赤泥对堆肥腐熟度及其产品对稻米Cd阻控效果

周红燕,陈 喆,2*,李侃麒,冷为贵,王宗抗

赤泥对堆肥腐熟度及其产品对稻米Cd阻控效果

周红燕1,陈 喆1,2*,李侃麒1,冷为贵3,王宗抗3

(1.桂林理工大学环境科学与工程学院,广西 桂林 541004；2.广西师范大学,珍稀濒危动植物生态与环境保护教育部重点实验室,广西 桂林 541004；3.贵港市芭田生态有限公司,广西 贵港 537000)

以赤泥作为堆肥添加剂进行鸡粪好氧堆肥试验,分析了堆肥过程中赤泥对温度、pH值、电导率(EC)及种子发芽指数(GI)的影响.通过三维荧光与紫外-可见光光谱特征解析堆肥过程中腐殖酸(HA)组分的演变特征.并利用盆栽实验探索堆肥产品对矿区土壤中稻米镉的阻隔效果.结果表明,赤泥的添加提高了堆体高温期的温度.EC较堆肥前均显著降低,但赤泥堆肥EC(4.29mS/cm)显著高于鸡粪堆肥EC(3.59mS/cm).鸡粪堆肥与赤泥堆肥的GI随着堆肥时间的增长而升高,在堆肥结束时分别达到100.2%和96.44%,说明2种堆肥产品均未表现出植物毒性.2种堆肥HA中类蛋白质等物在堆肥过程中均转化为较为稳定的类腐殖质物质,HA的SUVA254、SUVA280和A226～400在堆肥结束后也显著提高,表明堆肥的腐殖化程度升高.此外,赤泥的添加提高了腐殖化参数数值,证明赤泥的加入能够加速堆肥腐殖化进程.在盆栽实验中,鸡粪堆肥与赤泥堆肥均提高了土壤的pH值,降低了土壤中DTPA-Cd和糙米中Cd的含量.其中,施用2g/kg赤泥堆肥后,糙米Cd含量降低幅度最大,为58.68%,水稻糙米中Cd的含量由未施用堆肥的0.42mg/kg降低至0.17mg/kg.因此,赤泥的添加可以在一定程度上提高堆肥效率,同时施加堆肥产品对土壤Cd的生物有效性及水稻植株内Cd起到抑制与阻隔作用,且施加赤泥堆肥效果更为显著.

赤泥；鸡粪；堆肥；水稻；镉污染；重金属

赤泥是制铝工业提取氧化铝时排出的工业固体废弃物,广西平果铝是中国特大型铝土矿,年产氧化铝高达200万t,年排放赤泥量也高达约200万t[1].铝矿开采造成赤泥的露天随意堆放不仅占用矿区大量耕地资源,而且雨水对赤泥堆场的不断冲刷与淋溶作用已造成矿区大面积重金属污染,其中以镉(Cd)污染耕地超标问题尤为突出.目前,赤泥的主要利用途径包括生产水泥、混凝土[2],提取有价金属[3]等.研究表明,赤泥因其碱性可有效增加土壤pH值,有利于铁锰氧化物的形成[4],能够降低金属的溶解度和生­­­物有效性.同时,由于赤泥中富含的铝硅酸盐可吸附或络合重金属[5],使得赤泥可被作为土壤钝化剂[6]使用,但仍需通过预处理与无害化等手段确保赤泥投加的安全性.通过堆肥可将畜禽粪便中的病原体、抗生素和重金属等物质无害化,使之成为优质的肥料[7].堆肥过程中通常添加秸秆、粉煤灰[8]和石灰[9]等作为膨胀剂和钝化剂,加速堆肥发酵,钝化重金属,提升堆肥品质.土壤中施用有机肥不仅可以调节土壤养分含量和pH值,还可以固定、吸附土壤中的重金属离子.Wang等[10]研究表明,施用有机-无机复混肥可以增加土壤pH值2～3个单位,并降低玉米植株各部位中Cd的浓度;Zhang等[11]的研究也表明,猪粪堆肥可以代替化肥施用于土壤,并能够显著降低小麦籽粒中重金属镍、铜、锌、镉和铅的含量,且低于我国小麦食品标准限值.赤泥对土壤中的重金属也可以起到较好的钝化作用[12].

