应用负载型纳米铁去除饮用水中砷的研究

砷是毒性最强的元素之一，水体中砷的污染己经引起人们广泛的关注。我国的新疆、内蒙、山西和台湾等省和地区地下水砷含量严重超标。全球共有5,000多万人遭受高砷饮用水的威胁，其中中国有1,500多万，是饮用水砷污染最严重的国家之一。WHO推荐饮用水砷的最高允许浓度从原来的50 µg•L-1已降至10 µg•L-1。更为严格的砷卫生标准的颁布，对作为饮用水源的地下水中的砷去除工艺提出了更高的要求。吸附法除砷比膜法、混凝法和离子交换法更安全、简便，是砷去除工艺中最有效的方法之一。 首先，本研究通过优化制备条件（包括炭种类的选择、炭的粒径大小、还原剂的浓度及滴定速率、反应温度、铁盐的种类及浓度、分散剂的比例及浓度），制备了负载型纳米铁。考虑到砷的去除效率、工程应用的可行性以及经济性，最优的制备条件如下：选用粒径为20~40目煤质炭，在室温、一定的分散剂比例及浓度，0.2 M KBH4滴速为20 d•min-1时所制备的Fe/炭为82.0 mg•g-1；纳米铁在活性炭孔内呈针状，其直径为30~500 nm，长度为1,000~2,000 nm。绝大多数的铁都负载到活性炭内部，这在处理水时铁不流失很重要。 其次，利用制备的负载型纳米铁作吸附载体，进行了饮用水中As(Ⅴ)的吸附去除实验。研究了该吸附剂对As(Ⅴ)的吸附等温线、动力学以及影响动力学的各种因素（包括As(Ⅴ)的不同初始浓度、吸附剂用量、pH值、共存离子和不同温度）、pH值、共存离子等环境条件对As(Ⅴ)去除的影响；以及吸附剂的再生及再生后的吸附效率等。研究发现在前12 h内吸附较快，72 h时达到了平衡。用Langmuir 吸附等温式估算出As(Ⅴ)的吸附量为12.0 mg•g-1。该吸附剂在pH 6.5， (25±2)℃， As(Ⅴ)初始浓度为2 mg•L-1，吸附剂用量为1.0 g•L-1时，As(Ⅴ)的去除率为75.2%；当把吸附剂的用量增加到1.5 g•L-1时，As(Ⅴ)的去除率可达99.9%以上。吸附剂可以用0.1M的NaOH浸泡12 h后即可再生，再生效率较高。常见的阴离子中PO43-、SiO32-对As(Ⅲ)的去除抑制较大，而SO42-、CO32-、C2O42-等离子对砷的去除影响较小。Fe2+对As(Ⅲ)的吸附抑制作用较大而其它阳离子影响不大。吸附剂可用0.1 M NaOH 有效再生，并且具有良好的机械性能。实验室初步实验数据表明，该吸附剂对饮用水除砷具有较好的应用前景。 第三，利用实验室制备的负载型纳米铁对饮用水中As(Ⅲ)的吸附去除也进行了研究。考察了吸附等温线、动力学以及影响动力学的各种因素、pH值、共存离子等环境条件对As(Ⅲ)去除的影响；以及吸附剂的再生及再生后的吸附效率等。研究发现，该吸附剂在pH 6.5， (25±2)℃， As(Ⅲ)初始浓度为2 mg•L-1，吸附剂用量为1.0 g•L-1时， 对As(Ⅲ)的去除率为99.8%；其吸附容量为1.996mg•g-1。吸附过程中部分As(Ⅲ)被氧化。与As(Ⅴ)的吸附相比，该吸附剂对As(Ⅲ)的效率比较高-而常见的其它除砷吸附剂如载铁纤维棉等，对As(Ⅴ)的效率比As(Ⅲ)高，为有效去除As(Ⅲ)，常常需要专门加上氧化这一过程。 最后，利用负载型纳米铁对饮用水中As(Ⅲ) 的氧化性能进行考察，发现该吸附剂不但能够有效吸附去除饮用水中的砷，而且还能把As(Ⅲ)有效地氧化为As(Ⅴ)。经过对吸附剂的构成组分分析发现，活性炭表面因富含多种官能团而对三价砷的氧化作用最大；其次是纳米铁也能把As(Ⅲ)氧化为As(Ⅴ)。

Arsenic is one of the most toxic elements. The pollution of arsenic from aqueous systems arose people's attention. The concentration of arsenic is much higher than MCL(minimum concentration limit) from groundwater of XinJiang, Neimeng, Shanxi province and Chinese Taiwan. There are more than 50 million people are threaten by the arsenic contamination drinking water and 15 million people in Chian where is one of the seriousest arsenic contamination area. WHO recommended the arsenic MCL of drinking water from 50 µg•L-1 to 10 µg•L-1. The stricter MCL of arsenic is promulgated, the higher efficiency of removal is demanded. Among the possible treatment processes, adsorption is considered to be less expensive procedure that is easier and safer to handle as compared to precipitation, ion exchange, and membrane filtration. First, We optimize parameters (including the kind and the grain size of Active carbon, the concentration and titration rate of reductant, reaction temperature, the kind and concentration of the ferric salt, the proportion and concentration of the dispersant ) that are the most effective in the loading ferric onto the AC and find out maximum mass of iron Fe/AC 82.0 mg•g-1 will reach (1) when the particle size is 