Zhiying Guo ,Liping Ma, ,Quxiu Dai ,Xinbo Yang ,Ran Ao ,Jie Yang ,Jing Yang,Wengang Li

1 Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

2 Yunnan Tianlang Energy Saving &Environmental Protection Group Co.Ltd,Kunming 650300,China

3 College of Resources and Environment,Chengdu University of Information Technology,Chengdu 610225,China

4 School of Environmental and Municipal Engineering,Xi’an University of Architecture and Technology,Xi’an 710000,China

Keywords:Municipal sludge Corn-core powder Pollution Polymers Waste treatment

ABSTRACT The high water content of sludge has always posed significant challenges for its treatment.Synthetic flocculants,which are widely used in sewage treatment plants,can cause secondary pollution during their production and use.Thus,natural polymer flocculants made of natural materials have received increased research attention recently.Corn-core,an agricultural waste,was modified through alkalization with sodium hydroxide (NaOH) and etherification with cetyltrimethylammonium bromide (CTMAB) in this study.The L16(45)orthogonal array was used to study the modification conditions.The moisture content(MC)of treated sludge was reduced by approximately 37%with the addition of modified corn-core powder (MCCP),which was synthesized under optimal conditions.In addition,changes in the functional groups of the material before and after modification were determined by FT-IR.Thermogravimetric analysis indicated that MCCP was steady at room temperature,moreover,BET analysis showed that MCCP had greater surface area.The microstructures of material before and after modification were examined by scanning electron microscopy,revealing that MCCP had a flake-shaped structure and had an increased area of contact area with sludge particles.MCCP is a dehydrating agent that can enhance sludge dewaterability through charge neutralization and re-agglomeration and provide superior economic benefits.?2021 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

1.Introduction

Problems relating to municipal sludge treatment and disposal have become increasingly common in recent years,and treatment costs account for up to 40–50% of the total cost of sewage treatment plants [1–3].Furthermore,making sludge innocuous via a single treatment route is difficult as the by-products of sludge treatment are complex and diverse [4,5].Therefore,sludge treatment and disposal are key challenges for sewage water treatment.Because sludge is a heterogeneous colloidal system with extremely high water content and consists of bacterial cells,refractory organics,and some colloids,separating sludge flocs from the water phase is difficult [6,7].For this reason,reducing sludge moisture content(MC) has become one of the most challenging technical tasks in sewage treatment[4,8].The addition of flocculants and coagulants is an effective method for reducing MC through charge neutralization,sweeping,and bridging,which agglomerates the sludge particles into large flocs,disrupt the steady sludge network,enhance the flow of the bound water,and facilitate dehydration treatment[9,10].

Cationic polyacrylamide (CPAM) has become a widely used sludge conditioner because it can be used at low doses and does not strongly alter the pH [11].However,synthetic flocculant,which is a refractory degradation product and has residual monomer toxicity,is potentially harmful.Naturally modified polymer flocculants are worth studying because they are non-toxic and easily degradable unlike traditional sludge conditioners [10,12–16].Therefore,recently,several natural modified polymer flocculants,such as hydrophobic cationic chitosan[12]and rice husk biochar modified by FeCl3[17],have been studied and used to improve sludge dewaterability before mechanical dehydration.

Corn is a common crop worldwide [18].Thereinto,corn-core,which accounts for approximately 20%–30% (in volume percent)of total corn,is easily degradable because of the presence of zein,a protein colloid [19].Here,we used alkalization-etherification to modify corn-core powder (CCP).The alkalization process can change the physical morphology of the fibers in the corn-core and enlarge the amorphous region,which is facilitates the penetration of the etherifying agent and enhance the thoroughness of the etherification reaction [20,21].In the etherification process,the reaction rate is faster than the diffusion rate of reagents,and most of the etherification reactions are surface reactions,thus resulting in poor reaction uniformity [22].To increase the diffusion rate of the etherifying agent,the raw material should be alkalized before etherification.The etherification process can then load cationic quaternary ammonium groups on the surface of CCP [23,24],finally permitting the modified corn-core powder(MCCP)to confer positive charges that neutralize the negative charges in sludge.Therefore,MCCP can serve as a dehydrating agent for sludge while retaining the advantages of CCP,including its high heating value and high biodegradability.