本研究以鸡粪与米糠为基质,以赤泥为辅料进行鸡粪好氧堆肥试验,通过三维荧光和紫外-可见光光谱,分析堆肥腐殖酸中有机物质的变化,探究赤泥对堆肥效率与质量的影响;并对水稻进行盆栽实验探索堆肥产品对矿区Cd污染土壤中水稻稻米Cd的阻隔效果,为利用赤泥提高堆肥效率与品质提供参考.

1 材料与方法

1.1 供试材料

堆肥实验采用鸡粪与米糠为原料,以赤泥为辅料.鸡粪取自广西桂林丰悦农业科技发展有限公司,米糠取自广西贺州农丰宝公司,赤泥取自广西平果铝业公司.盆栽实验供试土壤采自某矿区周边Cd污染耕地土壤.堆肥原辅材料及供试土壤的基础理化性质如表1所示.

表1 堆肥原辅料与供试土壤的基础理化性质

1.2 堆肥实验

1.2.1 实验设计 堆肥发酵桶有效体积为50L,堆肥桶高为41cm,上口直径为40cm,下口直径为33cm,桶底铺一层多面空心球,上覆双层纱网.根据研究目的共设置2个处理,鸡粪堆肥(CM):将鸡粪:米糠以质量比5:1均匀混合;赤泥堆肥(RM):将鸡粪:米糠:赤泥以质量比5:1:0.75均匀混合,每个处理设置3个重复.调节含水率和碳氮比分别为60%和25/1,采用强制通风好氧堆肥方式,使用氧气泵由下部筛网向上通风供氧,通气量为2L/min.

1.2.2 采样与前处理 堆肥历时约45d,分别在堆肥的第0,5,11,19,28,36,45d采集样品各约600g,采用五点法[13]对堆体中心部位进行取样,混合均匀后采用四分法[13]将样品分为3份各200g,样品于４℃冰箱保存,测定pH值、电导率(EC)、含水率及种子发芽指数(GI).一份样品经－54℃冻干后研磨过100目筛,用于重金属分析与腐殖质的提取实验.另一份样品保存于－20℃冰箱中.

1.2.3 基础理化性质测定 每天早(08:00)、晚(18:00)分别对堆体进行温度测定,从上到下依次测定5,15,25,35,40cm等5个层次的温度,取其算术平均值为当天测量温度,同时记录环境温度.取不同时期新鲜堆肥并按固液比1:10(/)加入超纯水[14],150r/min混合振荡浸提30min,取出并静置30min,待测pH值.取不同时期新鲜堆肥样品并按固液比1:10(/)加入超纯水[14],混合振荡浸提30min,取出在4000r/min,25℃条件下离心10min,待测电导率.

1.2.4 种子发芽指数(%) 取新鲜堆肥样品并按固液比1:10(/)加入超纯水,混合振荡浸提30min,取出并静置浸提1h,在4000r/min,25℃条件下离心10min后,用定性滤纸过滤,吸取10mL 滤液于垫有滤纸的培养皿内,均匀放入20粒小白菜种子(种子需先置于蒸馏水中浸泡2h,挑选饱满种子),盖上皿盖,在25℃培养箱中避光培养72h后测定发芽率和根长[15].每个样品3次重复,同时以去离子水做空白试验.GI(%)=(处理种子发芽率´处理种子根长)/(对照种子发芽率´对照种子根长)´100%.

1.2.5 堆肥腐殖酸(HA)的提取 堆肥腐殖酸提取按照国际腐殖质协会提供的方法[16]进行提取.

1.2.6 HA三维荧光(3D-EEM)测定与分析[17]3D-EEM采用仪器Perkin Elmer Lumines-cence Spectrometer LS50B测定.激发光源:150W氙弧灯;PMT电压:700V;信噪比>110;带通(Bandpass):激发波长(x)＝10nm;发射波长(m)＝10nm;扫描速度:1200nm/min.激发发射波长范围x:300～ 450nm,m:350～600nm.平行因子分析(PARAFAC)采用MATLAB7.11中DOMFluor工具包分析.

1.2.7 HA紫外-可见光光谱(UV-Vis)测定[18]UV-Vis采用UV-8000a光度计测定.以纯水作为空白对照,在紫外吸收区,扫描波长范围为190～ 700nm,扫描间距为1nm.首先将待测样品的TOC调整为20mg/L以下,扫描全谱.

