Effect of black carbon on sorption and desorption of phosphorus onto sediments

Marine Pollution Bulletin 146 (2019) 435–441

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Effect of black carbon on sorption and desorption of phosphorus onto sediments

T

Hong-Hai Zhanga,b,d, Xiao-Yan Caoa,b,d, , He Wangd, Zhun Mac, Jing Lid, Li-Min Zhoud, ⁎ Gui-Peng Yanga,b,d, ⁎

a

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education/Institute for Advanced Ocean Study, Ocean University of China, Qingdao 266100, China b Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266100, China c College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China d College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China

ARTICLE INFO

ABSTRACT

Keywords: Marine sediment Black carbon Phosphorus Sorption Surface property

The sorption behavior of phosphorus onto sediment was investigated with the addition of BC derived from incomplete biomass combustion (PC). The sorption kinetic curves of phosphorus onto PC and sediment could be described by a two-compartment first order equation, and the sorption isotherms fit the Freundlich model well. With increasing amounts of PC added, the sorption capacity increased while the HI did not change much. The distribution of phosphorus forms showed that CaeP (ACa-P plus DAP) constituted the highest fraction in the sediment samples. Throughout the sorption process, CaeP and OP changed very little, but the Ex-P and FeeP increased obviously, and the presence of PC made this increase more significantly. The high specific area and the presence of iron and aluminum, as well as the modification of the sediments surface properties, make the addition of PC be favorable for the sorption of phosphorus onto sediments.

1. Introduction Phosphorus (P) is an essential and basic nutrient for phytoplankton growth in the oceans. It can influence marine ecosystem structure by controlling the primary production. As more and more phosphorus entering the aquatic systems, the excess phosphorus concentration can cause eutrophication, leading to algae over blooming and water quality degradation (Sharpley and Smith, 1989; Young et al., 1999). Marine sediment is regarded as an important carrier of phosphorus cycling. The sorption and desorption of phosphorus on sediment plays a significant role in phosphorus biogeochemical cycle by controlling P transport and bioavailability (Meng et al., 2014; Wang and Li, 2010). It is well known that sorption characteristics are closely related to the composition of sediments. Besides calcite, Al and Fe oxides, and clays, organic matter also has been reported to strongly affect P sorption in sediment surfaces (Jalali and Peikam, 2013). Phosphate sorption by organic matter (OM) is complex and is affected by both the “quantity” and “quality” of the OM (Wang et al., 2006). Organic matter in sediments is usually classified into two states: a rubbery state, mainly formed by amorphous organic matter (AOM) such as humic/fulvic

substances and lignin, which is also known as “soft” domain, and a glassy state, also known as “hard” domain, formed by condensed materials such as black carbon (BC), coal, and kerogen (Lou et al., 2014; Zhang and He, 2013). Black carbon (BC) is the residues of incomplete biomass and fuel combustion, characterized by its intrinsically high micro-porosity and surface activity. In recent years, BC has been attracting more and more attention due to its extraordinary sorption performance and the resulting potential role in controlling the fate and toxicity of many environmental organic contaminants (Kasozi et al., 2010; Xia et al., 2011; Lou et al., 2012). It has been reported that BC was responsible for > 80% of the total sorption for PAH, PCB and PCDD by harbor sediments (Lohmann et al., 2005). The sorption of hydrophobic organic compounds by BC could be up to 10–100 times greater than sorption by other types of organic matter (Cheng et al., 2014), or several orders of magnitude higher than in natural sediments (Lou et al., 2014). Besides organic compounds, BC may also show strong sorption affinity for metal ions. It has been found that the maximum capacity for Cu2+sorption in BC-containing soils was > 4 times greater than those for the adjacent soils, and the sorption of Cu2+ by historical BC was substantially higher than that of fresh BC (Cheng et al., 2014). It

⁎ Corresponding authors at: Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education/Institute for Advanced Ocean Study, Ocean University of China, Qingdao 266100, China. E-mail addresses: [email protected] (X.-Y. Cao), [email protected] (G.-P. Yang).

https://doi.org/10.1016/j.marpolbul.2019.06.059 Received 18 February 2019; Received in revised form 18 June 2019; Accepted 20 June 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.