As an environmentally friendly sludge conditioner,modified natural polymer flocculants can also enhance sludge dewaterability[6,10,25].The aims of this study were the following:(1)to synthesize MCCP by an alkalization-etherification process,and optimize the synthesis conditions;(2)to characterize the physicochemical properties of the products using various analytical methods;(3) to evaluate the dewatering performance of sludge before and after treatment;and(4)to conduct an environmental and economic benefits analysis to evaluate the feasibility of MCCP pretreatment.

2.Materials and Methods

2.1.Material and instruments

Waste CCP with a size of 100 mesh(i.e.,150 μm)was used as the raw material for the sludge dehydrating agent.Meanwhile,the following reagents were used:sodium hydroxide (NaOH) (analytical reagent,Chengdu Kelong Chemical Reagent,Chengdu,China);cetyltrimethylammonium bromide (CTMAB) (analytical reagent,Chengdu Kelong Chemical Reagent,Chengdu,China).

The sludge samples used in this study were collected from the thickening tank of a municipal wastewater treatment plant in Kunming,China.Once the sludge samples were transported to the lab-oratory,these collected samples were stored at 4 °C and used within three days to minimize the activity of microbes and experimental error [26].Moreover,the basic characteristics of the raw sludge are shown in Table 1.

Table 1 Characteristics of raw sludge samples collected from a municipal wastewater treatment plant in Kunming,China

2.2.Modification of CCP

To remove the excess moisture,CCP was dried at 105°C for 2 h before use.NaOH and CTMAB were used as alkalinizing and etherifying agents,respectively,and the proposed reaction routes are shown in Fig.1.The optimum synthetic conditions of MCCP were confirmed by the L16 orthogonal array,which had five factors and four different levels,and MC and SRF were set as the target indexes.The details of the orthogonal experimental design are provided in Table 2.MCCP synthesized under different conditions was added to the raw sludge and stirred at 200 rpm for approximately 5 min.The treated sludge was then left to stand for approximately 1–2 h until solid–liquid stratification was achieved.Finally,the treated sludge was filtrated at the same pressure to determine sludge dewatering performance.All experimental data consisted of averages from repeated experiments and were conducted at room temperature.

Table 2 Factors for the orthogonality experiment

Fig.1.The reaction routes of (a) alkalization and (b) etherification.

2.3.Characterization of MCCP

2.3.1.Fourier Transform-Infrared (FT-IR) spectroscopy

To confirm differences in the functional groups of MCCP and CCP,both MCCP and CCP were characterized using an FTIR spectrometer(ALPHA,BRUKER,Germany).Thereinto,the spectrometer was operated with a wavenumber range of 400–4000 cm-1and a scan speed of 40 sheets﹒s-1.

2.3.2.X-ray photoelectron spectroscopy (XPS)

To analyze the elemental composition and the local functionality of CCP and MCCP,X-ray photoelectron spectroscopy (XPS) patterns were obtained using an EscaLab250Xi instrument (Thermo Fisher Scientific,USA) equipped with an Al Kα X-ray source(1486.6 eV).The XPS spectra peaks were fitted using CasaXPS Version 2.3.16 software (Casa Software Ltd,UK).

2.3.3.Thermogravimetric-differential thermal analysis

Thermogravimetric analysis (TGA) was used to determine the stability of MCCP using a Thermogravimetric-Differential Thermal Analyzer(HCT-3,HENVEN,Beijing).The addition of a certain mass of the sample into equipment resulted in the appearance of thermogravimetry(TG)curves to conduct differential thermal analysis(DTA),which was related to the temperature and time (heating rate:10°C﹒min-1;temperature range:25–900°C;sweeping speed of nitrogen (N2) flow:40 ml﹒min-1).

2.3.4.Scanning electron microscope (SEM) measurement

SEM(Quanta Q400,FEI,USA)was used to study the morphological structure.This test was conducted at room temperature,and 2000× and 5000× magnifications of the SEM were used.

2.3.5.BET analyses

Average pore diameter and the surface area of CCP and MCCP were determined using the Brunauer–Emmett–Teller (BET)method with an Automatic Specific Surface Area and Pore Size Distribution Analyzer (Autosorb1-iQ-MP,Quantachrome,America).

