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Article

Synergistic Effects of Calcium Peroxide and Fe3O4@BC Composites on AVS Removal, Phosphorus and Chromium Release in Sediments

by
Yintian Li
1,2,
Yanchun Huang
1,2,
Xueying Wang
1,2,
Ge Gou
1,2,
Chao Liu
1,2,
Jun Li
1,2,
Yuxin He
1,2 and
Naiwen Li
1,2,*
1
College of Water Resource & Hydropower, Sichuan University, Chengdu 610065, China
2
State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(10), 1626; https://doi.org/10.3390/w14101626
Submission received: 21 March 2022 / Revised: 10 May 2022 / Accepted: 13 May 2022 / Published: 18 May 2022
(This article belongs to the Special Issue Water Environment Governance and Restoration)
Browse Figures
Versions Notes

Abstract

:
Black odorous sediment pollution in urban areas has received widespread attention, especially pollution caused by acidified volatile sulfide (AVS), phosphorus and heavy metals. In this study, an Fe3O4@BC composite was fabricated by the coprecipitate method of Fe3O4 and biochar (BC) and was mixed with calcium peroxide (CP) for sediment pollution treatment. The results showed that the AVS removal rate could reach 52.8% in the CP+Fe3O4@BC system and −18.1% in the control group on the 25th day. AVS was removed in the following three ways: AVS could be oxidized with oxygen produced by CP; H2O2 produced from CP also could be activated by Fe2+ to generate hydroxyl radicals that have strong oxidation properties to oxidize AVS; AVS could also be removed by bacterial denitrification. As for phosphorus, total phosphorus (TP) content in overlying water remained at 0.1 mg/L after CP and Fe3O4@BC were added. This is due to the conversion of NH4Cl-P and Fe/Al-P into Ca-P in sediments, which inhibited the release of phosphorus. Simultaneously, the release and migration of heavy metal chromium (Cr) were slowed, as demonstrated by the results (the acid extractable and reducible states of Cr in the sediment decreased to 0.58% and 0.97%, respectively). In addition, the results of the high-throughput genetic test showed the total number of microorganisms greatly increased in the CP+Fe3O4@BC group. The abundance of Sulfurovum increased while that of sulphate-reducing bacteria (SRBs) was inhibited. Furthermore, the abundance of denitrifying bacteria (Dechlorominas, Acinetobacter and Flavobacterium) was increased. In brief, our study showed the synergistic effect of Fe3O4@BC composites and CP had a remarkable effect on the urban sediment treatment, which provides a new way to remove sediment pollution.

1. Introduction

In recent years, the massive discharge of pollutants into water bodies has caused great harm to urban rivers and formed black odorous sediment with the rapid development of industry and agriculture in China [1,2,3]. The main pollutants include AVS, phosphorus and heavy metals. Among them, AVS mainly causes black and odorous water bodies [4] and the release of P in sediments could lead to eutrophication [5], which could affect water quality and the aquatic ecosystem. In addition, there are high levels of chromium (Cr) in sediment in many places, which can affect biological and human health. Therefore, it has become a research hotspot for the treatment of AVS, P and Cr.
The current approaches to removing AVS are chemical methods (e.g., chemical oxidation, chemical precipitation and flocculation). AVS removal through denitrification enhanced by nitrate addition has been considered as a cost-effective technology for black odorous sediments control [6]. However, the addition of nitrate would cause a surge of nitrogen in the water, which leads to eutrophication and affects human health and safety [7]. Meanwhile, nitrate has a dual effect on the release of P, which can reduce the release by promoting the oxidation of sediments and can facilitate oxidation by stimulating the growth of phytoplankton [8]. Besides, the addition of nitrate would cause a morphological change of heavy metals in the sediment and lead to the re-release of heavy metals into the overlying water [9]. CP can slowly liberate hydrogen peroxide (H2O2) and O2 at a “controlled” rate when contacting with water (detailed as shown in Equations (1) and (2)), during which H2O2 can further generate free radicals [10]. Therefore, CP can effectively control black odorous sediments [11,12]. As for phosphorus and chromium pollution, we mainly inhibit their release from sediment [13,14].
CaO2 + 2H2O → Ca(OH)2 + H2O2
2CaO2 + 2H2O → 2Ca(OH)2 + O2
Nowadays, nano-composites, especially Fe3O4 nano-composites, are increasingly applied in environmental engineering. Various valence states of iron existing in Nano-Fe3O4 composites play a key role in microbial growth and can promote the metabolic activity of the microbial community [15,16]. Furthermore, as a high-performance catalyst of the Fenton reaction, Nano-Fe3O4 is widely used for H2O2 activation to degrade organic pollutants [17,18,19]. However, nano-Fe3O4 particles tend to attract each other and aggregate into many larger-size particles, which causes a significant decrease in the specific surface area [20] and reduces the catalytic performance. To address this issue, researchers have chosen different materials as a carrier to stabilize and scatter the nano-Fe3O4 particles. Compared with other Support Materials, biochar (BC) is a readily available porous carbon-rich material with a higher specific surface area [21]. BC is widely applied in soil improvement [22,23], sediment treatment [24] and environment recovery [25]. BC can be obtained from a variety of precursors. Humic acid is a carbon-rich material that contains a large number of functional groups, such as hydroxyl, carboxyl and aromatic groups. Biochar derived from humic acid not only has a larger specific surface area, but also retains organic functional groups, which can solidify heavy metals through cation exchange and complexation [26]. Herein, we consider that Fe3O4 loaded onto humic acid biochar can solve the agglomeration problem of nano-Fe3O4 and adsorb heavy metals to reduce their harm.
In this study, humic acid was selected as the raw material to produce biochar at a temperature of 600 °C. The nano-Fe3O4@BC composites were successfully prepared by the coprecipitation method. The synergistic effect of CP and Fe3O4@BC composites contributes to restoring the black odorous sediment. This study mainly focused on the removal effect and mechanism of AVS. Besides, we also study the effects of composites on the removal of P and release of Cr. The variation of the microbial community in the sediment was also investigated. In short, we propose a safe, high-efficiency and eco-friendly method for black odorous sediment pollution remediation in urban rivers.

