Background/Objectives

Decades of rising temperatures, drought, and shifting rainfall patterns in Cambodia pose a threat to food security and human health due to diminished freshwater availability and reduced fish production. In the northeast, both industrial and artisanal mining activities, particularly in Chong Phlas, Mondulkiri province, involve the extraction of gold and copper. Local miners use both physical (gravity) and chemical separation methods, discharging the leftover materials directly into nearby streams, resulting in discolored and contaminated water. Direct contact with this water can immediately harm the skin. The gold processing plant releases sludge, a mixture of sediment and chemicals, into the environment without proper sediment collection or water treatment, posing a potential risk to the surrounding freshwater ecosystem.

Approach/Activities

In situ characterization of acid mine drainage (AMD) involved measuring physicochemical parameters using a multimeter and probes (pH, oxidation-reduction potential [ORP], electrical conductivity [EC], dissolved oxygen [DO]) at nine sampling points. Laboratory analysis included heavy metals (arsenic, iron, and manganese) measured in AMD sample. Eggshells from household waste and limestone as the low-cost geomaterial were ground to 75 μm for X-ray refraction and X-ray diffraction testing. Eggshell and limestone, mainly composed of CaCO₃, showed over 95% CaO from XRF analysis. Batch analysis aimed to remove heavy metals from artisanal mine wastewater, optimizing adsorbent dose and contact time (0.2 g, 0.4 g, 0.6 g, 0.8 g, 1 g, 1.2 g, 1.5 g and 5, 10, 15, 30, 60, 180, 300, 480, and 1440 min). The optimum conditions were determined for effective treatment of AMD.

Results/Lessons Learned

In the dry season, on-site measurements of pH, DO, ORP, and EC ranged from 5.93 to 7.23, 6.06 mg/l to 7.32 mg/l, 24.67 mV, 282.67 mV, and 0.05 to 830.00 mS/cm, respectively. During the rainy season, these parameters varied from 2.31 to 6.19, 1.87 mg/l to 6.87 mg/l, 32.00 mV to 243.00 mV, and 0.06 mS/cm to 4.42 mS/m. Paired averages of these parameters for dry and wet seasons were 6.3 and 3.68; 6.42 mg/l and 5.75 mg/l; 109.81 mV and 149.71 mV; and 332.33 mS/cm and 2.16 mS/cm, respectively. The average iron content was 20.67 mg/l. Manganese concentration in both seasons ranged from 0.1 mg/l to 5.00 mg/l and 0.10 mg/l to 7.50 mg/l, respectively. Arsenic levels were 0.10 mg/l to 1.00 mg/l in the dry season and 0.035 mg/l to 67.5 mg/l in the rainy season. In batch analysis, the AMD water sample with 61.875 ppm of As, 1340 ppm of Fe, and 18.4 ppm of Mn underwent a kinetic study on metal removal using pseudo-first order and pseudo-second order methods.

The adsorbent, as determined through chemical and mineralogical analysis, predominantly consisted of calcite (CaCO₃). The presence of calcite, releasing Ca²⁺, led to an increase in the pH of AMD from 2.4 to beyond 4.0 with 0.2 g of limestone and eggshell adsorbent doses. Consequently, elevating the adsorbent dose resulted in a pH increase approaching 6 for both eggshell and limestone, exceeding 5.5. Effective removal of heavy metals (As, Fe, and Mn) surpassed 95% for both eggshell and limestone adsorbents. The optimal conditions were identified at a 0.6 g adsorbent dose, achieving a remarkable percentage removal of 99.0% for As and Fe, and 97.0% for Mn.

The pH rapidly increased from the initial condition of 2.42 to above 5.5 within the first 5 minutes when using eggshell or limestone. Subsequently, the pH continued to rise, reaching 6.4 after 24 hours of application. In the initial 5 minutes, removal efficiency exceeded 99.0%, 45%, and 95.0% for As, Fe, and Mn, respectively. The optimal removal time was observed at 300 minutes, achieving 99.9% removal for As, over 96% for Fe, and 98.8% for Mn at 0.6 g of adsorbent doses. Increasing contact time enhanced Fe removal, while for As and Mn, the efficiency plateaued with longer contact times. Treatment of AMD wastewater, especially AMD, demonstrated over 95% removal efficiency with the application of low-cost adsorbents like eggshell and limestone at a dose of around 0.2 g and a contact time of 5 minutes for As and Mn, extending to 300 minutes for Fe. The AMD wastewater treatment adhered to the standards set by the Ministry of Environment, Cambodia. Due to their cost-effectiveness, accessibility, and suitability for AMD treatment, eggshells and limestone show promising potential for removing trace elements such as As, Fe, and Mn. Future research should explore their effectiveness in eliminating co-existing ions, including sulfate and magnesium, found in high concentrations in AMD.