Preparation of Sodium Lignosulfonate/Chitosan Adsorbent and Application of Pb 2+ Treatment in Water

: Industrial wastewater has brought huge disasters to water resources and soil and has seriously affected the growth of animals and plants. There is an urgent need for a green and efficient adsorbent to solve the problem of water pollution. Sodium lignosulfonate and chitosan undergo free radical polymerization to form a lignin/chitosan adsorbent, which is used to treat Pb 2+ in water pollution. An orthogonal experiment was used to optimize the content of sodium lignosulfonate, chitosan, cross-linking agent and initiator to obtain the adsorbent with the best adsorption performance. The adsorbents were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), thermal analysis and zeta potentiometry. The influence of different conditions on the adsorption of heavy metal ions by lignosulfonate/chitosan adsorbent was explored, and a kinetic and isotherm model was established. The results showed that the adsorption capacity of Pb 2+ was 345 mg g − 1 when the adsorbent was 0.01 g, the concentration of heavy metal ions was 100 mg L − 1 and pH was 7. The adsorption process of lignosulfonate/chitosan is a kind of spontaneous adsorption mode, which is mainly composed of electrostatic adsorption and chemical adsorption. ions attached by monolayer chemical adsorption. The thermodynamic results show that the adsorption process is a spontaneous endothermic reaction. The changes of the surface functional groups measured by FT–IR, the surface charge measured by the Zeta potential and the isotherm and kinetics results demonstrated electrostatic attraction and metal chelation between Pb 2+ and LS/CS. In summary, LS/CS is a new and efﬁcient adsorbent for Pb 2+ removal from water.


Introduction
Water health is very important to human and environmental safety. Today's freshwater resources are extremely polluted, and water-pollution treatments have been investigated worldwide. One cause of water pollution is domestic and industrial sewage. Heavy metal ions are among the most serious pollutants [1][2][3][4], as they are rich in nature and do not easily degrade in the environment. Lead is a toxic metal that can accumulate in human and animal tissues. Its sources include paints, coatings, batteries, smelting, hardware, machinery, electroplating, cosmetics, hair dyes, glaze dishes, tableware, coal, puffed food and tap water pipes [5][6][7]. After entering the body through the skin, digestive tract or respiratory tract, lead interacts with various organs, causing anemia, neurological disorders and kidney damage [8,9].
Currently, lead is processed by oxidation-reduction [10], electrolysis [11], ion exchange [12] and adsorption [13][14][15]. The oxidation-reduction method efficiently precipitates heavy metal ions, but produces a large amount of waste residue that causes secondary pollution. The electrolysis method converts heavy metal ions into easy-to-handle valence states through the electrolytic cell principle. This method efficiently treats a large amount of sewage, but its application is limited by low cost. The ion-exchange method separates heavy metal ions through ion-exchange resins. Although the separation of heavy gold ions approaches 100%, the resin is expensive and not easily recycled. Among the abovelisted methods, adsorption is the expected main method for future processing of heavy metal ions. The adsorption method has several advantages-high efficiency, economy and greenness-which are not shared by traditional methods [16][17][18][19]. from aqueous solutions. Chitosan is biocompatible, lipid-lowering, antibacterial, non-toxic and possesses other properties that are desired in various industries. Chitosan is slightly soluble in water and soluble in weak acid. After dissolution, it becomes gelatinous and has a strong adsorption capacity by virtue of its polar groups (hydroxyl and amino groups), which have a water absorption capacity above 500%. Therefore, these groups are used as the components of various humectants. The -NH 2 ortho-position on chitosan is-OH, which can coordinate with divalent heavy metal ions. This group can be modified or loaded on other substances to form composite materials that adsorb heavy metal ions. Chitosan has broad application prospects in papermaking, textiles, agriculture, food processing and treatment of heavy metal ions [28][29][30][31].
In this study, a sodium lignosulfonate-chitosan (LS/CS) adsorbent for Pb 2+ removal was synthesized from sodium lignosulfonate and chitosan, and the synthesis conditions were optimized by an orthogonal design. Sodium lignosulfonate enables the fabrication of low-cost adsorbents, and both sodium lignosulfonate and chitosan are easily obtained and biocompatible. Therefore, LS/CS adsorbent is the expected future trend of adsorbent developments.