2. Materials and Methods

2.1. Reagents and Materials

CP (≥50%) was purchased from Zhoujian Animal Health Technology Co., Ltd. (Guangzhou, China). Humic acid (≥40%) was purchased from the Changbai Mountain Nutrient Soil Plant in Dalian, China. All the other chemicals used in the work were analytical and purchased from Kelong chemical company (Chengdu, China). Deionized water was applied by a water purification system (ULUPURE, Chengdu, China).
The sediment used in this experiment was collected from the Jiang’an River at the Jiang’an Campus of Sichuan University, which is a typical black and odorous body of water. The collected sediment was stored in the laboratory in dark conditions. The content of water content, NH4+-N, NO3−-N, NO2−-N, AVS, phosphorus and Cr in the initial sediments was 111.5%, 8.309 mg/kg, 37.352 mg/kg, 3.095 mg/kg, 0.744 mg/g, 0.686 mg/g and 16.284 mg/kg, respectively.

2.2. Material Preparation

2.2.1. Preparation of Biochar

To obtain the biochar, humic acid was firstly crushed and sieved. Then, the humic acid was transferred into a porcelain boat and calcined at 600 °C for 2 h (10 °C/min) under an N2 atmosphere (200 mL/min) in a tube furnace. The obtained powder was marked as BC.

2.2.2. Preparation of Fe3O4@BC

The Fe3O4@BC nanocomposites were synthesized by the co-precipitation method [27,28]. Firstly, 10.2 g FeCl3·6H2O, 4.975 g FeCl2·4H2O and 7 g of pre-prepared BC were dissolved in 200 mL ultrapure water and the solution was stirred continuously for 24 h. Secondly, a peristaltic pump was used to slowly drop 5 M NaOH into the above solution (4 mL/min) until the solution’s pH reached 11. The whole preparation process was purged with N2 to eliminate the interference of dissolved oxygen in the water. Finally, the black material obtained was filtered and washed with ethyl alcohol several times. After drying in the vacuum oven at 65 °C for 24 h, we successfully prepared Fe3O4@BC.

2.2.3. Characterization of the Biochar and Fe3O4@BC

The surface morphology of the biochar and the Fe3O4@BC composites were observed using scanning electron microscopy (SEM, JSM-7500F, Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDS) was coupled with SEM to examine the surface elemental composition and obtain surface elemental distribution maps. The chemical structures were identified by Fourier-transform infrared spectroscopy (FT-IR, Nicolet 6700, Watertown, MA, USA). Analysis of the crystalline structures of the catalyst powders was performed by X-ray diffraction (XRD, EMPYREAN, Almelo, The Netherlands).

2.3. Experiment Design

The experiment was conducted with a 250 mL serum bottle, and 7 groups of experiments under different conditions were set at the same time: CK group, BC group, CP group, CP+BC group, CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group. Three parallel experiments were set for each group. To prevent the influence of sampling from the water body and sediment for the subsequent measurement of the experiment, three serum bottles were taken for each treatment in the experiments and were not returned. The total number of experimental serum bottles was 147. The amount added to each group is shown in Table 1.
After each group was mixed, 150 mL ultra-pure water was slowly added and placed in the incubator (HWS-350) under dark conditions at 25 °C. The sediment was sampled on days 2, 4, 6, 8, 13, 18 and 25.

