Waste Management & Research 2016, Vol. 34(2) 129–138 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15615698 wmr.sagepub.com Introduction Heavy metals present in water bodies are of enormous concern to public health. Cadmium (Cd) is a heavy metal of environmen- tal and occupational concern and has been classified as a human carcinogen by several regulatory agencies (Nordberg, 2009). Major sources of Cd that is released into the environment include the manufacturing of alloy, batteries, pigment and plastic, min- ing and metallurgical industry. A classic example of Cd contami- nation is the Jinzu River in Japan. Many people have been affected by Cd poisoning, which causes the Itai-itai disease, causing severe pain, bone fractures, proteinuria and osteomala- cia (Nordberg, 2009). Therefore, the World Health Organization and the American Water Works Association set the limit of Cd concentration in drinking water to 0.003 mg L −1 (World Health Organization, 1993). Numerous techniques have been developed to eliminate Cd from water and wastewater, including chemical precipitation, membrane, ion exchange and adsorption. Among these methods, adsorption is considered the most economically favourable method for the removal of Cd at low concentrations. Activated carbon (AC) possesses a high surface area and is widely used to remove organic and inorganic contaminants. AC is much more expensive than biochar and therefore its use on a large scale is limited. The estimated cost of biochar is US$246 per ton, which is approximately one-sixth the cost of commercially available AC (approximately US$1500 per ton) (Ahmad et al., 2014) because biochar requires less energy than AC and can be used without further activation or modification. The global production of fresh citrus fruits is approximately 98.7 million tons per year. This includes 62% orange, 17% man- darin and tangerine, 5% citron, 11% lime and lemon, and 5% grapefruit (Zirebwa et al., 2012). In general, orange peel (OP) is an agricultural and industrial waste product in the orange juice and soft drink industries. Therefore, OP is considered an abun- dant source of sorbents, such as biosorbent, biochar, hydrochar and even AC. Unlike biosorbent, biochar is a stable carbon-enriched and porous material. It is synthesized using biomass produced through thermal decomposition or carbonization under limited oxygen conditions or nitrogen atmosphere. Thus, biochar might be a Effect of pyrolysis temperatures and times on the adsorption of cadmium onto orange peel derived biochar Hai Nguyen Tran 1,2 , Sheng-Jie You 2 and Huan-Ping Chao 2 Abstract The mechanism and capacity of adsorption of cadmium (Cd) on orange peel (OP)-derived biochar at various pyrolysis temperatures (400, 500, 600, 700 and 800°C) and heating times (2 and 6 h) were investigated. Biochar was characterized using proximate analysis, point of zero charge (PZC) analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Equilibrium and kinetic experiments of Cd adsorption on biochar were performed. The results indicated that the pH value at PZC of biochar approached 9.5. Equilibrium can be reached rapidly (within 1 min) in kinetic experiments and a removal rate of 80.6–96.9% can be generated. The results fitted the pseudo-second-order model closely. The adsorption capacity was estimated using the Langmuir model. The adsorption capacity of Cd on biochar was independent of the pyrolysis temperature and heating time (p>0.01). The maximum adsorption capacity of Cd was 114.69 (mg g −1 ). The adsorption of Cd on biochar was regarded as chemisorption. The primary adsorption mechanisms were regarded as Cπ–cation interactions and surface precipitation. Cadmium can react with calcite to form the precipitation of (Ca,Cd)CO 3 on the surface of biochar. The OP-derived biochar can be considered a favourable alternative and a new green adsorbent for removing Cd 2+ ions from an aqueous solution. Keywords Biochar, orange peel, cadmium, surface precipitation, adsorption, waste management 1 Department of Civil Engineering, Chung Yuan Christian University, Chungli 320, Taiwan 2 Department of Environmental Engineering, Chung Yuan Christian University, Chungli 320, Taiwan Corresponding author: S-J You and H-P Chao, Department of Environmental Engineering, Chung Yuan Christian University, 200 Chung Pei Road, Chungli 320, Taiwan. Emails: sjyou@cycu.edu.tw and hpchao@cycu.edu.tw 615698WMR 0 0 10.1177/0734242X15615698Waste Management & ResearchTran et al. research-article 2015 Original Article