Material Preparation
Preparation of LS/CS Adsorbent LS/CS adsorbent was synthesized by neutralizing acrylic acid, followed by freeradical reaction with sodium lignosulfonate and chitosan. For this purpose, 2.744 g of KOH was dissolved in 5 mL of water and neutralized with 5 mL of acrylic acid added through a dropper. To the resulting solution, LS, CS, potassium persulfate and NMBA were added in succession, along with 10 mL of water with stirring to dissolve. The mixture was placed in a sonicator and reacted at 70 • C for 1 h. The obtained product was washed with absolute ethanol, dried and ground to obtain the LS/CS adsorbent.
To maximize the adsorption performance of the adsorbent, the dosages of LS, CS, K 2 S 2 O 8 and NMBA in the synthesis were optimized. A full factorial experimental design with various levels of each factor requires numerous groups of experiments that consume manpower and time and increase the error in the experimental process. An orthogonal design experiment is a multi-factor, multi-level design method performed on representative groups selected from the full-factor experiment. The selected groups should be evenly dispersed, neat, and comparable. Orthogonal experimental design effectively reduces the number of experiments and is an efficient, fast and economical approach for optimizing the synthesis scheme. To find the best adsorbent, the LS, CS, K 2 S 2 O 8 and NMBA contents were optimized by Orthogonal Experiment Assistant II (V3.1) software. Table 1 shows the data of the orthogonal design experiment.

Characterization of Adsorbents
The Fourier transform infrared (FT-IR) spectra of the samples were measured by a Nicolet nexus 470 spectrometer (Nicolet, USA) in the wavenumber range 400-4000 cm −1 . For these measurements, the sample mass was thoroughly ground with dry KBr (mass ratio 1:100). The sample microstructures were studied by emission scanning electron microscopy (FE-SEM) JSM-7800F (JEOL Co., Ltd., Mitaka, Japan). The dried samples were gold-coated and imaged at an acceleration voltage of 10.0 kV. The thermal gravimetric analysis (TGA) of the samples was evaluated by a 449 C integrated thermal analyzer (Netzsch, Germany). During the TGA measurements, the samples were heated from 20 to 800 • C at 58 • C min −1 in a nitrogen environment. The Zeta (ζ) potentials were measured by a Nano ZS90 potential analyzer (Malvern, UK) at room temperature (20 • C). The Pb +2 contents before and after adsorption were measured by the flame method using a TAS-986 atomic absorption spectrophotometer (Beijing General Instrument Co., Ltd., Beijing, China). The wavelength was 283.3 nm, the combustion gas was 0.09 MPa acetylene flowing at 1.5 L min −1 and the combustion height was 6 mm.

Adsorption of Pb 2+
All adsorption experiments were carried out in 50 mL salt solution (salt solution is formed by Pb(NO 3 ) 2 dissolving in water). During the experiments, the pH of the solution was adjusted with 0.1 mM HCl and 0.1 mM NaOH. The parameters of the kinetic experiment were set as follows: adsorption time = 5-240 min, initial concentration of heavy metal ions =100 mg L −1 , LC/CS adsorbent dosage = 0.01 g, pH = 7.0, temperature = 20 • C. The parameters in the adsorption isotherm model were set as follows: initial concentration of heavy metal ions = 100-900 mg L −1 , amount of LC/CS adsorbent = 0.01 g, pH = 7.0, temperature = 20 • C. The parameters of adsorption thermodynamics were as follows: initial concentration of heavy metal ion = 100 mg L −1 , amount of LC/CS adsorbent = 0.01 g, pH = 7.0, temperature = 20-60 • C. The pH effect on the adsorption was obtained by varying the pH from 1.0 to 7.0. This experiment was performed in a solution of heavy metal ions (100 mg L −1 ) with an LC/CS adsorbent dosage of 0.01 g at 20 • C. Finally, the effect of adsorbent dosage on the adsorption was investigated by varying the amount of LS/CS adsorbent from 0.01 to 0.07 g. The heavy-metal ion concentration, pH, and temperature were set to 100 mg L −1 , 7.0 and 20 • C, respectively. The residual concentrations of Pb 2+ were determined by a TAS-986 atomic absorption spectrophotometer. The adsorption capacity Q e at (equilibrium (mg g −1 )) was calculated as where C e and C o are the equilibrium and initial concentrations, respectively (both in mg L −1 ), V (L) is the volume of the system and M (g) is the mass of the adsorbent.