2.4. Measuring Method

The pH and oxidation-reduction potential (ORP) were measured with a portable analyzer (Multi3610, WTW, Munich, Germany). For the determination of AVS, 2.0 g wet sediments were added into a round-bottom flask filled with N2. Then, the sediment suspension was stirred and acidified for 45 min with 20 mL HCl (6 mol) at room temperature. Finally, with N2 as a carrier gas, the H2S was absorbed by 0.5 mol NaOH solution and determined via the methylene blue method, using a Techomp UV1000 spectrophotometer. According to the four-step extraction method proposed by Hieltjes and Lijklema [29], different forms of P were extracted from the sediment and the content of P was analyzed by the molybdate salt photometer (T3200, Shanghai, China). In addition, the Cr in different forms in the sediment was detected by the BCR three-step extraction method and their content was measured by the Atomic absorption spectrophotometer (AA-6880G, Tokyo, Japan). The extraction steps of different forms of P and Cr are provided in the Support Materials (Text S1). Ammonia nitrogen (NH4+-N), nitrate-nitrogen (NO3−-N) and nitrite-nitrogen (NO2−-N) analyses were conducted according to national standard methods issued by the State Environmental Protection Administration of China [30]. NH4+-N, NO3−-N and NO2−-N of sediment were extracted by 1 mol/L KCl and then analyzed by colorimetric analysis with a spectrophotometer (T3200, Shanghai, China).
The total DNA in the sediment was extracted using the E.Z.N.ATM Mag-Bind Soil DNA Kit (Omega, New York, NY, USA) and its completeness was tested by agarose gel electrophoresis. The microbes in the sediment were measured via high-throughput sequencing conducted with the Illumina MiSeq system (Shanghai, China) [31].

2.5. Statistical Analysis

The results were presented as average values and standard deviations of three replicate samples. Significant differences among the average values in different treatments were identified through one-way ANOVA and then Tukey’s test was conducted. All statistical analyses were performed using SPSS 21.0 (IBM, New York, NY, USA) and significant levels were reported at p < 0.01.

3. Results

3.1. Characterization of Fe3O4@BC

3.1.1. SEM and EDS

As shown in Figure 1a, the surface morphology of biochar mainly presents a flaky structure with porous structure. However, it could be clearly observed that the surface was loaded with many nano-particles when the Fe3O4 was loaded on the biochar in Figure 1b.
The mass fraction of Fe in Fe3O4@BC increased from 0.78% to 22.30% after loading Fe3O4 onto biochar (Figure 1c,d). The results showed that Fe3O4@BC was successfully prepared and the Fe3O4 was loaded onto biochar in the form of nano-particles.

3.1.2. XRD and FT-IR

The crystalline properties of BC and Fe3O4@BC were analyzed by XRD (Figure 2a). The characteristic peaks at 2θ = 21.2°, 35.1°, 41.4°, 50.4° and 67.2° were indexed as the (111), (220), (311), (400) and (511) planes of Fe3O4, respectively (PDF#19-0629). Thus, the results of XRD were consistent with the SEM image which indicated that Fe3O4 was successfully loaded onto the biochar.
The FT-IR spectra of the synthesized BC, Fe3O4 and Fe3O4@BC are shown in Figure 2b. Each band in the spectrum represents the vibration of functional groups in BC, Fe3O4 and Fe3O4@BC: -OH (3270 cm–1), C=O (1630 cm–1), C-O (1022 cm–1) and Fe-O (555cm–1). The addition of Fe oxides to the biochar made the biochar more hydrophilic, and the oxygen-containing functional group was enhanced, so -OH was produced [32]. The Fe-O stretching vibration peak also shows that Fe3O4 was successfully loaded onto the biochar [33].

3.2. pH and OPR

The variations of pH in overlying water are presented in Figure 3a. The CP+Fe3O4@BC group changed from 7.3 to 7.4 over time and had no significant difference compared with the CK group, but almost all other experimental groups had significant differences compared with the CK group (Support Material Table S5). The pH of each group with BC or CP increased and finally remained at about 8.0. In the CP+BC+Fe3O4 group, the pH remained stable in the early stage and began to rise after the 6th day, eventually reaching 8.3.
From Figure 3b, it was found that the ORP of all groups (excluding the CP+Fe3O4@BC group) remained positive during the entire process of the experiment. However, in the CP+Fe3O4@BC group, the ORP was negative in the first 8 days and then started to rise to a positive value after the eighth day.

3.3. Removal of AVS in the Sediment

As shown in Figure 4a, the content of AVS in the CP+Fe3O4@BC group decreased from 0.75 to 0.35 mg/g during the experiment and the AVS removal rate reached 52.77%. The AVS concentration in the sediment in all of the CP, CP+BC, CP+Fe3O4 and CP+BC+Fe3O4 groups decreased to 0.68 mg/g, 0.62 mg/g, 0.55 mg/g and 0.46 mg/g, respectively. All experimental groups showed significant differences on day 6, especially the CP+Fe3O4@BC group and the CK group (Support Material Table S7). Compared with the CK group, the AVS removal rate of CP+Fe3O4@BC reached 71%. However, AVS in the CK and BC groups increased to 0.88 mg/g and 0.86 mg/g, respectively. Compared with the CK group, the removal of AVS in the CP+Fe3O4@BC group reached 71%, indicating that the combined CP and Fe3O4@BC can effectively reduce the formation of AVS in sediment.