1.3 盆栽实验

1.3.1 实验设计 本实验共设7个处理,不添加堆肥为对照(CK),施加纯鸡粪0.5,1和2g/kg风干土壤(CM0.5、CM1和CM2),施加赤泥堆肥0.5,1和2g/kg风干土壤(RM0.5、RM1和RM2).每个处理设3个重复.将15kg风干土装入25L培养桶,桶高为30cm,上口直径为32cm,下口直径为29cm,在淹水状态下(土面水深 3～4cm)平衡30d后移苗.本实验采用的水稻品种均为陆两优996(Lu Liang You996).每盆移栽4株水稻秧苗,移栽后90d(抽穗期)收获.整个生育期保持土面水深3～4cm.实验期间水稻无病虫害发生.在移苗21d后施入堆肥.

1.3.2 采样与预处理 水稻收获后,将水稻籽粒脱壳,用不锈钢粉碎机粉碎至约1mm的粉末,测定Cd的含量.水稻收获后,采集培养桶中土样,风干,混匀,过20目筛,测定有效态Cd的含量.

1.3.3 有效硅的测定[19]采用0.025mol/L柠檬酸缓浸提,进行钼蓝比色.

1.3.4 重金属总量的测定[20]重金属总量采用三酸(HNO3-HF-HClO4)消解法进行消解,使用ICP- OES待测(ICP-OES,Opetima 7000DV).

1.3.5 重金属有效态的测定[21]称取风干土壤样品并按固液比1:10(/)加入DTPA溶液,在180r/min条件下室温振荡反应2h,然后在4000r/min,25℃条件下离心10min,过0.45μm滤膜,使用 ICP-OES待测.

1.4 数据统计与分析

数据统计分析用Microsoft Excel 2010以及SPSS11.5完成,用Duncan法进行显著性多重比较分析,用Pearson系数法(<0.05)进行相关性分析.数据绘图用Origin 8.0完成.

2 结果与讨论

2.1 赤泥对鸡粪堆体温度、pH值、EC、GI的变化

如图1a所示,堆肥初期温度迅速升高,CM与RM这2个处理的温度均在堆肥第1天达到50℃以上,维持5d,即已达到堆肥无害化卫生要求[22].其中,CM最高温度平均达到62.1℃,RM最高温度平均达到66.8℃,说明赤泥的添加可以显著提高堆体在高温期的温度(<0.05),这可能是因为在堆肥过程中,赤泥的碱性避免了pH值的降低并为微生物提供了足够的钙,增强了微生物的代谢活性[23].

如图1b所示,CM与RM处理的pH值均呈先上升后下降的趋势,这与前人研究[24]结果一致.在堆肥结束时,CM与RM的pH值分别为8.14和8.27.由于微生物分解蛋白质类有机物,含氮物质在微生物作用下被分解并产生大量氨氮,在堆体内积累使 pH值上升.随着堆肥的进行,堆体有机物分解并产生小分子有机酸,导致pH值下降[25].

如图1c所示,在堆肥过程中,CM处理的EC从6.13mS/cm降到3.59mS/cm;RM处理由5.25mS/cm降到4.29mS/cm.2种堆肥的EC值在堆肥过程中均呈现先上升,再下降的趋势.堆肥完成时,RM的EC值要显著高于CM的EC值(<0.05),表明赤泥的添加会增加堆体的EC值[26].

如图1d所示,在堆肥初期,2种堆肥处理的GI增长速率较慢.随着堆肥时间延长,两种处理堆肥过程中GI增长迅速.CM与RM堆肥的GI最终分别达到100.21%和96.44%,这可能是由于RM的EC值大于CM的所导致.堆肥完成时,CM与RM这2种堆肥的GI均超过80%,表明随着堆肥的进行,堆肥产品均已完成无害化并达到腐熟,对植物的毒害作用逐渐减少,且赤泥的投加并未造成堆肥产品的植物毒性[27].