Results of the Orthogonal Experiment
The synthesis was optimized for the adsorption capacity of Pb 2+ by an orthogonal experimental design. The results of the orthogonal experiment are listed in Table 2.
In the orthogonal experimental results, changing the level of a factor with a high extremum exerts a greater effect on the experimental results than changing a factor with a low extremum. The factor with the maximum extremum is the most important factor. The influence of the factors on the adsorption decreased in the order K 2 S 2 O 8 > (C 6 H 11 NO 4 ) n > C 20 H 24 Na 2 O 10 S 2 > NMBA. The orthogonal experiment does not consider the interactions between factors, so summing the experimental results of each factor at each level deter-Sustainability 2021, 13, 2997 5 of 13 mines the best plan. As clarified in Supplementary Materials Figure S1, the best value is the value that maximizes the sum of the experimental results of each factor at each level. The best levels of the factors were thus determined as follows: C 20 H 24 Na 2 O 10 S 2 = 0.3 g, (C 6 H 11 NO 4 ) n = 0.3 g, K 2 S 2 O 8 = 0.07 g and NMBA = 0.0115 g. By synthesizing the factors at their best levels, we could maximize the adsorption performance of the absorbent. The adsorption capacity of LS/CS adsorbent for Pb 2+ was 345 mg g −1 , obtained by measurement and calculation (the detailed results of this experiment are listed in Supplementary Materials Table S1). k i -The sum of the experimental results at level i of each factor; R-The extremum of the data.