20-40 mesh; (2) when the titration velocity of 0.2 M KBH4 is 20 d/min; (3) at room temperature and appropriate dispersant. The supported NAVI particles in the pores of AC are needle shaped and the particle size is 30 - 500 nm in diameter and 1000 - 2000 nm in length. The majority of the NZVI particles are <100 nm in diameter. In comparison, the nano-sized zero-valent iron synthesized in solution was cluster of aggregates of round-shaped particles with the diameter < 100 nm. It can also be seen that the most of the zerovalent iron particles were loaded into the pores and cracks rather than onto outer surfaces. This is very important for repeated use in water treatment facilities without lose of the iron particles. Second, adsorption kinetics and isotherms of arsenate As(Ⅴ) from aqueous solutions onto a novel adsorbent, nano zero-valent iron supported on activated carbon (NZVI/AC), was investigated in this work. The effects of various experimental parameters on adsorption kinetics, including initial As (V) concentration, adsorbent dosage, pH, coexisting ions and temperature were investigated using a batch-adsorption technique. Kinetics study revealed that adsorption of arsenate by NZVI/AC was fast in the first 12 h and the equilibrium was achieved in ~72 h. The adsorption capacity of the synthesized sorbent for arsenate at pH6.5 calculated from Langmuir adsorption isotherms in batch experiments was 12.0 mg•g-1. Phosphate and silicate markedly decreased the removal of both arsenite and arsenate, while the effect of other anions and humic acid was insignificant. Common metal cations enhanced arsenate adsorption but ferrous iron was found to suppress arsenite adsorption. NZVI/AC can be effectively regenerated by elution with 0.1 M NaOH. The adsorbent had ideal rigidity and abrade enduring performance. The results suggest that NZVI/AC is an ideal candidate for the treatment of arsenic contaminated drinking water. Third, adsorption kinetics and isotherms of arsenate As(Ⅲ) from aqueous solutions onto NZVI/AC was investigated in this work, too. The adsorption capacity for arsenic was approximately 1.996mg/g activated carbon supported nano zero-valent iron (NZVI/AC) in the 2mg•L-1 As(Ⅲ) solution at pH 6.5 and (25±2) ºC. As(Ⅲ) was partly oxidized by the absorbent in the process of absorption. The efficiency adsorption of As(Ⅲ) was higher than that of As(Ⅴ) while other common absorbent, such as the bead cellulose adsorbent loaded with β-FeOOH, was easier to adsorb As(Ⅲ) than As(Ⅴ) and it was oxdated before adsorption. Last, As(Ⅲ) was partly oxidized by the NZVI/AC in the process of absorption. The activated carbon, which has many oxidative functional groups can oxidize As(Ⅲ) and nano zero iron also oxidize it partly.