2.3.6.Fractal dimension

Fractal dimension (DF) characterizes the degree of irregularity and the spatial filling of materials.The two-dimensional DFis defined by a powder law relation between the area (A) and the perimeter (P) of materials [27].It can be calculated by Eq.(1):

where C is a constant.The area(A)and perimeter(P)of the material were obtained using Image Pro Plus 6.0 (IPP 6.0) through pixel calculation.

2.4.Sludge dewatering experiments

Several indices can be used to determine the sludge dewatering performance during sludge conditioning,including MC,the specific resistance of filtration(SRF),and bound water content[28].MC is a basic indicator used in sludge dewatering experiments.Lee [29]indicated that the remaining water of the sludge after vacuum filtration can be considered bound water.SRF measures the hydraulic resistance of water from cake solids when it moves in the porous filter cake,and SRF is unrelated to the dewatering method used[30–32].Here,MC and SRF were used as indics to estimate sludge dewatering performance after adding MCCP into the raw sludge.

MC in the dried sludge and bound water content could be calculated by Eqs.(2) and (3),respectively:

where m1is the mass of the evaporating dish from the dryer,m2is the mass of the evaporating dish and the sludge after air pump filtration,and m3is the mass of the evaporating dish and the sludge after drying in a dry box with temperatures between 105 and 110 °C.

SRF was determined using Eq.(4) [33]:

where P is the filtration pressure,μ is the liquid viscosity,A is the filtering area,b is the slope of the filtrate flow rate curve,and ω is the mass of filter cake that corresponds to a liter of filtrate.

3.Results and Discussion

3.1.Preparation conditions of MCCP

These five factors listed in Table 2 were the main factors considered for the modification of CCP.Range (R) expressed the significance of parameters,and a larger R indicates higher significance[34].MC was used as the indicator,and RA>RE>RC>RB>RDindicated that the concentration of NaOH played the most significant role in the orthogonal experiment (Table 3).The alkalizing time,etherifying temperature,and etherifying time played comparatively insignificant roles.The value of Kiwas the average MC of treated sludge;smaller Kicorresponded to improved dewaterability.For instance,K1A(=64.464)

Table 4 Variance analysis of treated sludge

Fig.2.Effect curves of the specific resistance of filtration(SRF)with various factors.

Table 3 Average (Ki) at each level of each factor and the range of the moisture content (MC) of treated sludge

Fig.3.Effects of (a) NaOH concentration and (b) alkalizing time on moisture content (MC) and specific resistance of filtration (SRF).

Fig.4.FT-IR spectra of(a)the modified corn-core powder(MCCP)and(b)corn-core powder (CCP).

Fig.5.XPS-wide scans of the corn-core powder (CCP) and the modified corn-core powder (MCCP).

To determine the optimal NaOH concentration and alkalization time,single-factor experiments were conducted,revealing that 8%and 0.5 h,respectively,were the optimal conditions for synthesizing MCCP(Fig.3).In the lye,hydrated ions were formed by pairs of separated sodium ions and hydroxide ions[35].When NaOH is low in concentration,the size of hydrated ions is too large to enter the fiber crystal region and exert their effects [36].As the NaOH concentration gradually increased,the water molecules,which are available to form hydrated ions,decreased,thus reducing the size of the hydrated ions and allowing these ions to penetrate the fiber crystal region.The concentration of NaOH then further increased,and the size of the hydrated ions became smaller.These observations suggested that the amount of sodium ions increased and that these sodium ions had a competitive effect on the bound water.Hydrated ions that were smaller in size can reduce the destructionefficiency of the crystal region,thereby reducing the degree of fiber swelling.Thus,excessive alkalization can result in negative effects.Further experiments that synthesize MCCP under the aforementioned conditions are needed.

The distinctiveness of these five factors was verified by consulting the F-distribution laws,Fa(f1,f2),where f1and f2were the freedoms of the numerator and denominator in the ratio of variance,respectively [37].The consulting value of the F-distribution used here included F0.01(3,3)=29.46,F0.05(3,3)=9.28,and F0.1(3,3)=5.39 [38].The most important factors affecting the function of MCCP for sludge dewatering were the concentrations of NaOH and CTMAB,while the alkalizing time and the etherifying time had comparatively less important effects (Table 4).

