Treatment Of Wastewater Containing Heavy Metals With Modified Spent Grain

Wastewater containing heavy metals from extensive sources is the significant pollution source of heavy metals. It seriously endangers drinking water safety and public health. Currently, traditional lime neutralization method can not meet the increasingly stringent environmental requirements in China. It is urgent to develop a novel and efficient method for the advanced treatment of wastewater containing heavy metals. China is the world's largest producer of beer. Large amount of spent grain has been produced from the brewing industry, which lead to not only serious environmental pollution but also waste of valuable resources. In order to solve the problems of depth removal of heavy metal ions from wastewater, the brewing industry waste—spent grain was chosen as raw materials to prepare effective adsorbents. Moreover, the theoretical basis and heavy metal wastewater treatment were also studied. The main research contents and creative achievements are as follows:Mondern analysis—13C NMR (Nuclear Magnetic Resonance) was used to determine the composition and structure of spent grain. The main component of spent grain was determined as cellulose (23.6%), hemicellulose (18.9%), lignin (11.5%), and protein (22.3%). It can be found that spent grain is rich in hydroxyl groups, showing the nature of polyols. Thus, it possesses adsorption capacity of heavy metal ions and also susceptible to a series of chemical reactions like esterification, which provide favorable conditions for grafting functional groups to enhance its adsorption capacity.Based on the composition and structure of spent grain, DMol module of Materials Studio 4.0 program was used to construct stable structures of complexes formed between a variety of functional groups present and introduced in spent grain (Hydroxyl, amido, acetamido, carboxyl, and sulphydryl groups) and different heavy metal ions (Cu2+ Pb2+, Zn2+, Cd2+, and Ag+). Moreover, the energies of the frontier molecular orbital for the complexes were calculated to guide the modification of spent grain. The criterion for adsorption of heavy metals via different groups was proposed, which provide the important theoretical basis to prepare efficient heavy metal ions adsorbent by modification of spent grain.Raw spent grain has a certain adsorption capacity of heavy metal ions, but its adsorption capacity is low. Hence, modification of spent grain was systematically investigated. The salt immersion method was tried to modify spent grain. The results indicate that salt immersed spent grain (MSG) has a certain adsorption capability for Pb2+and Ag+. The theoretical maximum adsorption capacity of Pb2+and Ag+was 34.2 mg·g-1 and 158.23 mg·g-1, respectively. The adsorption mechanisms mainly included as follows:a little physical adsorption; ion exchange between Na+and Pb2+, Ag+; as well as complexing between oxygen, nitrogen and chlorine containing functional groups and Pb2+, Ag+ions.Based on the results of molecular design, a new method for fast esterification of spent grain was explored. Sodium hypophosphite monohydrate (NaH2PO2·H2O) and N, N-dimethylformamide (DMF) were chosen as catalyst and reaction medium, respectively; m(citric acid):m(spent grain):m(NaH2PO2·H2O):v(DMF)= 1:5:1:25; modification temperature of 140-150?; and reaction time of 2 h. Esterified spent grain (ESG) was successfully prepared under the above optimal conditions. Then using ESG as an adsorbent to remove various heavy metal ions was investigated. The results indicate that the adsorption capacities of ESG for Cu2+, Pb2+, Zn2+, Cd2+, Ag+are 129.03 mg·g-1,393.70 mg·g-1,269.54 mg·g-1,471.70 mg·g-1,239.23 mg·g-1, respectively, which are higher than that of the majority of other adsorbents. It can be concluded that ESG has great adsorption capability for various heavy metal ions. The adsorption mechanisms mainly involve physical electrostatic attractions as well as the complexing of hydroxyl groups present in spent grain and carboxyl groups introduced by esterification modification. The oxygen atom of C-O in carboxyl group was directly attached to the heavy metal ions that lead to most of the adsorption.A new method for efficient functionalization of spent grain with thiol groups was explored. Sodium bisulfate monohydrate(NaHSO4-H2O) and N, N-dimethylform-amide (DMF) were chosen as catalyst and reaction medium, respectively; m (thioglycollic acid):m (spent grain): m (NaHSO4·H2O):v (DMF):m(Na2S-9H2O)=25:5:0.125:12.5:30; modification temperature of 120?; and reaction time of 3 h; followed by treatment with sodium sulfide nonahydrate (Na2S·9H2O) in ethanol (95%) for 1 h to reduce the disulfides to free thiol groups. Thiol-functionalized spent grain (TSG) was successfully prepared under the above optimal conditions. Then using TSG as an adsorbent to remove various heavy metal ions was investigated. The results indicate that the adsorption capacities of TSG for Cu2+, Pb2+, Zn2+, Cd2+, Hg+are 188.32 mg-g-1, 300.30 mg-g-1,353.36 mg-g-1,420.17 mg-g-1,221.73 mg-g-1, respectively. The adsorption mechanisms mainly involves physical electrostatic attractions as well as the complexing of hydroxyl groups present in spent grain and thiol groups introduced by functionalization. In particular, the valence state of Cu changed from+2 to 0 during the adsorption of Cu2+ onto TSG; at the same time, a ring compound formed, the ring compound has higher stabilization energy.The kinetic and thermodynamic results are similar when adsorbing various heavy metal ions using different modified adsorbents such as MSG, ESG, and TSG. The kinetics of heavy metal ions adsorption followed pseudo-second-order model; the activation energy (Ea) of adsorption process was in the range of 8.4-83.7 kJ·mol-1, showing that heavy metal ions adsorption onto the above three adsorbents were activated chemical adsorption; the positive values of enthalpy change and the negative values of Gibbs free energies demonstrated that adsorption processes were all endothermic and spontaneous in nature.Compared with the ESG, TSG has higher theoretical maximum adsorption capacity of Cu2+, Zn2+. This can be explained that the stabilization energies of the complexes formed between Cu, Zn and thioglycollic acid are higher than that of the complexes formed between Cu, Zn and citric acid. The competitive adsorption capacity of ESG and TSG are slightly different as Pb> Cu> Cd> Zn> Ag and Cu> Pb> Cd> Zn, respectively. These competitive adsorption results of each adsorbent are generally in agreement with the stabilization energies of its complexes.ESG and TSG developed in this paper have great potential for the advanced treatment of wastewater containing heavy metals.