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is generally accepted that the strong sorption affinity of organic contaminants to BC is attributed to hydrophobic effect, the interaction between BC surface active sites and the contaminants, and/or filling of micropores (Wang et al., 2017). The surface aggregation due to the latter two effects may play a particularly significant role, leading to a nonlinear sorption character (Lou et al., 2014). As for Cu2+ mentioned above and other heavy metals, the higher sorption capability can be attributed to the surface negative charge resulted from the formation of surface O-containing functional groups, as the sorption is mainly driven by electrostatic interactions and sorption-precipitation between the functional groups of BC and heavy metals. Long term natural oxidation may enhance this effect and then strengthen the sorption on BC (Cheng et al., 2014; Lian and Xing, 2017). The wide spread combustion of biomass and fuel make BC widely distributed in the atmosphere, soils, water and sediments, and eventually be transferred into the ocean by riverine discharge, and atmospheric deposition (Hu et al., 2016; Huang et al., 2016). Black carbon has been found in marine dissolved organic carbon (DOC), particulate organic carbon (POC), and sedimentary organic carbon (SOC). Marine sediments contain a significant amount of BC in organic carbon, with BC/OC% values ranging from15 ± 2% to 21 ± 6% in abyssal sediments and up to 50 ± 40% in coastal sediments (Coppola et al., 2014). Although the results may vary depending on the methods to quantify BC, the black carbon content is still considerable in general. It is estimated that about 90% of the global BC burial occurs on the continental shelf, though the area accounts for only10% of the world ocean (Huang et al., 2016; Hu et al., 2016; Suman et al., 1997). The East China Sea (ECS) is one of the largest shelf seas in the world with significant anthropogenic impacts on the ecosystem. The continental shelf width is about 640 km and the mean depth is 72 m (Song and Liu, 2015). It receives annually 5×108 t of terrestrial particulates including 2–5×106 t of particulate OM (Liu et al., 2014). A survey of BC distribution in the East China Sea sediments showed that the BC concentrations were in the range of 0.30–1.52 mg g−1 and accounted for 12–65% of the total organic carbon (TOC). The percentage of BC derived from biomass combustion was 29–48% (Huang et al., 2016). In addition, excess input of N and P and changed nutrient structure have led to severe eutrophication and frequent breakout of harmful blooms in inshore waters in the ECS (Yu et al., 2012; Zhou et al., 2008). Phosphorus has been shown to be a limiting nutrient for phytoplankton growth in the ECS shelf (Liu et al., 2003; Fan and Song, 2014). Therefore, it is necessary to study phosphorus sorption on the sediment in the presence of black carbon to clarify the BC effects on the sediment phosphorus retention abilities. In the present paper, we studied the sorption kinetic curves and isotherms to characterize the retention of P on sediments with different content of BC, which is prepared from biomass combustion. Moreover, as BC may also contain some metal contaminants (e.g., Cu, Cd, and Pb) originated from heavy metal-containing feedstocks, which may affect the combination of sediment and phosphorus (Lian and Xing, 2017), we also studied the distribution of phosphorus fractions after phosphorus sorption.