Table 5 DF of corn-core powder (CCP) before and after modification

3.2.Characterization of MCCP

3.2.1.IR spectrum

Fig.6.XPS spectra of(a)C(1 s)and(c)N(1 s)of the corn-core powder(CCP);and XPS spectra of(b)C(1 s),(d)N(1 s),(e)Na(1 s),and(f)Br(3d)of the modified corn-core powder (MCCP).

Fig.7.Thermogravimetric curves of the modified corn-core powder (MCCP).

The FT-IR spectra of the CCP and MCCP from 400 to 4000 cm-1are shown in Fig.4.The characteristic peaks at 3425.8 cm-1(MCCP) and 3414.3 cm-1(CCP) corresponded to the hydroxyl groups.The absorption peaks at 2920,1730,1512,1263,and 1042 cm-1corresponded to the stretching vibration of —C—H,—C=O,and rings containing O,—C—O,and —C—O—C,respectively(Fig.4(b)),which were the main ingredients of cellulose and starch[39,40].After being modified,the absorption peaks at 1512 and 1263 cm-1disappeared,indicating that the ring containing O had opened [41].The peaks of N—CH3(2927.2 cm-1),—CH2—N—CH2—(2854.8 cm-1),and C—Br (901 cm-1) in MCCP indicated that the quaternary ammonium groups were loaded on the surface of CCP[42]and caused massive positive charges to be loaded on the structure of MCCP,thus promoting charge neutralization in the sludge network.CTMAB was a fungicide that could crack cells because of quaternary ammonium groups.Therefore,MCCP could rupture the cells in the sludge network and neutralize the negative charges simultaneously,thus disordering the steady sludge network and further releasing the bound water contained in the inner portion of sludge and enhancing sludge dewaterability.The absorption peaks at 1630 cm-1(MCCP) and 1637 cm-1(CCP) corresponded to the stretching vibrations of carbonyl groups,and the absorption peaks at 1387 cm-1of CCP and MCCP were the alkyl groups [40].These findings verified that MCCP had a carbon chain structure analogous to that of CCP and indicated that MCCP had high calorific values similar to CCP,thus demonstrating that its application in the sludge dewatering treatment aided the incineration and pyrolysis of sludge disposal.

Fig.8.SEM of corn-core powder (CCP) (a) magnified 2000 times;(b) magnified 5000 times,CCP after alkalization (c) magnified 2000 times;(d) magnified 5000 times,and modified corn-core powder (MCCP) (e) magnified 2000 times;(f) magnified 5000 times.

3.2.2.XPS analyses

To analyze the surface conditions of CCP and MCCP,XPS spectra of the samples were studied over the energy range of 0–1400 eV,and the main elements of the samples C,N,and O were probed(Fig.5).Compared with the full spectrum of the CCP,the MCCP sample showed a clear Na (1s) peak and Br (3d) peak,indicating that the elements Na and Br were introduced through the alkalization-etherification process.There were several chemical bonds with carbon in the materials (Fig.6(a) and (b)).Thereinto,the C—C bond was predominant;the C—N bond and the C=O bond were also detected in the MCCP[43].After modification of the CCP,the proportion of C—N bonds decreased,and the additional C—N+species confirmed that the CCP had been quaternized (Fig.6(c)and (d)).Meanwhile,the Na (1s) spectra of MCCP presented two peaks near 1070.8 and 1071.7 eV (Fig.6(g)),which might stem from NaN3and NaBr,respectively [44,45].The above results were consistent with the infrared analysis (Fig.4),indicating that the proposed reaction routes (Fig.1) were accurate and that the modification process met expectations.

3.2.3.TG-DTA analyses

Fig.9.Fluorescence density of corn-core powder (CCP) and modified corn-core powder (MCCP).

