Due to its high activation efficiency, waterglass has been widely used for alkali activations in geopolymer. In this study, the n(SiO₂)/n(Na₂O) (Ms) of waterglass was selected as the variable to investigate the role of the silicate structure on the mechanical properties of harden pastes. Ms was changed by the addition of NaOH to obtain the different silicate group, structure and experiments were performed by employing the liquid-sate ²⁹Si nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and gel permeation chromatography (GPC) techniques. Furthermore, selected dissolution, scanning electron microscope (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and FTIR experiments were used to measure the development of the amorphous gel and other materials with different curing condition. Results show that silicate structure of the waterglass was changed via the Si-ONa⁺ formation and the electric charge effect of Na⁺. Under the lower Ms waterglass, the Q⁰, Q¹ and Q_C² structure reverted to the main structure of the silicate group, which was kind of lower seize, molecule weight, linear or circular chain lower geopolymerization degree silicon structure. It would accelerate the geopolymerization speed of prepolymer formation. In addition, higher activity degree of Q⁰ and Q¹ were useful to increase the formation amount of the gel structure with a low Si/Al ratio and size. Thus, silicate structure of waterglass controls the amorphous gel properties to adjust the compressive strength of alkali-activated materials.

Irreversible pollution by heavy metals such as lead (Pb) and cadmium (Cd) adversely affects the ecological environment and human health. Due to its high adsorption, microporosity, and specific surface area, biochar possesses excellent potential for use in heavy metal pollution remediation. The preparation of mixed-based biochar from sludge and cotton stalk can solve the problems inherent to pure sludge biochar, such as undeveloped pore structure and a small specific surface area, while resourcefully utilizing both waste biomass types. This study investigated the adsorption capacity for Pb²⁺ and Cd²⁺ of mixed-based biochar prepared at different pyrolysis temperatures, different pyrolysis residence times, and different cotton stalks percentages. Response surface experiments revealed the optimum process conditions for preparing mixed-based biochar, which included a pyrolysis temperature of 638 °C, a pyrolysis residence time of 86 min, and an addition ratio of 50% for cotton stalks. The isothermal adsorption experiments revealed that the maximum adsorption capacities of mixed-based biochar for Pb²⁺ and Cd²⁺ were 111.11 and 86.21 mg/g, respectively. Our findings suggest the co-pyrolysis of sludge and cotton stalk as a green and sustainable method for safely disposing of Pb and Cd.

Biochars were prepared via the pyrolysis of sewage sludge at temperatures ranging from 350–550 °C. The properties and behaviors of heavy metals in the biochars were investigated. The results indicated that the pH values and ash contents of the biochars increased, while biochar yield and C, H, and N contents decreased with the increasing temperature. A high pyrolysis temperature contributed to a developed biochar pore structure. The specific surface area and pore volume of the biochars increased, while the average pore width decreased, with the increasing temperature. Heavy metals in the biochars were further enriched with the increasing temperature. TCLP tests demonstrated that the leaching potential of heavy metals from the biochars significantly decreased with the increasing temperature, indicating the decrease of potential ecological risks of heavy metals to the environment. Additionally, BCR tests confirmed the transformation of heavy metals from mobile fractions (F1 and F2) to stable fractions (F3 and F4). The evaluation results showed that a high pyrolysis temperature can effectively inhibit the ecological risks of heavy metals in the biochars. Thus, the conversion of sewage sludge into biochar via pyrolysis is a promising method for the safe disposal of sewage sludge.

Phosphate ore is a non-renewable resource, so finding a replacement is necessary. Municipal sludge has significant recycling potential because of its high phosphorus content and large discharge characteristics. The migration and transformation of phosphorus in municipal sludge treated with different concentrations of HCl were studied using the standards, measurements, and testing phosphorus extraction protocol from two aspects: phosphorus complexation and mineral form. After the hydrothermal carbonization treatment without HCl, the hydrochar retained 99.7% of phosphorus in the sludge, and the organophosphorus percentage was about 30%. In the hydrothermal carbonization treatment with the addition of 0.5–2.5% HCl, the phosphorus content in the hydrochar decreased gradually from 99.5% (46.18 mg/g) to 91.8% (64.17 mg/g) that of the original sludge, and the proportion of non-apatite inorganic phosphorus increased from 34% to 94%. Hydrochloric acid provides a low-pH environment and promotes the dissolution of calcium-related phosphorus precipitates and enhances the dehydration reaction. This study provides technical support for the recovery of phosphorus resources from municipal sludge.