Assessment of the occurrence, spatiotemporal variations and geoaccumulation of fifty-two inorganic elements in sewage sludge: A sludge management revisit

The limited information about the sludge quality has made its management a top environmental challenge. In the present study, occurrence and the spatiotemporal variations of 52 inorganic elements were investigated in the sludge samples from three wastewater treatment plants (WWTPs) in Xiamen city, China. The results showed, the occurrence of 49 elements with the concentrations in the range of >125–53500 mg kg−1 dry sludge (DS) for commonly used industrial metals, 1.22–14.0 mg kg−1 DS for precious metals, and 1.12–439.0 mg kg−1 DS for rare earth elements. The geo-accumulation studies indicated a moderate to high levels of buildup of some elements in the sewage sludge. Principal components analysis (PCA) indicated strong spatial and weak temporal variations in the concentrations of the elements. Therefore, the sludge disposal operations, based on the element concentrations, geoaccumulation and economic potential are suggested for each WWTP. Sludge from W1 and W2 were found suitable for agricultural usage, while that from W3 showed a higher economic potential for the recovery of precious metals. This study concludes that a comprehensive analysis of the elements in the sewage sludge could provide critical information for the disposal and management of the sludge.


Materials and Methods
Study area. Three municipal WWTPs (W1-3) located in Xiamen, China were investigated in this study. The wastewater in W1 was nearly 100% domestic wastewater, while in W2 and W3, the industrial wastewater contributed to 20% and 50%, respectively along with domestic wastewater. W1, W2, and W3 served around 1.0, 0.35, and 0.30 million inhabitants, respectively 26 . The average daily excess sludge was 1691.7, 214.5, and 243.7 tones, with the dry weight of 53.3, 4.5 and 8.1 tones, respectively for W1, W2 and W3 26 . The treatment process varied from one plant to another. Briefly, in W1, the treatment included screening, grit chamber, biological aerated filter, and UV disinfection. In W2, screening, grit chamber, orbal oxidation ditch, secondary settling tank, and UV disinfection were involved; while in W3, screening, grit chamber hydrolyzing pond, anaerobic/anoxic/oxic (A2O) reactor, secondary settling tank, and chemical disinfection were the main treatment processes 26 . Sludge sampling and pretreatment. Activated sludge samples in W1 and the return sludge samples in W2 and W3 were seasonally collected from each WWTP on February 20 th (F), May 8 th (M), August 11 th (A), and November 12 th (N) 2014 26 . Sludge samples were concentrated by centrifugation (5000 g) at 4 °C for 15 min, and dried via lyophilization. The dried sludge was then grounded in clean mortar and sieved by passing through a mesh size <0.15 mm. The obtained powders were kept in polyethylene bags for physicochemical characterization and determination of elements.
Physicochemical characterization. Physicochemical parameters, including pH, electrical conductivity (EC), and C, N, and S, were analysis. The values of pH and EC were determined in the filtrate of dissolved sludge sample in the milliQ water at ratio 1/5 (v/v) 27 using a multi-parameter meter (HACH, HQ40d). The total carbon (TC), total nitrogen (TN), and total sulfur (TS) were determined by a macro elemental CNHS/O Analyzer (Vario MAX; Elementar, Germany). The pre-detection of the elements in the sludge samples was performed using scanning electron microscopy (SEM, HITACHI S-4800) with energy dispersive X-ray (EDX) spectroscopy (Genesis XM2).
Metals contents in the sludge. Vessels and cleanup procedure. All digestion vessels (polytetrafluoroethylene) were cleaned by leaching with hot 10 mL aqua regia (mixture of HNO 3 and HCl at the ratio of 3:1) at 90 °C for a minimum of 2 h according to the European Norm 3051A 28 . The cleanup was then completed following the recommendations by Westerhoff et al. 17 . All volumetric wares, including glass calibrate cylinders, were carefully acid washed and rinsed following the same recommendations. The nitric acid and hydrochloride acid were analytical grade (Merck KGaA, Darmstardt Germany).
Sludge digestion and quality control (QC). Sludge sample (0.100 g) and freshly prepared aqua regia (12.0 mL) were placed in the digestion tube. The reaction mixture was kept for 30 min at room temperature, and then the digestion was completed under microwave at 180 °C according to the European Norm 3051A 28 . The digestion program is described in the Table S1 in the SI. After cooling, the samples were filtrated using a Millipore filter (0.45 µm) and collected in a 50 mL polypropylene centrifugation tube and diluted to 50.0 mL with 2% nitric acid solution. Samples were kept at 4 °C prior to the analysis. For quality control, an instrument blank, procedural blank and certified reference material samples with known concentrations of elements (GBW07309, GSD-9, Inspection and Quarantine of the People's Republic of China) were applied for each sample batch. All experiments were performed in triplicate.

Results and Discussions
Physicochemical characterization of the sludge. The physicochemical parameters of sludge are shown in     ( Fig. 1) emphasizes the presence of a broad range of elements, including Fe, Al, Mg, Ca, Mn, Zn, P, V, Ba, Cr, Zn, Ag, etc. in the sludge samples. To be noticed, the high peak of gold observed in Fig. 1 is due to the coating process for the SEM/EDX analysis.