elemental compositions were determined by an element analyzer (PE2400II). Some metal elements, such as Fe and Al, were determined by ICP as they may remain after treatment and affect the bonding state of phosphorus: First, 150 mg sample was burned at 550 °C for 2 h, then digested in 1:1(v:v) concentrated HCl and HNO3 at 190 °C for 3 h. After the digestion, the acid is completely volatilized at 120 °C. The residue was rinsed and the supernatant was fixed and tested by Thermo ICAP6300. The sediment sample was collected from the East China Sea (ECS) shelf at 31°29.757 N, 124°30.396E. The sample was sealed in plastic bags and stored at −20 °C. Prior to the experiments, the sediment sample were air-dried and ground. The part that pass 80 mesh sieves (< 178 μm) was used for the sorption experiments. 2.2. Physicochemical properties of BC and sediment samples The morphological characterization of BC was examined by scanning electron microscopy (SEM, Hitachi S-4800). The specific surface area (SSA) was obtained by nitrogen adsorption based on BET isotherm utilizing a Quantachrome NOVA 2000e gas sorption analyzer. The surface acidity and basicity were determined using the Boehm's titration method. 0.500 g sample was reacted with 50 mL of 0.1 mol L−1 NaHCO3, Na2CO3, NaOH and NaOC2H5 for the acidity determination as carboxyl group, carboxyl and lactonic groups, carboxyl, lactonic and phenolic groups, and the total acidic groups, respectively. A similar process was carried out using HCl solution for the acidity determination of all basic groups. The excess base or acid was calculated through the back-titration using 0.1 M HCl solution or NaOH solution (Boehm, 2002). Surface charge properties of the sediment samples were investigated by acid-base titrations using a Metrohm automatic potentiometric titration system (Metrohm 809) at 25 °C. 100 mg sediment was hydrated in 40 ml of NaNO3 solution (0.7 mol L−1) for 15 h before titration. The pH was initially adjusted to 3.0 by HNO3 solution, then a two-backtitration process under N2 was performed to pH 10 and then down to pH 3 with the addition of NaOH and HNO3 solutions, respectively. The base-back-titration data was used to analyze the proton consumption of the sample. The classical 2pK-CCM surface complexation model was used to calculate the protonation and deprotonation constants (Ka1int, Ka2int), respectively. Point of zero net proton charge pHPZNPC was obtained as pHPZNPC = 1/2(|pKa1int| + |pKa2int|) (Cao et al., 2017; Gao and Mucci, 2001; Tombácz and Szekeres, 2001). The cationic exchange capacity (CEC) was analyzed by EDTA ammonium acetate method (NJISS, 1978). 2.3. Sorption experiments Natural seawater (NSW) from Qingdao inshore was used as the medium for phosphorus sorption experiments. The seawater was aged for more than1 month and filtered through 0.45 μm membrane before use. The salinity and pH values were 30 PSU (Practical Salinity Unit) and 8.0 respectively with the inorganic phosphorous content lower than 0.001 mg L−1. KH2PO4 was used to prepare the phosphate solutions. The sorption experiments were performed using a batch technique.

2. Materials and methods 2.1. BC and sediments

2.4. Kinetic sorption experiments

The BC sample was produced from a residue of incomplete biomass combustion of peach tree wood. In order to reduce the effects of compounds of Si and metals, which may be present with high contents in the residue, the coarse biochar was treated sequentially with 2 mol L−1 HCl and 1:1 (mol L−1) HCl–HF acid: first shaken in 2 mol L−1 HCl at 250 rpm for 24 h at room temperature, followed by leaching (rinse) with 1 mol L−1 HCl five times, then treated with 1:1 (mol L−1) HCl–HF acid at 250 rpm for 24 h. After the treatment, the samples were thoroughly rinsed with distilled water till the pH was near 7 and dried (Chun et al., 2004). The sample was named as PC. The C, H and N

A certain amount of adsorbent sample was taken into a series of 40 mL phosphate solution (1 mg L−1) at 25 °C in a temperature-controlled shaker (THZ-82, Changzhou Guohua Appliance Co., China). The agitation speed was 150 rpm. At appropriate time intervals, the samples were withdrawn and the aqueous media were separated by centrifuge at 3500 rpm (Model LG10–2.4A, Beijing Physical Utilization Centrifuger Factory, China). The phosphate concentration in the supernatant was analyzed with an UV spectrometry (UV-2550 spectrophotometer, Shimadzu Corporation, Japan). The phosphorus sorbed on the samples 436

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was calculated based on the initial phosphate concentration before sorption.