3.4. Changes of TP Content in the Overlying Water and the Content of Different Fractions of Phosphorus in the Sediments

As shown in Figure 5, the TP content in overlying water remained between 0.05 mg/L and 0.35 mg/L. In the CK group, BC group and CP group, the TP content increased in the early stage and reached the highest value on the eighth day, which was 0.331 mg/L, 0.288 mg/L and 0.296 mg/L, respectively. Then, it began to slowly decrease. The total phosphorus content in the overlying water of the other four groups was relatively stable, ranging from 0.1 mg/L to 0.2 mg/L. However, the TP of the CP+Fe3O4@BC group was maintained at 0.1 mg/L on average, which met the first class water standard [34].
The main cause of overlying water eutrophication is phosphorus release from sediment [35]. The form of sediment P is divided into: weakly adsorbed phosphorus (NH4Cl-P), iron/aluminum bound phosphorus (Fe/Al-P), calcium bound phosphorus (Ca-P) and residual phosphorus (Res-P). NH4Cl-P and Fe/Al-P are easy to migrate, while Ca-P and Res-P are stable and difficult to migrate. On the second and eighth days, phosphorus morphological changes are shown in Figure 6a,b. Although NH4Cl-P and Fe/Al-P contents decreased in the CP+Fe3O4@BC group, Ca-P content increased to 27.86%. Therefore, the TP content of overlying water in the CP+Fe3O4@BC group was maintained at 0.1 mg/L. However, the NH4Cl-P content in the sediments of the CK group decreased from 23.25% to 10.61%, while the other contents remained unchanged, indicating that the release of NH4Cl-P into water led to the increase of TP content in the overlying. The content of Fe/Al-P in the BC group also decreased from 28.14% to 12.96%, while the other contents remained unchanged, which increased the content of TP in the overlying water. The P content of different forms in different experimental groups can be seen in supporting materials Tables S1 and S2.

3.5. Changes of Cr Content of Different Fractions in the Sediments

Different forms of chromium in sediments were measured by the BCR method to explore the transformation of different forms. The different forms of Cr in sediments are divided into: acid extractable state (exchangeable state and carbonate combined state), reducible state (iron-manganese oxide combined state), oxidizable state (organic state) and residue state. In each experimental group, the different fractions and contents of Cr in the sediment are shown in Figure 7.
Among them, the acid extractable state and the reducible state are unstable and easily release and migrate, which are the main fractions released into the water [36,37]. On the sixth day, the acid extractable state content of the CK group and the BC group was almost the same, accounting for about 8%, but the reducible state content was different at 10.31% and 5.20%, respectively. On the 25th day, the acid extractable state and reducible state in the CK and BC groups still remained at about 5%. However, the content of the acid extractable state and reducible acid in the CP+Fe3O4@BC group was 1.6% and 2.51% on the 6th day and decreased to 0.97% and 0.58% on the 25th day, respectively, which could greatly reduce the harm of Cr release and migration. The Cr content of different forms in different experimental groups can be seen in supporting materials Tables S3 and S4.

3.6. Changes in Nitrogen Form

3.6.1. Variation of NH4+-N, NO3−-N and NO2−-N in the Sediment

The experimental changes of NH4+-N, NO3−-N and NO2−-N in the sediment are shown in Figures S1, S3 and S5. The content of NH4+-N in the sediment showed a downward trend on the whole, but it could be seen that the content of the CP+Fe3O4@BC group was lower than the other groups. The content of NO3−-N in the sediment also showed a downward trend during the experimental study and was finally maintained at 80 mg/kg. NO2−-N showed a downward trend as a whole, but the content of the CP+Fe3O4@BC group was the lowest compared with other groups.

3.6.2. Variation of NH4+-N, NO3−-N and NO2−-N in the Overlying Water

The experimental changes of NH4+-N, NO3−-N and NO2−-N in the overlying water are shown in Figures S2, S4 and S6. As shown in Figure S2, the content of NH4+-N decreased gradually (except CP+Fe3O4@BC group) in all groups. The content of NH4+-N in the CP+Fe3O4@BC group increased from the 2nd day to the 11th day, with a peak at 2.4 mg/L followed by a slow drop. The overall nitrate nitrogen of each group in the experiment showed an upward trend and the final content remained at 2.5 mg/L, but the nitrate-nitrogen content in the CK group was the least. As shown in Figure S6, the content of NO2−-N in all experiment groups basically reached the highest value on the eighth day and then began to decrease to nearly 0 mg/L (except CP+Fe3O4@BC group). However, the NO2−-N content in the CP+Fe3O4@BC group was almost 0 mg/L in the early stage and slowly increased to 0.467 mg/L on day 8, then began to decrease to 0 mg/L.

3.7. Analysis of Microbial Changes in Sediments

3.7.1. Microbial Diversity

Table 2 shows the analysis of the biodiversity index. The coverage rate of the samples had reached 99%, which can truly reflect the microorganisms in the sediment. The Chao index is commonly used in ecology to estimate the number of species. From the below table, it can be found that the Chao indexes between the experimental groups were 2900~3200. The Chao index in the CK group was only 2977.78, while the Chao index increased after adding CP, BC, Fe3O4 and Fe3O4@BC, which proved that the addition of CP, BC, Fe3O4 and Fe3O4@BC can promote the growth of microorganisms.
Shannon and Simpson’s indexes represent the diversity of the community. The higher the Shannon index, the higher the community diversity, while the lower the Simpson index, the higher the community diversity. The Shannon index of the CP+Fe3O4@BC group was 5.97, which was larger than that of other experimental groups. Furthermore, the value of the Simpson was 0.1, which was smaller than the value of other experimental groups. It can be found that the addition of Fe3O4@BC material and CP can increase the diversity of the community. The Shannon Even index represents the uniformity of the species; the larger the value, the better the uniformity. The Shannon Even index of the CP+Fe3O4@BC group was larger than the value of other experimental groups, indicating that the uniformity was great.