The thermal analysis of MCCP by TGA is shown in Fig.7.There were four main stages of thermal decomposition and mass loss of MCCP.The first stage occurred in the temperature range of 25–160 °C and resulted in a mass loss of 2.86%;furthermore,it involved the evaporation of intermolecular moisture,which was absorbed by the strong hydrophilic groups of the MCCP.These findings indicated that MCCP was relatively stable at room temperature.In the second stage,the temperature range was 160–340°C,and the mass loss was 70.23%,which likely stemmed from the thermal decomposition of methyl in the quaternary ammonium groups [46].Saka et al.[47] suggested that the decomposition of cellulose and lignin occurred at 400–600 °C,which happened in the third stage (340–620 °C).In the third stage,the mass loss was 20.71%,which resulted in the fracturing and decomposition of the main chain.Finally,the TG curve showed almost no changes in the fourth stage when the temperature was beyond 620°C,indicating that the MCCP had completely decomposed and that carbonation began.

3.2.4.SEM measurement

SEM analysis was used to further study the surface microstructure of samples before and after modification by the alkalizationetherification process (Fig.8).CCP had an ordered filamentous arrangement (Fig.8(a) and (b)),as cellulose and lignin account for 60% of the total ingredients [48–50].Many channels appeared in the CCP after alkalization(Fig.8(c)and(d)).The alkalization process could enhance the reactivity of materials,intenerate lignin,swell cellulose,and loosen the binding between the components,thereby facilitating the subsequent etherification[51].After etherification,MCCP had a flake-shaped structure with many fine particles (Fig.8(e) and (f)).Compared with CCP,the material after alkalization and etherification had many channels for the flow of water;consequently,water could penetrate the inner structure,thereby increasing the solubility of MCCP [6,52].However,corncore,which is rich in cellulose and lignin [53],is the raw material of MCCP.Cellulose and lignin are insoluble in water,permitting MCCP to reach a slightly soluble state.This multi-layered threedimensional network structure of MCCP could confer absorption and bridging properties [4,54] as well as reinforce the interaction between MCCP and sludge flocs,thus disordering the sludge colloid network and releasing tiny sludge particles.Under electrical neutralization,these sludge particles could be flocculated into larger flocs.Guo et al.[55]demonstrated that sludge flocs with larger particle sizes could enhance sludge dewaterability.

Fig.10.Aperture distribution of modified corn-core powder (MCCP).

To study the uniformity and stability of CCP before and after modification,surface multi-component analyses of samples were conducted.Geddes et al.[56] indicated that a lower fluorescence intensity resulted in a denser structure.Compared with MCCP,the surface structure of CCP was denser (Fig.9).The fluorescence intensity of CCP was lower than 200,indicating that its pore structure was extremely dense.After alkalization-etherification,MCCP assumed a porous structure with better solubility,favoring the reaction between MCCP and sludge flocs and thereby enhancing dewaterability.The results of these analyses were consistent with the results of the SEM analyses.

3.2.5.BET analysis

The average pore diameter and surface area of CCP and MCCP were determined using the BET method with a surface area and pore size analyzer [47].Compared with CCP,the average pore diameter and surface area of MCCP increased,which was consistent with the results of the SEM analysis.The average pore diameters of CCP and MCCP were 1.749 nm and 8.499 nm,respectively,and the BET surface areas were 1.072 m2﹒g-1and 2.101 m2﹒g-1,respectively.Nearly all of the pore diameters of MCCP were distributed between 2 nm and 50 nm (Fig.10),indicating that MCCP was a mesoporous material.Therefore,this poriferous structure could provide more sites for counteracting partly negative charges.

3.2.6.Fractal dimension

In addition to material particle size,the shape also affects the behavior of aggregated particles,especially with respect to collision efficiency and settling rates.Recently,irregular shapes have been described in terms of fractal geometry concepts;consequently,a complete characterization of the material particles should include a description of the DFof the aggregates.Generally,densely packed aggregates have a high DF,while lower DFresults from loosely bound structures [27].After the process of alkalization-etherification,the DFof samples was reduced from 0.0698 to 0.0498 (Table 5).This reduction in DFindicated that the surface structure of MCCP became loose and thus provided wider space for the reaction between MCCP and sludge flocs to occur,thus favoring the release of bound water in the sludge.

Table 6 Benefit-cost analysis of the pretreatment process

Fig.11.Filtration rate of different types of sludge.

Fig.12.Moisture content(MC)and specific resistance of filtration(SRF)of different types of sludge.