Characterization of the LS/CS Adsorbent
The LS/CS microstructure Figure 1A reveals a large number of small pore structures distributed over the surface. The pore depth differed from the SEM color depth, and the pore size was approximately 1.0 µm. After adsorption, the pores in the LS/CS microstructure disappeared and were replaced by a sheet structure Figure 1B. The energy-dispersive X ray spectroscopy (EDS) spectra (colored panels in Figure 1) confirmed a large amount of Pb on the surface of LS/CS after adsorption, suggesting that the sheet structure was formed by Pb adhesion. Figure 2 shows the FT-IR spectra of sodium lignosulfonate, the LS/CS adsorbent before adsorption, chitosan and the adsorbent after Pb 2+ adsorption. The absorption peaks at 3400, 1100, and 589 cm −1 in the spectrum of sodium lignosulfonate were attributed to O-H tensile vibrations, vibrations of fatty ether, and vibrations of the sulfonic acid group, respectively. In the chitosan spectrum, the O-H stretching vibration peak appeared at 3430 cm −1 , and the peaks at 2890 and 1590 cm −1 were assigned to vibrations of the amino group. Comparing the infrared absorption spectra of sodium lignosulfonate and chitosan, the peaks in the LS/CS spectrum were assigned as follows: an O-H vibration absorption peak at 3450 cm −1 , a fatty ether vibration peak at 1108 cm −1 , amino vibration peaks at 2893 cm −1 and 1585 cm −1 and a sulfonic group at 590 cm −1 . LS/CS exhibited the characteristic absorption peaks of both sodium lignosulfonate and chitosan, confirming that LS/CS was successfully synthesized from these two constituents. After the adsorption of metal ions, the infrared absorption peaks of O-H, N-H, C-O-C and C = C in the LS/CS adsorbent shifted to low energies, and their intensities were diminished. Combining the FT-IR results with the images in Figure 1, one observes that when Pb adsorbed to LS/CS, it affected the functional groups on the surface, leading to changes in their electron clouds and hence in their absorption peaks.  Figure 2 shows the FT-IR spectra of sodium lignosulfonate, the LS/CS adsorbent before adsorption, chitosan and the adsorbent after Pb 2+ adsorption. The absorption peaks at 3400, 1100, and 589 cm −1 in the spectrum of sodium lignosulfonate were attributed to O-H tensile vibrations, vibrations of fatty ether, and vibrations of the sulfonic acid group, respectively. In the chitosan spectrum, the O-H stretching vibration peak appeared at 3430 cm −1 , and the peaks at 2890 and 1590 cm -1 were assigned to vibrations of the amino group. Comparing the infrared absorption spectra of sodium lignosulfonate and chitosan, the peaks in the LS/CS spectrum were assigned as follows: an O-H vibration absorption peak at 3450 cm −1 , a fatty ether vibration peak at 1108 cm −1 , amino vibration peaks at 2893 cm −1 and 1585 cm −1 and a sulfonic group at 590 cm −1 . LS/CS exhibited the characteristic absorption peaks of both sodium lignosulfonate and chitosan, confirming that LS/CS was successfully synthesized from these two constituents. After the adsorption of metal ions, the infrared absorption peaks of O-H, N-H, C-O-C and C = C in the LS/CS adsorbent shifted to low energies, and their intensities were diminished. Combining the FT-IR results with the images in Figure 1, one observes that when Pb adsorbed to LS/CS, it affected the functional groups on the surface, leading to changes in their electron clouds and hence in their absorption peaks.   Figure 3 shows the thermogravimetry curves of chitosan, sodium lignosulfonate and the LS/CS adsorbents and their differentials. During the heating process, chitosan lost 5% of its weight by water evaporation at 100 • C. The weight loss was maximized at 210 • C, when the external small molecules were decomposed. High-temperature thermal decomposition continued until 500 • C, and the weight remained at 5%. No obvious boundary appeared between the dehydration stage and the low-temperature decomposition stage of the LS/CS adsorbent, because the first two stages of sodium lignosulfonate were staggered with those of chitosan; consequently, the two phases were merged into one large phase. The weight loss of the LS/CS adsorbents continued beyond 500 • C, indicating that the molecules in the adsorbent decomposed at higher temperatures after adding the sodium lignosulfonate. This result further proves that the LS/LC absorbent was a compound of sodium lignosulfonate and chitosan.  Figure 3 shows the thermogravimetry curves of chitosan, sodium lignosulfonate an the LS/CS adsorbents and their differentials. During the heating process, chitosan lost 5 of its weight by water evaporation at 100 °C. The weight loss was maximized at 210 °C when the external small molecules were decomposed. High-temperature thermal decom position continued until 500 °C, and the weight remained at 5%. No obvious boundar appeared between the dehydration stage and the low-temperature decomposition stag of the LS/CS adsorbent, because the first two stages of sodium lignosulfonate were sta gered with those of chitosan; consequently, the two phases were merged into one larg phase. The weight loss of the LS/CS adsorbents continued beyond 500 °C, indicating th the molecules in the adsorbent decomposed at higher temperatures after adding the s dium lignosulfonate. This result further proves that the LS/LC absorbent was a compoun of sodium lignosulfonate and chitosan.  Figure 4A shows the effect of adsorbent dosage on the adsorption performance of th LS/CS adsorbent. When the adsorbent dosage increased from 0.01 to 0.15 g, the adsorptio capacity for metal ions increased, but at dosages of 0.02 g and higher, the adsorption c pacity showed a steady decline. At low dosage (0.01-0.15 g), increasing the amount adsorbent increased the number of adsorption sites and hence the adsorption capacit However, at dosages exceeding 0.15 g, an increasing number of adsorption sites were lef and the average adsorption capacity of adsorbent decreased accordingly. Figure 4B shows the effect of pH on the adsorption capacity of the LS/CS adsorben The inset plots the Zeta potential of the adsorbent as a function of pH. The pH of the zer potential was 4.1. When the pH exceeded 4.0, the surface of the LS/CS adsorbent was ne atively charged and attracted heavy metal ions. Increasing the pH increased the negativ  Figure 4A shows the effect of adsorbent dosage on the adsorption performance of the LS/CS adsorbent. When the adsorbent dosage increased from 0.01 to 0.15 g, the adsorption capacity for metal ions increased, but at dosages of 0.02 g and higher, the adsorption capacity showed a steady decline. At low dosage (0.01-0.15 g), increasing the amount of adsorbent increased the number of adsorption sites and hence the adsorption capacity. However, at dosages exceeding 0.15 g, an increasing number of adsorption sites were left, and the average adsorption capacity of adsorbent decreased accordingly. Figure 4B shows the effect of pH on the adsorption capacity of the LS/CS adsorbent. The inset plots the Zeta potential of the adsorbent as a function of pH. The pH of the zero potential was 4.1. When the pH exceeded 4.0, the surface of the LS/CS adsorbent was negatively charged and attracted heavy metal ions. Increasing the pH increased the negative charge and hence the adsorption capacity. In contrast, the adsorbent surface was positively charged at pH below 4.0 and repelled heavy metal ions. The smaller the pH, the greater the rejection and the smaller the adsorption capacity.   Figure 5A,B plots the linear fittings of the data to the pseudo-first-order and pseudosecond-order models for Pb 2+ adsorption to LC/CS. It is seen from Figure 5A that the adsorption capacity increased over time until the adsorption equilibrium was reached. While  Figure 5A,B plots the linear fittings of the data to the pseudo-first-order and pseudosecond-order models for Pb 2+ adsorption to LC/CS. It is seen from Figure 5A that the adsorption capacity increased over time until the adsorption equilibrium was reached. While the adsorption capacity increased, the rate first increased and then decreased; this trend was related to the surface adsorption sites on LS/CS and their interaction with heavy metal ions. This relationship was explored by dynamic modeling.  Figure 5A,B plots the linear fittings of the data to the pseudo-first-order and pseudosecond-order models for Pb 2+ adsorption to LC/CS. It is seen from Figure 5A that the adsorption capacity increased over time until the adsorption equilibrium was reached. While the adsorption capacity increased, the rate first increased and then decreased; this trend was related to the surface adsorption sites on LS/CS and their interaction with heavy metal ions. This relationship was explored by dynamic modeling. The pseudo-first-order kinetics are given by

