%, and 24.6%, respectively. With the rapid development of economy and urban, the demand of construction materials is increasing, as a result, the scale and amount of brick making plants are increasing. Generally, the brick making plants are located at suburban area where close to the cultivation area, the TE released from brick plants could be accumulated in soil and crops by precipitation thus finally lead to adverse effects on environment and human health.^5,8,14 Therefore, it is of extreme significance to evaluate the status of TE emissions and adopt adequate countermeasures.

Table 3 Toxic element emissions from brick-making processes in China, 2008–2013 (t per year)

	2008	2009	2010	2011	2012	2013
As	187.99	321.93	389.66	445.79	536.42	644.05
Cd	24.22	44.05	56.56	71.20	76.66	94.96
Hg	3.38	5.86	7.09	7.46	8.01	9.71
Pb	872.04	1511.12	1859.95	2156.73	2630.75	3269.79

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The historical distribution of TE emissions from various raw materials from 2008 to 2013 are illustrated in Fig. 2, coal ash is the mainly source of As, Cd, and Pb emissions, increasing from 159.01 t, 21.07 t, and 737.56 t in 2008, to 566.02 t, 84.99 t, 2891.38 t in 2013, accounting for 87.9%, 89.5%, and 88.4% of the respective total emissions, and with annual growth rate of 23.6%, 26.1%, and 25.6%, respectively. The high emissions of As, Cd, Pb from coal ash are attributed to the extensive consumption (ESI Fig. S1†), high concentrations of As, Cd, Pb in coal ash (ESI Table S8†), the high emission factors and the less application of APCDs. For Hg, the emission sources are coal, coal gangue and coal ash, which account for 44.8%, 32.8% and 16.8% of the total emissions.

Fig. 2 Historical distribution of toxic element emissions from various raw materials during brick making processes in China, 2008–2013.

Provincial emissions of As, Cd, Hg, and Pb from brick making plants in China for the year of 2013 are illustrated in Table 4 and Fig. 3, the emissions of TE are various among provinces. Shandong, Henan, Hubei, Hunan, Sichuan and Guangxi are the largest emitting provinces, which taken up 70%, 82%, 67%, and 73% of total As, Cd, Hg, and Pb emissions, respectively. The high emissions of TE in Henan are mainly due to the large amount of coal, coal gangue, and coal ash consumption. Provinces like Hubei, Hunan, Sichuan, and Guangxi have relatively higher emissions mainly attributed to high concentration of these TE in raw materials, especially in coal ash. Although the concentrations of TE in coal ash from Fujian, Jilin, Inner Mongolia and Yunnan are relatively higher, the emissions from these provinces are lower due to lower coal ash consumption.

Table 4 Provincial emission of As, Cd, Hg and Pb from brick-making processes in China in 2013 (t per year)

	As	Cd	Hg	Pb
Anhui	14.38	0.51	0.92	56.73
Beijing	0.04	0.01	0.00	0.31
Chongqing	4.58	1.48	0.23	27.28
Fujian	19.95	0.82	0.05	56.33
Gansu	1.55	0.04	0.07	3.43
Guangdong	16.34	0.65	0.04	52.23
Guangxi	71.25	2.36	0.47	145.28
Guizhou	15.68	2.45	0.24	60.68
Hebei	15.92	1.01	0.13	103.88
Heilongjiang	5.18	0.26	0.06	34.78
Henan	74.04	21.45	2.35	572.98
Hubei	94.14	8.00	1.06	841.71
Hunan	88.99	7.57	0.30	249.31
Inner Mongolia	11.35	0.56	0.13	54.02
Jiangsu	3.38	0.10	0.15	27.74
Jiangxi	7.60	0.74	0.05	20.49
Jilin	28.84	0.52	0.34	80.07
Liaoning	14.51	0.55	0.18	55.73
Ningxia	0.28	0.11	0.00	1.16
Qinghai	0.05	0.00	0.00	0.22
Shaanxi	18.27	4.81	0.34	180.49
Shandong	53.36	5.55	1.23	194.29
Shanghai	0.41	0.02	0.00	2.18
Shanxi	2.04	0.51	0.05	15.13
Sichuan	70.68	33.40	1.13	392.07
Tianjin	1.09	0.63	0.01	8.74
Xinjiang	0.55	0.03	0.01	0.62
Yunnan	4.69	0.57	0.05	24.17
Zhejiang	4.90	0.26	0.11	7.73
Total of China	644.05	94.96	9.71	3269.79

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Fig. 3 Provincial distribution of As, Cd, Hg, and Pb emissions from brick making plants in 2013.