3.7.2. Microorganisms in Sediments

Figure 8 shows the relative abundance of genus-level microorganisms in different experimental groups. The main bacteria in the CK group were Dechloromonas (5.6%), Arthrobacter (7.3%), Paenisporosarcina (3.5%), Sulfurovum (0.5%), Luteolibacter (2.5%) and Desulfobulbus (0.45%).
Sulfurovum can obtain energy by oxidizing AVS [38]. Compared with CK group, Sulfurovum abundance of the CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group increased to 0.9%, 1.5% and 1.7%, respectively. Sulphate-reducing bacteria (SRBs) use oxidized sulfur compounds as electron acceptors and produce sulfide while oxidizing organics for energy generation. As a type of SRB, Desulfobulbus content in the CK group and BC group were 0.45% and 0.31%, respectively, but the Desulfobulbus content in the CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group is 0.21%, 0.3% and 0.13%, respectively. Therefore, the introduction of CP and Fe3O4@BC composite materials can inhibit the growth of SRB and promote the growth of sulfur oxidizing bacteria, which can remove AVS [39].
Dechlorominas, Acinetobacter and Flavobacterium are all denitrifying bacteria [40]. Compared with the CK group, the abundance of Dechloromonas [41] in the CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group increased to 6.8%, 7.3% and 7.5%, respectively. Acinetobacter content in the CK group was only 0.043%, and 0.059%, 0.079%, 0.103% and 0.106% for the BC group, CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group, respectively. Flavobacterium mostly has the function of nitrogen and phosphorus removal [42]. Flavobacterium content in the CK group was only 0.068%, while in the BC group, CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group it was 0.097%, 0.079%, 0.098% and 0.103%, respectively. The introduction of BC and Fe3O4 can promote the growth of Dechlorominas, Acinetobacter and Flavobacterium, which can promote denitrification. Nitrospira is a kind of nitrite-oxidizing bacteria (NOB), which can oxidize ammonia nitrogen into nitrate nitrogen [43]. According to the measurement data, the abundance of Nitrospira in the CP+Fe3O4@BC group was 0.12%, while that in the CK group was only 0.05%. The results showed that CP and Fe3O4@BC could promote the transformation of nitrogen and reduce nitrogen, one of the main components of eutrophication.

4. Discussions

4.1. AVS Removal Mechanism

The excessive accumulation of AVS in the sediment could poison biological growth and impact environmental health [44], and AVS is the main cause of black and odorous water [45]. The removal rate of sediment AVS in the CP+Fe3O4@BC group (52.8%) was higher than in the CK group (–18.1%). The mechanism of AVS removal could be explained as follows:
Firstly, CP can slowly decompose to release oxygen at a “controlled” rate when in contact with hydrous media. AVS can be oxidized by oxygen to form SO42− (Equation (3)) [46,47,48]. The production of oxygen also led to the increase of Sulfurovum (Figure 8), which promoted the removal of AVS. The removal rates of AVS in the sediments of the CP group, CP+BC group, CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group were 8.74%, 17.3%, 26.3%, 38.6% and 52.8%, respectively. However, AVS concentration increased in the experimental group without CP (Figure 9a). The content of AVS in the CK group and BC group increased by 18.1% and 15.9%, respectively.
O2 + H2S → SO42− + H2O
Secondly, the hydroxyl radicals can efficiently oxidize AVS. Since the hydroxyl radicals have a high standard oxidation potential (2.80 V) and exhibit high reaction rates [49,50] (Equation (4)). The removal rate of AVS in the CP+Fe3O4 group, CP+BC+Fe3O4 group and CP+Fe3O4@BC group was 26.26%, 38.58% and 52.77%, respectively, which is probably attributed to the Fenton reaction to produce hydroxyl radicals (Figure 9b) [51]. Nano-Fe3O4 was loaded onto the BC by the coprecipitation method, which solved the agglomeration problem of nano-Fe3O4 and made it a full-contact reaction. Therefore, the removal rate of AVS from sediments in the CP+Fe3O4@BC group was 14.19% more than that in the CP+BC+Fe3O4 group. Simultaneously, the by-product of the Fenton reaction could produce H+, which reacted with OH− to maintain the pH value [52], therefore the pH of the CP+Fe3O4@BC group was unchanged (Figure 3a).
Fe2+ + H2O2 → Fe3+ + ∙OH + OH–
Finally, the autotrophic sulfide driving denitrification could be considered as the main theory of nitrate repairing AVS in sediment [53]. The addition of CP and Fe3O4@BC materials promoted the growth of nitrifying bacteria and transformed NH4+-N and NO2−-N into NO3−-N in the sediment. Meanwhile, denitrifying bacteria also increased, which could use the nitrate to remove AVS (Figure 9c).
S2– + NO3– + H2O → SO42– + N2
The removal rate of AVS in the CP group was only 8.74%, which was due to the removal of AVS by oxygen. However, the removal rate of the CP+BC group reached 17.3%, which had an increment of 8.56% compared with the CP group. The above phenomenon could attribute to the biochar containing a small amount of iron, generating hydroxyl radicals and then oxidizing AVS. Meanwhile, the removal rate of AVS in the CP+Fe3O4@BC group was much higher than that in the CP+BC+Fe3O4 group. We reasoned that this incremental phenomenon owing to the agglomeration of nano-Fe3O4 and its inability to react effectively with other substances. The uniform loading of Fe onto biological carbon can effectively solve the agglomeration phenomenon and promote the utilization rate of Fe.
Desulfobulbus is a type of SRB that can transform sulfate into sulfide in an anoxic environment. Desulfobulbus content in the CK and BC groups was 0.45% and 0.31%, respectively, thus increasing AVS content [54,55]. However, the Desulfobulbus content in the CP+Fe3O4@BC group is only 0.13%, proving that the addition of CP and Fe3O4@BC composite materials can inhibit SRBs and can effectively repair AVS over a long period.
In summary, there were three removal mechanisms of AVS by adding CP and Fe3O4@BC composite materials into sediments. The synergism of CP and Fe3O4@BC effectively removed the content of AVS in sediments, decreased the accumulation of NH4+-N and NO2−-N in sediments, inhibited the SRB growth and, therefore, achieved long-term effective control of odorous sediment.

