Preparation Of Sludge-derived Humic Acid-based Superabsorbent Hydrogel And Its Application In Soil As Water-retaining Agent

In recent years,the production of urban sewage sludge has been increasing with the development of citise,which will cause harm to the environment if not treated in time,therefore,the sludge needs to be reduced,harmless and resourceful.The composition of sewage sludge is complex and it is rich in organic matter,of which humic acid is one of the important constituents.The application of superabsorbent hydrogels as soil water-retaining agents is of great significance for the development of water-saving agriculture.And the hydrogel modified with humic acid not only improves the water absorption performance of the hydrogels,but also has the potential to promote crop growth.In this study,humic acid was extracted from sludge and used to prepare superabsorbent hydrogel,which was added to the soil to improve the water retention capacity and planting efficiency,making full use of the beneficail resources in sludge while reducing the amount of sludge.The main conclusions obtained are as follows:（1）Humic acid was extracted from sludge using acid pretreatment and alkali solubilization acid precipitation.The results showed that the sludge humic acid content was10.28%and the extraction rate was 14.94%.The main functional groups were carboxyl groups,alcohol groups,phenolic hydroxyl groups,amide bonds,etc.,as well as the bonds that might originate from fatty acids in the sludge.The elemental composition was C>H>N with high H/C atomic ratio,which indicates that the structure contains aliphatic chain structure with relatively few aromatic groups and low C/N atomic ratio.The E₄/E₆ ratio is between 3and 4,the relative molecular mass is larger and the condensation of aromatic components is higher.（2）The results showed that the best water absorption performance of the hydrogel was achieved with the dosage of initiator relative to the amount of acrylic acid（same later）of1.20%,the dosage of crosslinker of 0.30%and the neutralization degree of acrylic acid of60%.Under this ratio,the best water absorption performance of the modified highly absorbent hydrogel（HA1.0）was investigated at 1.0%of sludge-derived humic acid,and the water absorption rate was 663.61 g/g,which was 39.1%higher than that of the pure hydrogel（HA0.0）.Fourier transform infrared spectroscopy（FTIR）analysis showed that the sludge-derived humic acid was successfully introduced into the hydrogel,and scanning electron microscopy（SEM）showed that the introduction of humic acid changed the structure of the hydrogel,with smaller pore size and more pores,and increased cross-link density.Thermogravimetric analysis（TGA）showed that the introduction of humic acid decreased the stability of the hydrogel,and X-ray diffraction（XRD）showed that the hydrogel showed an amorphous state,and there was no obvious crystal diffraction surface after the introduction of humic acid.（3）The water uptake kinetics illustrates that the water uptake of all hydrogels conforms to the pseudo-second-order kinetic model,and the water uptake diffusion characteristics（the first 60%of water uptake）of pure hydrogels are SupercaseⅡdiffusion,while the hydrogels containing humic acid are non-Fickian diffusion.In addition,the hydrogels had the best water absorption performance in neutral p H environment,while the water absorption of hydrogels decreased with increasing ion concentration and valence charge number in different ion species and ion concentrations,and the hydrogels containing humic acid were more salt tolerant compared with pure hydrogels.The dehydration kinetics of hydrogels,on the other hand,showed that the dehydration of hydrogels was consistent with the Page drying kinetic model,i.e.,dehydration is an exponential power function model.Repeated water uptake-dehydration experiments indicated that the hydrogels have the ability to be reused;the humic acid hydrogels have the ability to slowly release humic acid,thus increasing the organic matter content of the soil.（4）Hydrogels applied as soil water retention agents.1)seedling germination experiments showed that hydrogel reduced seed germination index;2)the addition of sludge-derived humic acid hydrogel to soil increased lettuce plant height,leaf number and leaf area,indicating that hydrogel could improve lettuce growth;3)HA1.0 elevated chlorophyll in lettuce leaves slightly less than HA0.0;4)lettuce in the HA1.0 group had higher soluble protein content than the HA0.0 group,and both were higher than the soil-only group.The soluble protein content in the leaves of HA1.0 group was higher than that of HA0.0 group,and both of them were higher than that of the pure soil group.This is due to the fact that the sludge-derived humic acid hydrogel not only improves the water holding capacity and water retention properties of the soil,but also releases humic acids that are beneficial for improving crop growth;（5）HA1.0 has higher degradation performance than HA0.0.The content of heavy metals in sludge,sludge humic acid,humic acid-modified hydrogel and its leachate is lower than the national limit and meets the safety of agricultural applications.In summary,sludge-derived humic acid has different physicochemical properties from other sources of humic acid,and its introduction into hydrogels can improve the water absorption and water retention properties of hydrogels,and its application as a soil water retention agent can promote crop growth,which has potential value for application in the agricultural field.