pore size of 3.392 nm. The elemental compositions and Boehm titration results of the surface functional groups are present in Table 1.The C, H and N elemental compositions determined by PE2400II shows the contents of carbon was 79.85%. Despite the careful acid treatment, some metal elements remained in the PC such as Fe and Al (also shown in Table 1), which may affect P fraction distribution in sediment. The presence of oxygen-containing groups on the surface leads to surface acidity or basicity of the material. From the results of Bohem titration, it can be found that hardly any basic groups were detected. But there were abundant acidic functional groups on the surface of the material. The total acidic group content was 2.412 mmolg−1 and among the acidic functional groups, carboxylic group was the dominant. Fig. 2 shows the kinetic and equilibrium isotherms of phosphorus onto the PC material at room temperature. The kinetic experiment was performed with the initial phosphorous concentration of 1 mg L−1. Obviously, the sorption of phosphorus increased rapidly during the initial 5 h, then the sorption amount rose slowly. Compare with the sorption amount at 24 h, almost no further sorption could be found after 16 h. The sorption equilibration time was therefore determined to be 24 h to ensure a complete equilibration. To investigate the kinetic mechanism of the sorption process, a two-compartment first order equation was tried to interpret the kinetic curve, and a nonlinear method was used here to obtain the rate parameters of the model. The two-compartment first order equation is expressed as:

2.5. Equilibrium sorption experiments The isotherm experiments were conducted in various initial concentrations of the phosphorus from 0 to 1.0 or 1.6 mg L−1. A certain amount of BC or sediment was added to a series of 100 mL glassstopped flasks with each flask containing 40 mL solution. The flasks were agitated as above till equilibrium was established. After centrifugation, the phosphorus concentrations in the supernatants were analyzed, and the amount of sorption onto the sediment was determined as in the kinetic experiments. The sediment residues were collected and used for the desorption isotherms: 40 mL of NSW was added and the samples were shaken at 150 rpm till equilibrium. Then the suspensions were centrifuged, the supernatants were analyzed and the amount of phosphorus desorbed was calculated similarly to those described above. 2.6. Analysis of phosphorus fractions Phosphorus fractions were analyzed before and after sorption. The fractions were determined using a sequential extraction method (Ruttenberg, 1992; Huerta-Diaz et al., 2005; Song and Liu, 2015). The exchangeable or loosely sorbed P (Ex-P) and Fe-bound P (FeeP) were determined by sequential chemical extraction using 1 mol L−1 MgCl2 and citrate-dithionite-bicarbonate (CDB) with modifications following a Mg(OH)2 co-precipitation method (CDB-MAGIC). ACa-P was subsequently extracted with 1 mol L−1 acetate buffer, and the sediment residue was extracted with 1 mol L−1 HCl as for DAP. An extraction with 1 mol L−1 HCl was performed to determine inorganic phosphorus (IP). The total phosphorus (TP) was extracted with 1 mol L−1 HCl after treatment under 550 °C. The organic P (OP) was obtained through the difference between IP and TP. All extracted phosphates were analyzed by using the spectrophotometric method. For all samples, triplicates were analyzed and the average data were reported. All solutions were prepared using deionized water and all chemicals used were of analytical reagent grade.

Qe

Qt Qe

= Frap exp( k rap t ) + Fslow exp( k slow t )

(1)

where Qe and Qt (mg g−1) are the sorption amounts of phosphorus at equilibrium and various time t (min), Frap and Fslow are the rapid and slow fractions of the process, and krap and kslow are the rate constants of the rapid and slow sorption, respectively. The fitting results showed that the correlation coefficients, r2, was 0.996, and the theoretical Qe was 0.1829 mg g−1, which is in agreement with the experimental one. Therefore, it can be concluded that this model described the rate curve well. The kinetic rate constant of the rapid step krap was 3.7826 h−1, which was 20 times higher than the rate constant of the slow one kslow (0.1519 h−1), while the slow fraction of the process (0.6612) was twice as much as the rapid one (0.3388). The isotherm experiments were conducted in various initial concentrations of the phosphorus from 0 to 1.0 mg L−1. Langmuir and Freundlich models were used to describe the isotherms.