4.2. Mechanism of Phosphorus and Chromium Change in Different Fractions in the Sediment

4.2.1. Phosphorus

Phosphorus is an important nutrient, but excessive P will also lead to eutrophication of the water body, thereby destroying the water environment [56]. NH4Cl-P is unstable and easily released into overlying water under external disturbance, leading to eutrophication. Fe/Al-P is also unstable, but stable in an acidic environment, and easily released from sediment in a neutral or alkaline environment, resulting in an increase in overlying water phosphorus content. NH4Cl-P and Fe/Al-P are referred to as migrating phosphorus in this study since these two forms of P can easily migrate and release [57,58].
As shown in Figure 10, the phosphorus migration state showed a decreasing tendency from the second day to the eighth day, but on the contrary, the Ca-P content was increased (except CK group and BC groups). The content of the P migration state in the CK group and BC group decreased from 0.301mg/g and 0.273mg/g to 0.225mg/g and 0.16mg/g, while the content of Ca-P remained unchanged. This was attributed to the release of the phosphorus migration state in sediment, leading to the increase of P content in the overlying water (Figure 5) [59,60,61], and the Ca-P was relatively stable and difficult to release into water bodies. In the CP+Fe3O4@BC group, the pH value remained stable and the addition of calcium ions transformed the NH4Cl-P and Fe/Al-P into Ca-P, which led to low TP content in overlying water [62]. Therefore, adding CP and Fe3O4@BC composite to the sediment could effectively control internal P release by increasing the concentrations of Ca-P. Simultaneously we could observe the transformation between ammonia nitrogen and nitrate nitrogen and the change of the nitrite content of the intermediate product according to the ORP of the overlying water. In the reduction environment, the dissimilatory nitrate reduction to ammonium (DNRA) was prone to occur [63]. In the oxidizing environment, ammonia nitrogen is converted to nitrite nitrogen by microorganisms and eventually to nitrate nitrogen (nitrification reaction) [64]. In addition, denitrifying bacteria increased in the CP+Fe3O4@BC group, which promoted denitrification.
In conclusion, the introduction of CP and Fe3O4@BC composite materials can effectively inhibit the release of phosphorus in sediment and promote denitrification, which could reduce the eutrophication of water bodies [5].

4.2.2. Chromium

Cr is a well-known carcinogenic element present in drinking water. The high concentration of Cr also exerts strong toxic effects as it can diffuse through the cell membranes, oxidize biological molecules and create a potential risk of living being healthy. At the same time, Cr also is responsible for skin tumors in animals [65].
As shown in Figure 11, the contents of acid exchangeable Cr (Ex-Cr) and reducible Cr (Reducible-Cr) in the CK and the BC groups were high, and were easy to release and migrate to water bodies. The content of Ex-Cr and Reducible-Cr in the experimental group with CP was lower than 0.5 mg/kg. The Ex-Cr in CP group, CP+BC group, CP+Fe3O4 group and CP+BC+Fe3O4 group were 0.263 mg/kg, 0.251 mg/kg, 0.263 mg/kg and 0.694 mg/kg, respectively, on the sixth day. However, on the 25th day, they all increased to 0.433 mg/kg, 0.313 mg/kg, 0.342 mg/kg and 0.324 mg/kg, respectively. The addition of calcium peroxide leads to an increase in Ex-Cr and potentially to the release of chromium from the sediment [47]. Adsorption, ion exchange, complexation and precipitation are the major mechanisms involved in the conversion of soluble and potentially soluble forms of heavy metals to geochemically stable solid phases by biochar [66]. However, the content of Ex-Cr and Reducible-Cr in the BC group did not decrease, which may be ascribed to the fact that the BC had no effect on Cr fixation. When Fe3O4 was loaded onto BC by the coprecipitation method, -OH functional groups were formed, which can solidify Cr (Figure 2b). Therefore, the Ex-Cr and Reducible-Cr contents in the CP+Fe3O4@BC group were only 0.157 mg/kg and 0.0942 mg/kg at 25 days, respectively, which were far lower than those in other experimental groups (Figure 11).
In general, the addition of CP and Fe3O4@BC composites can change the easily migrating state of Cr into a stable state, thus reducing the harm of heavy metal Cr.

