In this study, adsorption of the heavy metal ions (Pb(II), Cu(II) and Cd(II)) from water by peanut shells (PS), sawdust (S) and commercial activated carbon (AC) were comparatively studied. Thus, the relationship between different adsorption parameters and different heavy metal ion removal rates was investigated. The adsorption capacity of the three adsorbents for heavy metal ions increased with an increase in temperature, pH value, contact time, adsorbent dosage, and heavy metal ion concentration, however, it decreased with an increase of adsorbent particle size. All the adsorption processes are better described by Langmuir isotherm or Freundlich isotherm. Thus, the results show good agreement with pseudo-second-order kinetics and the adsorption processes are spontaneous heat absorption processes. Herein, all adsorbents have higher affinity for Pb(II) ions, and hence possess higher removal rates. In addition, heavy metal ions were desorbed significantly at acidic conditions in the desorption experiments. The results demonstrate that PS can be used as a green adsorbent instead of AC for the adsorption of heavy metal ions from the water.

Agricultural waste materials (peanut shell and sawdust) were used as replacements for activated carbon to remove heavy metal ions from aqueous solution. Batch adsorption experiments were conducted in single, and competitive systems. The adsorption efficiencies of different heavy metal ions were, in descending order: lead, copper, cadmium. Activated carbon showed best adsorption efficiency (lead: 14.01 mg/g; copper: 13.1 mg/g; cadmium: 5.5 mg/g), followed by peanut shell (lead: 9.5 mg/g; copper: 49.1 mg/g; cadmium: 5.07 mg/g), and sawdust (lead: 5.5 mg/g; copper: 5.3 mg/g; cadmium: 3.99 mg/g), which showed the worst efficiency. The amount of heavy metal ions adsorbed onto activated carbon, peanut shell and sawdust increased with the increase of adsorption time and then reached equilibrium values. As the initial concentration of heavy metal ions increased, the amount of heavy metal ions adsorbed increased while the removal ratios thereof decreased. The adsorption processes in both single and competitive systems followed a Langmuir isotherm model and pseudo second-order kinetic model. The intraparticular diffusion process can be divided into two stages for the adsorption process of heavy metal ions. There were antagonistic effects among three kinds of heavy metal ions in a competitive system.

Industrial solid waste red mud discharge has caused serious environmental problems. This study utilized red mud as an additive to loessial silt being used for roadway subgrade material. In this study, unconfined compressive test, direct shear test, electrical resistivity test, and hydraulic conductivity test were conducted on red mud stabilized loessial silt (RMLS) with different red mud dosage (D_R) to investigate D_R effect on mechanical-electrical-hydro properties. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were carried out to reveal the mechanism from micro perspective. The results showed addition of appropriate amount of red mud (30–42%) effectively improved unconfined compressive strength of treated loessial silt but reduced resistivity and hydraulic conductivity. Significant correlation between resistivity and strength performance of RMLS mixture was developed. Microscopic analysis indicates red mud addition will promote generation of hydration products such as calcium silicate hydrations (C-S-H), calcium silicate aluminates hydrations (C-A-S-H), and ettringite (Aft), which will tightly connect surrounding particles of loessial silt and hydrates. Red mud particles adhere to surface of soil particles and fill in pores between them improving a compact and stable structure. This study demonstrated the feasibility of using red mud as a stabilization material for roadway subgrade and proved that resistivity measurement is a nondestructive testing method to evaluate mechanical properties for RMLS mixture.

Bentonite is widely used as a waterproof material in civil engineering. The swelling capacity and impermeability will be reduced in saline water conditions. A polymer-modified bentonite was used in this study, and the swelling characteristic and hydraulic conductivity in saline water were investigated. Results show that: the modified bentonite (MB) has good swelling characteristics and low permeability in saline water conditions; the unload swelling strain of MB in saline water conditions increased with rising swelling time; the final unload swelling strain of MB decreased with the increase of vertical pressure and increased with increasing initial dry density; and, as the swelling time increased, the swelling pressure increased rapidly at first and then decreased. Based on experimental results, this study obtained a linear relationship between the ratio of time to unload swelling strain and swelling time, a formula for load swelling strain that was related to initial dry density and vertical pressure, a linear relationship between the ratio of time to swelling pressure and swelling time, and an exponential relationship between swelling pressure and initial dry density. The MB can be used as a waterproof material in seepage-prone applications under saline environmental conditions in civil engineering works.