图1 不同处理在堆肥过程中温度、pH值、EC与GI的动态变化

CM:鸡粪堆肥;RM:含赤泥的鸡粪堆肥

2.2 赤泥对堆肥HA组分的平行因子分析

2.2.1 堆肥HA组分特征 如图2所示,经过PARAFAC得出两组HA的组分均主要有3种,分别为组分1(C1:x=395nm,m=480nm)、组分2(C2:x=380nm,m=420nm)和组分3(C3:x=285nm,m=340nm).HA的成分主要以C1和C2为主.C1和C2均属于类腐殖质物质,C1反映了长波类腐殖酸物质,C2反映了类富里酸物质[28];C3则反映了类色氨酸类物质(类蛋白物质)[29].CM与RM堆肥HA的荧光组分与已报道的市政污泥堆肥的荧光物质组成相似[30].

2.2.2 不同分子量荧光组分演变规律 如图3所示,CM处理中,堆肥初期的HA组分主要以C3为主,占比64.15%,随着堆肥进行,C3含量逐渐减少,堆肥结束时C3为22.52%,C1的含量则从13.89%增长至35.84%,C2从21.10%增长至41.64%.RM处理中,堆肥初期HA组分也以C3为主,但显著低于CM中C3含量(<0.05),仅为51.29%,随着堆肥进行,C3含量逐渐下降至20.43%.C1的含量从22.90%增长为38.97%,C2从25.81%增长至40.60%,与CM结果相似.从以上3种组分的含量变化可以得知有机物的降解由难到易为C1>C2>C3,即最易降解的为类蛋白质物质,这可能是由于C3的成分及结构简单,生物利用性较高导致.此外,由于类腐殖酸中含有大量的芳香基团,C1含量的增长说明HA中芳香官能团大量增长[31].而赤泥的添加导致初始C3含量减少,初始C1含量增加,说明赤泥不仅促进了不稳定C3含量的快速降低,还促进更稳定的C1含量增加,从而加速了堆体的解毒过程[32].然而,在该研究中进行PARAFAC分析的样品仅为14个,存在一定的局限性,在后续研究中应使用更多的样品数量进行PARAFAC分析以保证数据的代表性.

图2 三维荧光平行因子分析解析出不同HA的3种组分

图3 堆肥不同阶段中HA组分百分含量变化

2.3 堆肥HA的紫外特征分析

2.3.1 SUVA254和SUVA280不饱和C=C键会引起有机质在254nm波长下的紫外吸收,SUVA254(=254´100/TOC)被认为与芳香族化合物的含量呈正比[33].如图4a、b所示,CM与RM中HA的SUVA254初始值分别为0.76和1.65,堆肥结束时分别为2.50与2.32.SUVA254一直呈增长趋势,说明HA的C=C含量在堆肥过程中逐渐增加,HA物质稳定化程度得到了提高.同样,SUVA280(=280´100/TOC)也可以表征堆肥的腐殖化程度,在相同TOC浓度下,SUVA280值越大,表示堆肥过程中非腐殖物质向腐殖质转化,堆肥产品的稳定程度越高[34].CM与RM中HA的SUVA280初始值分别为0.67和1.51,堆肥结束时分别为2.35与2.30,进一步说明HA腐殖化程度提高.

2.3.2253/203和226-400253/203(有机物分子在紫外-可见光光谱的253nm和203nm处的吸光度之比)与芳香环上取代基的种类和取代程度相关,当芳香环取代基的脂肪链增加,该值将减小,若芳香环中羰基、羧基、羟基等取代基增多时,该值便随之增加[35].如图4c、d所示,CM与RM中HA的253/203(=253/203)初始值分别为0.14和0.17,在堆肥结束时分别为0.27和0.24.在堆肥过程中,CM与RM的253/203总体呈上升趋势,这是由于在堆肥前期,HA物质的耗氧反应剧烈,苯环取代基上的含氧基团相对含量快速升高,苯环上的脂肪链发生聚合反应转化为羟基、羧基等[36].据报道[37],堆肥物质的苯环结构是体现堆肥稳定化程度重要指标,226～400nm吸收带具有很强紫外吸收,与有机质中的多个共轭体系的苯环相关,CM与RM中HA的226-400(226～ 400nm吸收带的区域积分)初始值分别为2.11和5.22,堆肥结束后分别为8.30和7.82.226～400呈上升趋势,说明有机质的分子缩合程度增高,芳香化程度增高,HA物质的稳定程度提高,堆肥产品逐渐稳定.以上结果说明,由于赤泥的加入使得RM中各腐殖化参数初始值显著高于CM的初始值(<0.05),以此加速腐殖化过程.