Adsorption Properties of the LS/CS Adsorbent
and the pseudo-second-order kinetics are described by The standardized standard deviation (∆Q e ) is computed as In these expressions, q t (mg g −1 ) is the adsorption capacity of the adsorbent at adsorption time t (min), and k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the rate constants of the pseudo-first-and pseudo-second-order kinetics, respectively.
As shown in Figure 5A,B, the adsorption process can be roughly divided into three stages. During the first stage, metal ions are carried by convection-diffusion from the solution to the outer surface of the LS/CS adsorbent. During the second stage, the attached metal ions enter the micropores of the LS/CS adsorbent and then diffuse to the inner surface of the adsorbent. During the last stage, the metal ions affix to the adsorption sites on the LS/CS surface. The adsorption data of the heavy metal ions on the LS/CS adsorbent were fitted by the above kinetic models, and the fitting parameters are recorded in Table 3. The experimental data were better described by the pseudo-second-order kinetics (correlation coefficient R 2 closer to 1.0) than by the pseudo-first-order kinetics, indicating that the pseudo-second-order kinetics described the actual process well. The actual adsorption process is a multi-process adsorption, consistent with the results of intraparticle diffusion [19][20][21][22]. Table 3. Linear fitting data of pseudo-first-order and pseudo-second-order models.  Figure 6 plots the adsorption capacity of the LS/LC adsorbent for Pb 2+ as a function of initial concentration. The adsorption capacity initially increased and then plateaued. The adsorption process exhibited equilibrium-like trends. As the initial concentration increased, the adsorption equilibrium moved in the direction of increasing adsorption capacity. As the number of adsorption sites was fixed, the adsorption capacity eventually saturated. To determine the adsorption model of the LS/CS adsorbent for heavy metal ions, we investigated two isotherm models.
The standardized standard deviation (△Qe) is calculated as The Langmuir isotherm is given by The Freundlich isotherm is expressed as Sustainability 2021, 13, 2997 10 of 13 The standardized standard deviation (∆Q e ) is calculated as Here, Q m, (mg g −1 ) is the maximum adsorption capacity according to the Langmuir theory, K L (L mg −1 ) is the Langmuir correlation coefficient, C e (mg L −1 ) is the concentration of Pb 2+ at adsorption equilibrium, R L indicates the affinity of the adsorbent for the adsorbed substance, C 0 (mg L −1 ) is the highest initial concentration of Pb 2+ and K F (L g −1 ) is the Freundlich correlation coefficient. Table 4 shows the fitting results of the Langmuir and Freundlich isotherms. The Langmuir isotherm yielded a larger R 2 and smaller ∆Q e than the Freundlich isotherm, revealing that the Langmuir process more closely matched the actual adsorption process. Langmuir theory assumes a uniform surface of the adsorbent, meaning that the adsorption heat and areal coverage are unrelated, and there is no interactivity between the adsorbates. Therefore, the adsorption process of the LS/CS adsorbent was inferred to be similar to monolayer adsorption [23][24][25]. Table 4. Isothermal parameters of the adjusted nonlinear Langmuir and Freundlich fittings.