Concentration of target elements in the sludge. Major industrial elements.
Out of 52 investigated elements, 49 were detected in the sludge. The most widely used industrial elements like Al, Fe, P, Ca, K, Mg, Na, Mn and W etc. were detected at very high concentrations in the sludge samples with an average annual concentration ranging from 125 mg kg −1 DS (W) to 53500 mg kg −1 DS (Fe) ( Table 2). Concentrations of Ti, Ba, Sr, Zn, Cu, Sn, Ni, and Cr were relatively lower compared to the above-mentioned elements in the sludge, with concentrations ranged from 14.9 mg kg −1 DS (for Ni, in W1) to 2230 mg kg −1 DS (for Zn in W3). Regarding the elements with great environmental concerns such as As, Cd, Co, Cr, Cu, Ni, Pb, and Zn, the concentrations ranged from 3.9-5. Precious elements. Precious metals i.e. Pd, Ag, Au, Ru, and Pt were detected in the sludge samples (Table 2), while the concentration of Re and Ir were below the detection limits (Table S2) Rare earth elements. A total of 16 rare earth elements, including Ce, Nd, La, Y, Pr, Sc, Sm, Gd, Dy, Er, Yb, Eu, Ho, Tb, Tm, and Lu, were also analyzed in the present study. Fifteen were detected, while Lu was below the detection limit (Table S2). As shown in Table 2, the concentrations of rare earth elements ranged from 1.12 mg kg −1 DS (Ho) to 439 mg kg −1 DS (Ce). The application of rare earth elements in the manufacturing industry was probably the major source to the sewage sludge via wastewater treatment process. As shown in the Yearbook of Xiamen Special Economic Zone 30 , the mechanical and electronic are the most important industries in Xiamen, where the rare earth elements were widely used besides other wide range of technological applications 31 . In addition, it is known that the Chinese rare earth elements reserve represents 50% of the world reserves 32 , which makes China the leader of the world in the rare earth element production 33 . As a consequence, the residues of these metals could accumulate in the sewage sludge.