3. Result and discussion 3.1. Characteristics of PC The SEM image (Fig. 1) shows a fibrous microscopic appearance of PC. The surface area through BET isotherm was 232.84m2/g with mean

Qe =

Qm KL Ce 1 + KL Ce 1

Q e = KFCen

(2) (3)

where Qe and Qm are the equilibrium amount and the maximum amount of phosphorous sorbed onto PC (mg g−1), respectively. Ce is the equilibrium concentration of phosphorous in solution (mg L−1). KL (mg L−1) and KF ((mg g−1) (L mg−1)1/n) are the sorption coefficients indicating the phosphorous sorption efficiency of sediments. 1/n is the Freundlich exponent which reflects the sorption intensity. Although Langmuir and Freundlich isotherms are theoretically two models with different conditions, they are often used in the sorption of phosphorus. The parameters of the isotherms resulted from the Langmuir equation and Freundlich equation are listed in Table 2. It can be found that both the models fit the data well and the Freundlich equation seems better. The difference between the subsequent desorption curve of phosphorus and the sorption curve showed an obvious hysteresis of the sorption. The ratio of the Freundlich exponents for desorption and sorption (nD/nS) was usually used as hysteresis index to characterize the irreversibility of the process.

Fig. 1. Scanning electron microscopy image of PC. 437

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Table 1 Composition (%) and Boehm titration results (mmol g−1) of PC. C

H

N

Fe

Al

Carboxyl

Lactone

Phenolic

Quinonyl

Acidic

Basic

79.85

1.69

5.14

0.072

3.98

2.149

0.051

0.061

0.151

2.412

0

Fig. 2. Sorption kinetic curve (A, line: two-compartment first order model fitting) and sorption isotherm (B, line: Freundlich model fitting) of phosphorus onto PC.

HI =

nd ns

(4)

A larger HI (until it is close to 1), indicates that the process is more reversible, while a lower HI means that the sorbed phosphorous is more strongly bound to the adsorbent and resulting in a lower desorption. The smaller the HI, the more difficulty of the sorbed phosphorous would be desorbed from the adsorbent (Kan et al., 1998; Oren and Chefetz, 2005; Xu and Li, 2010). Here the value of HI was 0.80, suggesting an obvious desorption hysteresis accompanied with chemical binding effects. Some active sites on the organic matter surface, especially the acidic functional groups may be responsible for the hysteresis. 3.2. Kinetic and equilibrium isotherms of P onto sediment with PC

Fig. 3. Sorption kinetic curves of phosphorus onto sediment samples with and without PC (lines: two-compartment first order model fitting).

We used a sediment sample with 4% PC addition as an example to investigate the effect of PC on the kinetics of phosphorus sorption onto sediments. The kinetic curves can still be described well by the twocompartment first order equation, but the effect of PC was very significant. As shown in Fig. 3 and Table 3, the presence of PC increased the equilibrium sorption capacity of the sediment from 0.0184 to 0.0419 mg g−1, and at the same time, the slow adsorption fraction increased from 0.274 to 0.360 with the sorption rate constant also increased from 0.4915 to 0.9246 h−1. Fig. 4 shows the sorption isotherms of the sediment samples with different contents of PC. The contents of PC employed in these experiments were 2%, 4% and 7%, respectively. It can be found that there was an obvious desorption in the low initial phosphorus concentrations due to the native adsorbed phosphorus (NAP) of the sediment itself. The value of the X intercept is considered to be the equilibrium phosphorus concentration value EPC0, indicating that the sediment is in exchange equilibrium of phosphorous with seawater at such a concentration. A modified Freundlich model are used to describe the isotherms (Huang and Zhang, 2010, 2011). The equation is expressed as follows.

Table 3 Sorption kinetic parameters of phosphorus to sediment samples with and without PC. Samples

Two-compartment first order equation

Sediment Sediment+4%PC

1

Qe = KF Ce n

Frap

Fslow

krap/ h−1

kslow/ h−1

Qe/ mg g−1

r2

0.726 0.640

0.274 0.360

17.56 15.19

0.4915 0.9246

0.0184 0.0419

0.993 0.997

(5)

NAP

The sorption parameters derived from the fitting results are presented in Table 4. The modified Freundlich equation provided a good fit to the data. With the addition of PC, the native adsorbed phosphorus was lowered and became difficult to desorb, indicating that the presence of PC enhances the retention of phosphorus in sediments. With the increasing of PC addition, the capability of phosphorus sorbed onto