2.4 堆肥HA不同光谱特征参数相关性分析

如表2所示.CM与RM的C1、C2均在<0.05水平上与C3呈负相关性显著,这说明C3减小会引起C1和C2的增加,而C3可能是C1和C2的来源.CM与RM的SUVA254、SUVA280及226～400两两呈正相关性极显著(<0.01),说明HA中一些含有苯环的有机物可能在254nm及280nm处有极强的吸收峰,进一步验证了堆肥过程中芳香性增加,聚合度升高.此外,CM的SUVA254、SUVA280及226～400均与C1呈正相关性显著(<0.05),RM的SUVA254、SUVA280及226～400均与C2呈正相关性显著(<0.05),且相关性强度由大到小依次为226～400>SUVA280> SUVA254,226～400显示相关性更大的原因可能是该参数为堆肥有机物在226～400nm这一波段的紫外吸收区域积分,它包含了SUVA254及SUVA280的相关信息,虽然这3个参数均可在一定程度上反映堆肥腐熟度,但226～400能较为全面地反映堆肥腐殖化程度的紫外特征参数.在CM中,253/203与以上3个紫外参数呈现出正相关性极显著(<0.01),却与C1未达到显著性水平(>0.05),而在RM中,253/203与以上3个参数均未呈现出显著性水平(>0.05),却与C2正相关性显著(<0.05),表明该参数与SUVA254、SUVA280及226～400相比,不能较好地反映堆肥的腐熟程度.

表2 堆肥HA的荧光组成与光谱特征参数的相关性

注:*,<0.05:相关性显著;**,<0.01:相关性极显著.

2.5 堆肥对水稻植株内Cd的积累效果

表3 鸡粪堆肥与赤泥堆肥的理化性质

由表3可知,经过堆肥处理后,CM和RM处理中重金属的总量均未超过有机无机复合肥料(GB/T 18877-2020)[38]的限量标准.

由表4可知,随着2种堆肥施加剂量增加,土壤pH值逐渐增加.施加鸡粪堆肥与赤泥堆肥后显著降低了土壤中DTPA-Cd含量,土壤中Cd的活性随着施加剂量的增加而降低.未改良土壤中DTPA-Cd的含量为1.10mg/kg,施加鸡粪堆肥与赤泥堆肥后土壤中DTPA-Cd的降低幅度分别为11.92%～20.91%和14.09%～33.59%.施加赤泥堆肥后显著增加了土壤中有效Si的含量,且随着施加剂量的增加而增加,增加幅度为49.07%～75.21%.而施加鸡粪堆肥后对土壤中有效Si影响不显著.施加2种堆肥后,与CK相比,水稻糙米中Cd含量均显著降低,降低幅度为17.97%～58.68%.糙米Cd的含量在0.17～0.42mg/kg之间,水稻糙米Cd含量由大到小为:CK>CM0.5> CM1>CM2>RM0.5>RM1>RM2,其中RM2处理下的糙米Cd含量低于国家《食品安全国家标准食品中污染物限量》中的规定值(0.2mg/kg,GB 2762- 2017)[39].

表4 不同剂量的鸡粪堆肥与赤泥堆肥对土壤性质及糙米中Cd含量的影响

注:同列不同小写字母表示不同处理间存在显著差异(<0.05,=3).

土壤中有效态Cd与糙米Cd含量的降低,可能是由于堆肥的碱性有效增加了pH值,有利于重金属氢氧化物复合物沉淀的生成[40].而赤泥中还含有硅酸盐矿物,SiO32-水解产生的OH-对中和土壤酸性、提高土壤pH值有着至关重要的作用.此外,硅酸盐矿物中的有效硅可与土壤中移动态Cd2+形成CdSiO3沉淀物,使重金属沉积在土壤和植物根系表面[41].

同时,因堆肥过程中形成大量腐殖质,腐殖质和土壤中重金属离子发生螯合与络合反应[42].腐殖质中的羧基、酚羟基以及羰基等活性功能基团增加,促进腐殖质提供更多可络合重金属的吸附点位[43],形成有机金属配合物.其中,氨基和巯基基团对Cd2+的络合与羟基对Cd2+的共沉淀均起重要作用.可见,施加堆肥对土壤Cd的生物有效性有抑制作用,对水稻植株内Cd起到有效的阻隔效果,且施加赤泥堆肥效果更为显著.