Spatiotemporal variation. Temporal variation. The relative distribution of element concentrations
amongst the four seasons, which highlight the temporal variation of the elements, is displayed in Fig. 2. Generally, the concentrations of the most elements within each WWTP did not show significant temporal variation as the elements were nearly equally distributed amongst the four seasons. Particularly, in W3 (Fig. 2c), the equal distribution was more pronounced. However, there were some exceptions. For example, in W1 (Fig. 2a), Mn, W, Zn, Cr, Sb, Re, Pt, Ce, La and Gd exhibited highest concentration in May. In W2 (Fig. 2b), elements generally exhibited lower concentration in August compared to the others seasons.
Spatial variation. The relative distribution of each element in W1-3 based on the average concentration from four sampling seasons is shown in Fig. 3 to indicate the spatial variations. Generally, the concentrations of most elements were higher in the sludge samples from W3, followed by W2, while lowest in W1. One-way ANOVA test showed significant spatial variations with p < 0.01 for the majority of the elements with the exception of Pr, Dy, Er, Yb, Eu, Tb, and Tm with 0.01 < p < 0.05; and Al, Fe, V, Ir, and Pt with p > 0.01. The significant spatial variation of the elements was likely due to the difference of the wastewater sources and the difference in the treatment processes in WWTPs. Indeed, the land usage type in W1 is mainly residential or commercial and service, while there are some manufacturing and industrial areas in W2 and W3 26 . The wastewater was nearly 100% domestic wastewater in W1, while W2 and W3 contained 20% and >50% industrial wastewater besides domestic wastewater, respectively. Consequently, the sludge from W3 showed higher concentration of most of the metals, as they are mainly used in the manufacturing and industrial processes 17,[20][21][22]34 . In addition, as previously stated, the treatment processes in the WWTPs also vary from one WWTP to another. Biological aerated filters + UV disinfection (BAF + UVdis) were used in W1, Orbal oxidation ditches + UV disinfection (O-OD + UVdis) in W2 while anaerobic/anoxic/oxic + disinfection (A 2 O + dis) were applied in W3 WWTP. Generally, the concentrations of the elements of greater environmental concern such as Cd, As, Ni, Cr, Cu and Zn in W1 and W2 met with Chinese standard (GB 4284-84, 1983) 25 . The regulated heavy metal concentrations for Cd, Cr, Cu Ni and Zn in the sludge adaptable for agricultural purposes are 5, 600, 800, 100 and 2000 mg kg −1 for acidic soils (pH ≤ 6.5) and 20, 1000, 1500, 200 and 3000 mg kg −1 for alkaline soil (pH ≥ 6.5), respectively. Therefore, the sludge from W1 and W2 could be used for the land application after properly stabilization. However, the heavy metals accumulation in the soil resulting from the repeated land application, together with the elemental bioavailability and bio-toxicity, needs to be furthered evaluated. In addition, the issue of emerging contaminants and persistent organic pollutants should also be taken into account and investigated during sludge stabilization before any application to the soil. Due to higher heavy metal concentrations, the sludge from W3 cannot be directly applied to the land. Due to the higher concentrations of precious elements in W3, the valuable elements are worthy to be recovered. Therefore, finding cost effective technology for the recovery of precious elements in W3 would be a good alternative.
Principal components analysis (PCA). PCA was performed to show the variation and correlation of the occurrence of elements in different samples. Figure 4a is plotted using the standardized concentrations of all the detected elements, with the principal component 1 (PC1) and PC2 explaining 57.71% and 34.94% of the variance in the elements distribution, respectively. Samples from the same WWTP could be clearly clustered into one group despite the different sampling seasons, indicating the strong spatial variations and little seasonal variations. Most of the elements were around the origin of the coordinates suggesting their minor correlation with PC1 and PC2. However, Al, Ca, P, Fe were far from the origin suggesting that these four elements contributed mostly for PC1 and PC2, which was mainly due to their significantly higher levels (>10 4 mg kg −1 ), although, the concentrations were standardized. Therefore, PCA was then performed based on the major industrial elements, precious metals, and rare earth elements.
As shown in Fig. 4b,c and d, samples from each WWTP could be clustered into separate groups based on the major industrial elements and rare earth elements. Al, Ca, Fe, and P mainly contributed to the distribution for the major industrial elements, while La, Nd, Ym, and Ce mainly contributed for the variations between samples from W1 and W3 in the case of rare earth elements. The results from PCA, based on the major industrial elements, precious elements and rare earth elements confirmed the high spatial variations and lower temporal variation, which was in accordance with the results in the previous sections.
Correlation between target elements and environmental factors. The correlation among the elemental concentrations and environmental parameters was carried out by SPSS. The heatmap (Fig. 5) was produced by R software version 3.2.2 based on Spearman's correlation matrix. The red and blue colors show the positive and negative correlation, respectively and the deeper color indicates the stronger correlation. The yellow color indicates no significant correlation with p value > 0.05. Generally, the elements were clustered into two apparent groups. Five rare earth elements (Pr, Nd, Sm, Eu, and Y) showed strong and positive correlation (r 2 > 0.5) with eight industrial heavy metals (Pb, Sn, Zn, Cd, Cr, Cu, Ni, Sb). These elements also showed a strong and positive correlation with pH. This could be explained by the fact that the pH plays an important role in the concentration and accumulation of elements in the soil or sludge 35 . Other physicochemical parameters didn't show any significant correlation with the elements concentrations in the sludge. In addition, rare earth elements, including Dy, Er, Ho, Yb and Tm, also showed strong correlation with each other. The strong correlations among the elements might be due to the similarity in their usage and origin.
Geoaccumulation index (Igeo). Igeo was used to evaluate the level of soil, sludge and sediment contaminations by comparing with the background concentration of a given element in the studied sample to its concentration in the upper continental crust (UCC) concentration 36,37 . Igeo was calculated via the equation (1) 38,39 .
Where, Cn is the detected concentration of a given element in the sludge, Bn is the background concentration in the UCC. The factor 1.5 is introduced to minimize the effect of possible variations in the background values which may be attributed to the lithogenic variations in the soil. The enrichment classes (Table S3) 40 were applied to evaluate the contamination of individual element in each sludge sample. Generally, the target elements were not accumulated in most of the studied sludge samples (Igeo ≤ 1) (Table 3). However, the accumulation of some specific elements was remarkable. For example, Sn showed moderate accumulation in the sludge (1 ≤ Igeo ≤ 2), while W and Cd indicated moderate to strong accumulation (1 ≤ Igeo ≤ 3) in all the WWTPs. In the samples from W3, there was moderate accumulation of Cr, Cu, and Zn, (0 ≤ Igeo ≤ 2). This is noteworthy that some precious metals, including Pd, Au, Ag and Ru, showed strong accumulation (Igeo ≤ 4) in the sludge from all WWTPs, indicating their enrichment in the sludge.

Conclusions
In this study, the occurrence of 52 elements was investigated in the sludge samples from three WWTPs in Xiamen city, China over one year. There were 49 elements detected, including major industrial elements, precious metals, and rare earth elements. The concentrations of most elements showed strong spatial variations but little seasonal variations. The spatial variations were mainly due to the difference of the wastewater source as well as treatment processes in WWTPs. Sludge from W1 and W2, which mainly received domestic wastewater, had relatively low elemental concentrations and could be suitable for the agricultural land application after a thorough stabilization and sequestration of potential emerging contaminants and persistent organic pollutants. However, sewage sludge from W3, which received more than 50% of the industrial wastewater, was found with relatively higher elemental concentrations and would pose an environmental risk for the agricultural usage. The high geoaccumulation index in W3 suggested that the elemental recovery might be a better option for the disposal of sewage sludge from W3. The investigation on a broad range of elements in this study provided useful information to assess the environmental risks and to evaluate the potential value of their recoveries, which can be very important for the sludge disposal and management.