Table 2 Sorption and desorption isotherm parameters of phosphorus on PC. Freundlich

Sorption Desorption

Langmuir

KF/(mg g−1)(L mg−1)1/n

n

r2

Qm/mg g−1

KL/L mg−1

Qm·KL/(mg g−1)(L mg−1)

r2

0.2611 0.2202

1.779 1.426

0.977 0.983

0.3427 –

2.2325 –

0.7651 –

0.967 –

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Fig. 4. Sorption (A) and desorption (B) isotherms of phosphorus onto sediment associated with different content of PC(lines: Modified Freundlich model fitting).

sediment increased obviously. For the Freundlich model, the larger the KF and the n values, the more effective the sediment samples for the sorption. When 7% PC was added, the KF value doubled and the Freundlich exponent n just decreased from 3.203 to 2.782. The results clearly indicated that the sorption of phosphorus on sediment was remarkably enhanced in the presence of PC. Furthermore, the sorbed phosphorus seemed more stable with the increase of PC content. As an active sorption component in the sediment, PC may have three effects on phosphorus sorption. One is its high specific area with more sorption site providing more affinity for phosphorus sorption, and the second is the acidic functional groups on its surface, which affect the acid-base properties of the sediment, thereby promoting the occurrence of chemisorption. The last may be that the metals in PC affect the combination of sediment and phosphorus. In order to verify the above ideas, we conducted the following experiments.

affecting phosphorus sorption on sediments. (Wei et al., 2014; Borgnino et al., 2009). At pH = 8, the main form of phosphate is HPO42− (pKa1 = 2.12，pKa2 = 7.20，pKa3 = 12.36), thus the physical sorption on account of electrostatic effects is more difficult to occur for the sediment with PC addition. However, the surface hydroxyl concentrations Hs and CEC increased obviously with the addition of PC in the sediment, which may play an important role in phosphorus sorption based on ligand exchange process and result in the enhanced chemi-sorption. The chemi-sorption is usually slow and irreversible. Therefore, with addition of PC, the promoted slow sorption showed an effect on reducing the HI values, which declined the enhancement of sorption reversibility caused by the high specific surface of PC, and resulted that the HI data did not change much eventually. The investigation of the change of phosphorus fractions in the sediment samples after sorption can also help us understand the above effects. Meanwhile, the distribution of phosphorus fractions is more reflective of the nutrient status of sediments than the total phosphorus. Therefore, we analyzed the distribution of phosphorus in sediments after sorption in the presence of PC. The total phosphorus (TP) fractions can be separated into exchangeable phosphate (Ex-P), Fe/Al-bound phosphate (Fe/Al-P), Calcium-bound phosphate (CaeP) and organic phosphorus (OP) (Ruttenberg, 1992; Huerta-Diaz et al., 2005; Song and Liu, 2015), and the sum of Ex-P, FeeP and OP is thought to be potentially bioavailable P (Song and Liu, 2015). In view of the fact that CaeP (ACa-P plus DAP) constituted the highest fraction in the sediments (which has also been found by Song and Liu (2015)) as about 0.367 mg g−1 and was far more than the other forms, Fig. 5 only shows the bioavailable phosphorus in the sediment before and after sorption in the seawater containing 1.0 mg L−1 phosphorus for 24 h. Throughout the sorption process, it was found that CaeP changed very little, which is another reason that we omitted it in Fig. 5. Compared with the sample before sorption, the Ex-P and FeeP increased obviously, and the presence of PC made this increase more significant. Ex-P was usually formed by physical function of phosphate with sediments based on electrostatic force and van der Waals force, thus high surface area is very favorable to the process. In our study, the high micro-porosity of PC in the sediment provided more surface active sites for phosphorus sorption, which increased the physical sorption