Until now, the comprehensive and systemic research on TE emissions from brick making plants is scare, but the emissions of TE from coal-fired plants have received fairly intensive study.^1,7,15,17 Tian et al. (2014) was calculated the emissions of TE from coal-fired plants and reported that the emissions of As, Cd, Hg, and Pb are 335.45 t, 13.34 t, 118.54 t, and 705.45 t in 2010, respectively,⁷ which is lower than that of the emissions from brick making plants. The higher emissions of TE in brick making plants are mainly attributed to the utilization of coal ash and the less application of APCDs. The removal efficiencies of electrostatic precipitators (ESP) for As, Cd, Hg, and Pb are 86.2%, 96.5%, 33.17%, and 97.2%, respectively.⁷ Once the ESP is installed at the brick making plants, the emissions of TE could be decreased obviously. Therefore, more efforts and countermeasures should be focus on the emission of TE from brick making industry.

3.3. Uncertainties

The certainties point to the estimation of emissions of As, Cd, Hg, and Pb from brick making plants may be influences by several factors such as: concentration of TE, raw materials consumption, emission factor of TE, and APCDs. The uncertainties of TE emissions were quantified by the Monte Carlo simulation and presented in ESI Table S16.†⁵⁰ The overall uncertainties are from −22% to 37% for As, from −28% to 45% for Cd, from −15% to 23% for Hg, and from −32% to 43% for Pb emissions, respectively. Emissions from coal and coal gangue are considered to have the low uncertainty, which may be attributed to the relatively more stable for the amount of consumption, the distribution of concentration and emission rate of TE among kiln types. The emission from coal ash is hypothesized to an important source of uncertainties, which may be due to the limited information on TE re-volatilization processes and the high variation of activity level. The emission from clay is demonstrated to the mainly contributor of uncertainties, which may be attributed to the few studies focus on the concentration and emission factor of TE in clay. The provincial concentration of TE in clay was represented by the background value of Chinese soil. Therefore, more detailed investigation and field tests for concentration and emission factors of TE in different raw materials are of extreme significance. Although there are some uncertainties caused by the lack of field test data in Chinese brick making plants, the investigations on the temporal and spatial characterization of TE emissions are of great importance to the environmental management and regularization.

3.4. Potential countermeasures

With the rapid urbanization, the demand of building materials and raw materials increasing. Although the utilization of mining wastes (coal gangue, coal ash) can relieve the exhaustion of clay partly, the corresponding environmental impacts should not be neglected. Based on aforementioned results, the emissions of As, Cd, and Pb are mainly from the re-volatilization of coal ash without APCDs, thus the installation of innovative atmosphere control technology and fine particle collection device is the countermeasure of priority. Currently, the downdraft kiln accounts for approximately 20% of the brick production, while the amount of manufacturers is taken up about 60%. Generally, the flue gas enters into atmosphere without APCDs, even not chimney. Therefore, the upgrading and renovating of the industrial structures should be accelerated, and the proportion of high emissions, high energy-extensive consumption with low output factories should be reduced. Additionally, the strict laws, standards, and regulations that prohibit or restrict emission of TE throughout their life cycles should be established. The construction industrial pollutant emission standards was protocoled in China in 2009, and the main restriction factors are NO_x, SO_x, and particulate matter. Finally, compared with pollutant control and reduction after their generation, the prevention of pollution is an effective approach. Reducing and eliminating the TE concentration in raw materials (coal, coal gangue, coal ash) via preliminary treatment (coal washing, adsorption) is the ultimate solution for TE emissions. Meanwhile, financial, technical, and education support should be provided to promote the development of industry and the implement of efficient pollutant control systems. To sum up, pollution prevention, renovation of firing equipment, and installation of effective pollutant control and purification systems, as well as financial, technical and education support, are the useful multi-countermeasures to reduce the TE emission from brick making plants.

Acknowledgements

The authors acknowledge the support from the National Basic Research Program of China (973 Program, 2014CB238903), the National Natural Science Foundation of China (no. 41173032 and no. 41373110). We acknowledge editors and reviewers for polishing the language of the paper and for in-depth discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14443k

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