5. Conclusions

The synergistic treatment of black odorous sediments with CP and Fe3O4@BC composites was researched in this study. The results showed that the addition of CP and Fe3O4@BC composites into the sediment could effectively remove AVS and the removal rate of AVS in sediments reached 52.77% on the 25th day. Compared with the CK group, the removal rate of AVS by CP and Fe3O4@BC reached 71%. The removal mechanism could be divided into three aspects: Firstly, oxygen produced by CP could oxidize AVS. Secondly, CP could produce H2O2, which was activated by Fe2+ to produce hydroxyl radical, and therefore oxidize AVS. Finally, the addition of CP and Fe3O4@BC could promote the growth of denitrifying bacteria (Dechlorominas, Acinetobacter and Flavobacterium) and strengthen denitrification to remove AVS. The addition of CP and Fe3O4@BC could transform the NH4Cl-P and Fe/Al-P into a stable Ca-P complex, greatly reducing the release of phosphorus. Meanwhile, the increase of denitrifying bacteria (Dechlorominas, Acinetobacter and Flavobacterium) could promote the denitrification reaction and further promote the conversion of nitrogen. Therefore, the addition of CP and Fe3O4@BC could effectively inhibit the eutrophication of the water body. Besides, after adding CP and Fe3O4@BC composites, the content of the Ex-Cr and the reducible-Cr decreased to 0.157 mg/kg and 0.0942 mg/kg on the 25th day, respectively, which could contribute to decreasing the harmfulness of the heavy metal Cr. The addition of Fe3O4@BC composites increased the total number of microorganisms, promoted the growth of Sulfurovum and inhibited SRBs, which could significantly inhibit the production of AVS. This treatment could be suitable for polluted sediment conditioning when dredging the sediment. Simultaneously, this study could provide a new idea to deal with black odorous sediment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w14101626/s1, Text S1: The extraction methods of different forms of phosphorus were introduced; Text S2: The extraction methods of different forms of chromium were introduced [67]; Text S3: Microbial index calculation equation; Table S1: The forms content of P in sediment on day 2; Table S2: The forms content of P in sediment on day 8; Table S3: The forms content of Cr in sediment on day 6; Table S4: The forms content of Cr in sediment on day 25; Table S5: Table of significant difference results (pH); Table S6: Table of significant difference results (ORP); Table S7: Table of significant difference results (AVS); Table S8: Table of significant difference results (TP); Figure S1: The variation of NH4+-N in the sediment; Figure S2: The variation of NH4+-N in overlying water; Figure S3: The variation of NO3-N in the sediment; Figure S4: The variation of NO3-N in overlying water; Figure S5: The variation of NO2-N in the sediment; Figure S6: The variation of NO2−-N in the sediment.

Author Contributions

N.L. and C.L. contributed to the conception of the study and provided financial support. Y.L., X.W. and G.G. performed the experiment. Y.L. and Y.H. (Yanchun Huang) contributed significantly to analysis and manuscript preparation. Y.L. performed the data analyses and wrote the manuscript. J.L., Y.H. (Yuxin He) and N.L. helped perform the analysis with constructive discussions. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by Major Scientific and Technological Special Program of Sichuan Province, China (2018SZDZX0027 and 2019-YF09-00081-SN), and the Strategic Cooperation Project of Sichuan University and Luzhou City, Sichuan Province, China (2020CDLZ-6).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this article.