3.3. Properties of sediment samples after P sorption First, we conducted desorption experiments to investigate the difference between sorption and desorption isotherms. The isotherms of desorption are shown in Fig. 4B. The values of HI were used to show the sorption–desorption hysteresis which resulted from chemical binding sorption. As shown in Table 4, the value of HI was 0.89 for the sediment without PC addition, while as the amount of PC added increased from 2% to 7%, HI did not change much. Thus, the addition of PC strengthened both the physical adsorption and chemi-sorption to a similar degree, resulting in little effect on the hysteresis of phosphorus sorption onto sediments. Table 5 lists the CEC and surface acid-base property parameters of the sediment samples with different contents of PC. The pHPZNPC of the samples were at pH = 5.68–6.41, means that the sediment surface was negatively charged at pH = 8. From the results of Bohem titration before, we found there were abundant acidic functional groups on the surface of PC, while the basic functional groups seemed negligible. The present acidic functional groups, such as carboxylic group, could reduce the pHPZNPC, making the surface of the samples more negative at pH = 8, just as the results shown in Table 5. It has been widely accepted that the surface properties are one of the important factors

Table 4 Sorption and desorption isotherm parameters of phosphorus to sediment with different contents of PC fitted by the modified Freundlich model. Contents of PC

Sorption

Desorption

KF/ (mg g−1)(L mg−1)1/n

n

NAP/mg g

r

KF/ (mg g−1)(L mg−1)1/n

n

NAP/mg g−1

r2

HI

0%PC 2%PC 4%PC 7%PC

0.0534 0.0492 0.0753 0.0955

3.203 2.800 2.795 2.782

0.0280 0.0054 0.0090 0.0096

0.998 0.981 0.991 0.993

0.0727 0.0774 0.110 0.113

2.860 2.616 2.464 2.461

0.0338 0.0202 0.0156 0.0096

0.989 0.982 0.979 0.993

0.89 0.93 0.88 0.88

−1

2

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Table 5 The CEC and surface acid-base properties of different samples. Samples

lgKa1int

lgKa2int

Vya

pHPZNPC

Hs/(mmol L−1)

(qH-qOH)pH=8 /(mmol g−1)

CEC/(mmol g−1)

DH2–5 DH2–5 + 2%PC DH2–5 + 7%PC

3.93 3.03 2.74

−8.90 −8.79 −8.62

64 127 131

6.41 5.91 5.68

0.58 0.62 0.66

0.034 0.061 0.121

0.0720 0.0888 0.0960

a

Vy is the overall variance of the fitting process.

Hs and CEC increased obviously with the addition of PC in the sediment. The analysis of the distribution of phosphorus forms showed that CaeP (ACa-P plus DAP) constituted the highest fraction in the sediments and was far more than the other forms. Throughout the sorption process, CaeP and OP changed very little, while the Ex-P and FeeP increased obviously, and the presence of PC made this increase more significant. Acknowledgments We sincerely acknowledge the support of the National Natural Science Foundation of China (No.41676064, No. 41830534) and the National Key Research and Development Program of China (Grant No. 2016YFA0601301). We also thank AoShan Talents Program supported by Qingdao National Laboratory for Marine Science and Technology (No. 2015ASTP).

Fig. 5. Contents of EX-P, Fe-P and OP of different samplese.

amount of phosphorus obviously. In addition, the Fe and Al in PC also play an important role in the process. FeeP, another important inorganic fraction, can be formed by ligand exchange of phosphate with Fe/Al oxide and hydroxide or precipitation with Fe/Al compounds in PC. As shown in Fig. 5, the presence of PC increased the FeeP content of the sediment after sorption more compared to the sample without PC addition. Therefore, the metal residual in PC may affect the combination of sediment and phosphorus, and then affect phosphorus bioavailability and migration in marine environment. As for OP, Fig. 5 showed it appeared stable for the whole process, means that the organic groups in PC did not react with phosphorus to form OP as we had expected. But we still don't know whether long-term interaction or PC aging will lead to the combination of P with PC, as the results of some metals mentioned before in the introduction (Cheng et al., 2014; Lian and Xing, 2017). In summary, due to the porosity of PC and the presence of iron and aluminum, as well as the modification of the sediments surface properties, PC can strengthen the sorption of phosphorus on sediments with a significant increase in Ex-P and FeeP.

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