Adsorption Thermodynamics
It can be seen from Figure 7 that the adsorption capacity of ions increases with the increase of adsorption temperature. In order to study thermodynamics, the data obtained by fitting with the following formula are shown in Figure 7. The corresponding thermodynamic parameters, Gibbs free energy ∆G 0 , entropy change ∆S 0 and enthalpy change ∆H 0 are shown in Table 5.
where C e (mg L −1 ) is the equilibrium ion concentration, Q e (mg g −1 ) is the equilibrium adsorption capacity, R is the universal gas constant, and T (k) is the thermodynamic temperature.

Adsorption Thermodynamics
It can be seen from Figure 7 that the adsorption capacity of ions increases with the increase of adsorption temperature. In order to study thermodynamics, the data obtained by fitting with the following formula are shown in Figure 7. The corresponding thermodynamic parameters, Gibbs free energy ΔG 0 , entropy change ΔS 0 and enthalpy change ΔH 0 are shown in Table 5.  It can be seen from Table 5 that ∆G 0 is negative and decreases with the increase in temperature, indicating that the adsorption process is a spontaneous process, and the increase in temperature is conducive to the adsorption, which is consistent with the trend of Q e -T. The positive value of ∆H 0 indicates that the adsorption process is accompanied by an endothermic process. The thermodynamic curve fitting shows that the adsorption process of the five ions adsorbed by LC/CS is accompanied by a confused spontaneous endothermic reaction.

Adsorption Mechanism
To reveal the adsorption mechanism of Pb 2+ on LS/CS, the adsorption behavior of Pb 2+ on LS/CS was analyzed by combining the above experimental results with the various characterization results (SEM observations, FT-IR spectra, and Zeta potentials). When the LS/CS was added to the solution, the Zeta potentials revealed a negative charge on the LS/CS surface that attracted the positively charged lead ions. Meanwhile, the SEM-EDS observations revealed a surface layer on the LS/CS, which was presumed as positively charged Pb. From the kinetics and isotherm analyses, the dominant adsorption mechanism of Pb 2+ to the LS/CS adsorbent was inferred as single-molecular-layer chemical adsorption. The absorption peaks of -NH 2 , -OH, C-O-C and C=C changed significantly after the Pb adsorption. The adsorption process likely involves the formation of a Pb-π structure between Pb 2+ and aromatic functional groups on LS/CS [8], and the formations of Pb-O and Pb-N structures with hydroxyl [6] and amino groups [7], respectively. Electrostatic interaction between Pb 2+ and the LS/CS surface also contributes to the adsorption process. The adsorption mechanism is shown in Figure 8.