Acknowledgments

The authors would like to acknowledge the financial support from the Major Scientific and Technological Special Program of Sichuan Province, China (2018SZDZX0027), the Key Research and Development Program of Sichuan Province, China (2019-YF09-00081-SN) and the Strategic Cooperation Project of Sichuan University and Luzhou City, Sichuan Province, China (2020CDLZ-6). At the same time, the authors are very grateful to Yanchun Huang and Ge Gou’s graduate students for accompanying me to take samples. I also thank Yanchun Huang for his help in the experiment.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 14 01626 g001 550
Figure 1. SEM images of (a) BC, (b) Fe3O4@BC; EDS spectrum of (c) BC, (d) Fe3O4@BC.
Figure 1. SEM images of (a) BC, (b) Fe3O4@BC; EDS spectrum of (c) BC, (d) Fe3O4@BC.
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Figure 2. (a) XRD patterns of BC and Fe3O4@BC; (b) FT-IR spectra of BC and Fe3O4@BC.
Figure 2. (a) XRD patterns of BC and Fe3O4@BC; (b) FT-IR spectra of BC and Fe3O4@BC.
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Figure 3. The variation of pH and ORP in overlying water during the experimental study: (a) pH. (b) ORP; ** indicates a significant effect at the 0.01 probability level between the control and different treatments.
Figure 3. The variation of pH and ORP in overlying water during the experimental study: (a) pH. (b) ORP; ** indicates a significant effect at the 0.01 probability level between the control and different treatments.
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Figure 4. (a) The variation of AVS in the sediment during the experimental study; (b) The proportion of AVS in each group relative to the initial sediment on the 25th day; ** indicates a significant effect at the 0.01 probability level between the control and different treatments; ns indicates no significant effect at the 0.01 probability level between the control and different treatments.
Figure 4. (a) The variation of AVS in the sediment during the experimental study; (b) The proportion of AVS in each group relative to the initial sediment on the 25th day; ** indicates a significant effect at the 0.01 probability level between the control and different treatments; ns indicates no significant effect at the 0.01 probability level between the control and different treatments.
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Figure 5. The variation of TP in overlying water during the experimental study; ** indicates a significant effect at the 0.01 probability level between the control and different treatments; ns indicates no significant effect at the 0.01 probability level between the control and different treatments.
Figure 5. The variation of TP in overlying water during the experimental study; ** indicates a significant effect at the 0.01 probability level between the control and different treatments; ns indicates no significant effect at the 0.01 probability level between the control and different treatments.
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Water 14 01626 g006 550
Figure 6. The variation of P in different forms in sediments: (a) the 2nd day. (b) the 8th day.
Figure 6. The variation of P in different forms in sediments: (a) the 2nd day. (b) the 8th day.
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Water 14 01626 g007 550
Figure 7. The variation of heavy metal Cr in different groups during the experimental study; (a) the 6th day; (b) the 25th day.
Figure 7. The variation of heavy metal Cr in different groups during the experimental study; (a) the 6th day; (b) the 25th day.
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Water 14 01626 g008 550
Figure 8. The relative abundance of microbial communities.
Figure 8. The relative abundance of microbial communities.
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Water 14 01626 g009 550
Figure 9. Synergistic reaction mechanism of Fe3O4@BC and CaO2; (a) Oxygen oxidizes AVS. (b) Hydroxyl radical oxidizes AVS. (c) Autotrophic sulfide drives denitrification.
Figure 9. Synergistic reaction mechanism of Fe3O4@BC and CaO2; (a) Oxygen oxidizes AVS. (b) Hydroxyl radical oxidizes AVS. (c) Autotrophic sulfide drives denitrification.
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Water 14 01626 g010 550
Figure 10. (a) The Phosphorus migration state changes on the 2nd day and the 8th day; (b) The Ca-P changes on 2nd day and 8th day.
Figure 10. (a) The Phosphorus migration state changes on the 2nd day and the 8th day; (b) The Ca-P changes on 2nd day and 8th day.
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Water 14 01626 g011 550
Figure 11. (a) The Ex-Cr fraction changes on the 6th day and the 25th day; (b) the reducible-Cr fraction changes on the 6th day and the 25th day.
Figure 11. (a) The Ex-Cr fraction changes on the 6th day and the 25th day; (b) the reducible-Cr fraction changes on the 6th day and the 25th day.
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Table
Table 1. Details of the batch experiments.
Table 1. Details of the batch experiments.
GroupsSediment
(g)		Water
(mL)		BC
(g)		CaO2
(g)		Fe3O4
(g)		Fe3O4@BC
(g)
CK75150
BC751504
CP75150 12
CP+BC75150412
CP+Fe3O475150 122
CP+BC+Fe3O4751504122
CP+Fe3O4@BC75150 12 6
Table
Table 2. Diversity Index Statistics in sediments.
Table 2. Diversity Index Statistics in sediments.
SampleShannonChaoSimpsonShannonevenCoverage
13th daysCK5.492977.780.030.700.99
BC5.633207.530.020.710.99
CP+Fe3O45.733030.680.020.740.99
CP+BC+Fe3O45.623068.400.030.720.99
CP+Fe3O4@BC5.973144.750.010.760.99
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Li, Y.; Huang, Y.; Wang, X.; Gou, G.; Liu, C.; Li, J.; He, Y.; Li, N. Synergistic Effects of Calcium Peroxide and Fe3O4@BC Composites on AVS Removal, Phosphorus and Chromium Release in Sediments. Water 2022, 14, 1626. https://doi.org/10.3390/w14101626

AMA Style

Li Y, Huang Y, Wang X, Gou G, Liu C, Li J, He Y, Li N. Synergistic Effects of Calcium Peroxide and Fe3O4@BC Composites on AVS Removal, Phosphorus and Chromium Release in Sediments. Water. 2022; 14(10):1626. https://doi.org/10.3390/w14101626

Chicago/Turabian Style

Li, Yintian, Yanchun Huang, Xueying Wang, Ge Gou, Chao Liu, Jun Li, Yuxin He, and Naiwen Li. 2022. "Synergistic Effects of Calcium Peroxide and Fe3O4@BC Composites on AVS Removal, Phosphorus and Chromium Release in Sediments" Water 14, no. 10: 1626. https://doi.org/10.3390/w14101626

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