3.3.Dewatering performance

3.3.1.Filtration performance of treated sludge

To study the function of MCCP,the filtration rate curve of three kinds of sludge (raw sludge,sludge treated by CCP (CCP-sludge),and sludge treated by MCCP(MCCP-sludge))were examined.Compared with raw sludge,CCP-sludge and MCCP-sludge had faster filtration rates,and MCCP-sludge had more filtrate.Compared with raw sludge,the final filtrate volume of CCP-sludge was lower(Fig.11).The addition of CCP could form a skeletal support that enhances the structure and the flow of internal water;furthermore,CCP could expand because of its water absorption properties,block the channels of internal water,and finally wrap the unflowed water in the sludge system.Contrarily,MCCP was an organic flocculant loaded with positive charges,which could diminish the repulsion force between colloidal particles in sludge as well as decrease the affinity between sludge colloidal flocs and bound water [41],thus accelerating the filtration rate to obtain more filtrate.

3.3.2.Effects of conditioning conditions on sludge dewatering

The main indicators MC and SRF of the three types of sludge samples were measured under the same pressure (at -0.3 MPa).MCCP-sludge resulted in lower MC and SRF (Fig.12).Lower MC and SRF corresponds to improved dewaterability [14].Thus,MCCP-sludge had better dehydration performance compared with raw sludge and CCP-sludge.

Fig.13.Effects of modified corn-core powder (MCCP) on moisture content (MC),specific resistance of filtration (SRF),and the reduction in the bound water of sludge.

To optimize the dosage of MCCP,MC,and SRF,the reduction of bound water was measured under the same experimental conditions.Bound water consists of water molecules that do not solidify at the temperatures below the freezing point of pure water.Because of intermolecular force,bound water is difficult to remove by centrifugation and other mechanical methods [57].Thus,the content of bound water can be used to determine the difficulty of mechanical dewatering.Generally,higher amounts of bound water correspond to the increased difficulty of mechanical dehydration[38].The reduction in bound water was negatively related to the MC of sludge with the addition of MCCP (Fig.13).MCCP could destroy the steady sludge colloid network,leading to the release of bound water.At a dosage of 20% DS,the reduction in bound water was the largest,and the MC was the lowest.This finding indicated that MCCP was positively charged after modification,and these positive charges could neutralize negative charges contained in the sludge system and break the structure of the sludge colloid network,thus releasing tiny sludge particles,enhancing the flow of bound water,and improving dewatering performance.Furthermore,the excessive addition of MCCP resulted in an excess of positive charges in the sludge network,thus hindering the release of bound water and decreasing sludge dewaterability.

3.4.Environmental and economic benefits analysis

The benefit-cost ratio(BCR),which can be calculated by Eq.(5),is a practical means for evaluating the feasibility of the sludge pretreatment method [58].When the value of BCR is greater than 1,the environmental benefits obtained from the treatment of sludge are greater than the economic costs expended[59].Higher BCR values correspond to higher feasibility.A higher BCR value was obtained with MCCP addition,indicating that MCCP pretreatment could enhance sludge dewaterability and provide superior economic benefits (Table 6).

4.Conclusions

This study was focused on synthesizing MCCP and evaluating its physicochemical properties using FT-IR,XPS,SEM,and BET.The orthogonal experiment showed that NaOH concentration had the largest effect on the MC.Furthermore,the FT-IR and XPS analyses indicated that the quaternary ammonium groups had loaded on the surface of MCCP.MCCP could then rupture the cells in the sludge network and neutralize the negative charges simultaneously because such samples possessed positive charges and high shear stress.TG-DTA showed that MCCP was steady at room temperature,and SEM analysis revealed that the structure of MCCP was in the shape of a flake,indicating that MCCP had a greater surface area and more channels for the flow of water,thereby increasing solubility.Meanwhile,the addition of MCCP could promote the charge neutralization of the sludge network,gather tiny sludge particles into large ones through re-agglomeration,and reduce MC and SRF by 40%and 55%,respectively.In sum,MCCP is an environmentally friendly dehydrating agent that could enhance sludge dewaterability and provide substantial economic benefits.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(No.21666016);the National Key R&D Program of China(2018YFC1900200);and the Key Laboratory and the Analysis and Testing Foundation of Kunming University of Science and Technology (2017M20162207011,2019P20183107007).