Adsorption Mechanism
To reveal the adsorption mechanism of Pb 2+ on LS/CS, the adsorption behavior of Pb 2+ on LS/CS was analyzed by combining the above experimental results with the various characterization results (SEM observations, FT-IR spectra, and Zeta potentials). When the LS/CS was added to the solution, the Zeta potentials revealed a negative charge on the LS/CS surface that attracted the positively charged lead ions. Meanwhile, the SEM-EDS observations revealed a surface layer on the LS/CS, which was presumed as positively charged Pb. From the kinetics and isotherm analyses, the dominant adsorption mechanism of Pb 2+ to the LS/CS adsorbent was inferred as single-molecular-layer chemical adsorption. The absorption peaks of -NH2, -OH, C-O-C and C=C changed significantly after the Pb adsorption. The adsorption process likely involves the formation of a Pb-π structure between Pb 2+ and aromatic functional groups on LS/CS [8], and the formations of Pb-O and Pb-N structures with hydroxyl [6] and amino groups [7], respectively. Electrostatic interaction between Pb 2+ and the LS/CS surface also contributes to the adsorption process. The adsorption mechanism is shown in Figure 8.

Conclusions
The synthesis of sodium lignosulfonate/chitosan (LS/CS) was optimized by an orthogonal design, and the adsorption characteristics and mechanism of LS/CS for Pb were studied. The FT-IR, TGA, SEM, and Zeta potentials showed a porous, negatively charged structure of LS/CS with multi-functional groups. The adsorption performance of LS/CS was controlled by many factors and was especially sensitive to pH. The adsorption capacity of LS/CS for Pb (100 mg L −1 ) was 345 mg g −1 at a pH of 7.0, an adsorbent dosage of 0.01 g and a reaction time of 120 min. The adsorption process of Pb on LS/CS accorded with pseudo-second-order kinetics and the Langmuir isotherm model, revealing that the lead ions attached by monolayer chemical adsorption. The thermodynamic results show that the adsorption process is a spontaneous endothermic reaction. The changes of the surface functional groups measured by FT-IR, the surface charge measured by the Zeta potential and the isotherm and kinetics results demonstrated electrostatic attraction and metal che-2+ Figure 8. Adsorption mechanism of Pb 2+ on the LS/CS adsorbent.

Conclusions
The synthesis of sodium lignosulfonate/chitosan (LS/CS) was optimized by an orthogonal design, and the adsorption characteristics and mechanism of LS/CS for Pb were studied. The FT-IR, TGA, SEM, and Zeta potentials showed a porous, negatively charged structure of LS/CS with multi-functional groups. The adsorption performance of LS/CS was controlled by many factors and was especially sensitive to pH. The adsorption capacity of LS/CS for Pb (100 mg L −1 ) was 345 mg g −1 at a pH of 7.0, an adsorbent dosage of 0.01 g and a reaction time of 120 min. The adsorption process of Pb on LS/CS accorded with pseudo-second-order kinetics and the Langmuir isotherm model, revealing that the lead ions attached by monolayer chemical adsorption. The thermodynamic results show that the adsorption process is a spontaneous endothermic reaction. The changes of the surface functional groups measured by FT-IR, the surface charge measured by the Zeta potential and the isotherm and kinetics results demonstrated electrostatic attraction and metal chelation between Pb 2+ and LS/CS. In summary, LS/CS is a new and efficient adsorbent for Pb 2+ removal from water.