3 结论

3.1 鸡粪堆肥与赤泥堆肥高温期均维持5d,已达到无害化卫生要求,且添加赤泥后,提高了堆体高温期的温度.2种堆肥的EC较堆肥前均显著降低,堆肥完成时,RM的EC要显著高于CM的EC,表明赤泥的添加会增加堆体的EC.鸡粪堆肥与赤泥堆肥的GI在堆肥结束时分别达到100.21%和96.44%,说明两种堆肥产品均没有植物毒性,且达到腐熟状态.

3.2 随着堆肥的进行,2个处理的腐殖酸中类蛋白质等物逐渐转化为较为稳定的类腐殖质物质,腐殖酸的SUVA254和SUVA280、253/203、226～400在堆肥结束后也均得到提高,说明堆肥的芳香性与腐殖化程度升高,且赤泥的添加会加速堆体的腐殖化进程.

3.3 鸡粪堆肥与赤泥堆肥提高了土壤的pH值,显著降低了土壤中DTPA-Cd的含量,施加2g/kg赤泥堆肥后,土壤中DTPA-Cd的含量降低了33.59%,且显著增加了土壤中有效Si的含量.施加2种堆肥后,糙米中Cd含量降低幅度为17.97%～58.68%,且施用2g/kg赤泥堆肥时,水稻糙米种Cd的含量降低至0.17mg/kg,并达到中国的食用安全标准.

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Effects of red mud on compost maturity and Cd resistance control of rice.

ZHOU Hong-yan1,CHEN Zhe1,2*,LI Kan-qi1,LENG Wei-gui3,WANG Zong-kang3

(1.College of Environmental Science and Engineering,Guilin University of Technology,Guilin 541004,China；2.Key Laboratory of Ecology and Environmental Protection of Rare and Endangered Plants,Ministry of Education,Guangxi Normal University,Guilin 541004,China；3.Guigang Patan Ecology Co.,LTD.,Guigang 537000,China).,2022,42(8)：3812～3821

Red mud was used as an additive to conduct aerobic composting experiments of chicken manure for evaluating effects of red mud on temperature,pH,electrical conductivity (EC),and seed germination index (GI) during composting. The evolution characteristics of humic acid (HA) components during composting were analyzed by combining three-dimensional fluorescence with ultraviolet-visible light spectrum; and the pot experiments were also conducted to explore the barrier effect of compost products on rice cadmium in the mining soil. The results showed that red mud increased the temperature of the compost during the high temperature period. The EC of both groups decreased significantly after composting,however,the EC of red mud compost (4.29mS/cm) was much higher than that of chicken manure compost (3.59mS/cm).The GI of chicken manure compost and red mud compost increased with composting time by; 100.2% and 96.44%,respectively,at the end of composting,indicating that the products of neither red mud compost nor chicken manure compost exhibited phytotoxicity. The protein-like substances in the HA of the two kinds of composts were converted into relatively stable humus-like substances throughout the composting process. A significant increase in SUVA254,SUVA280,and A226-400of HA indicates the elevated humification degree of compost. In addition,red mud could optimize the humification parameters,implying that the addition of red mud can accelerate the humification process of the heap. In pot experiments,both chicken manure compost and red mud compost increased soil pH and reduced the concentration of DTPA-Cd in soil and total Cd in brown rice. After applying 2g/kg red mud compost,the Cd content in brown rice decreased significantly (by 58.68%) from 0.42mg/kg to 0.17mg/kg after composting. Therefore,red mud can accelerate the composting efficiency to a certain extent. The application of compost products can inhibit the bioavailability of Cd in both soil and rice plants,and an addition of red mud can lead to a higher composting efficiency.

red mud；chicken manure；composting；rice；cadmium pollution；heavy metals

X705

A

1000-6923(2022)08-3812-10

2022-01-24

广西创新研究团队项目(2018GXNSFGA281001);广西科技基地和人才专项(桂科AD19110012);贵港市科技研发项目(贵科攻2021019);桂林市重大科技专项(20190219-3)

* 责任作者,讲师,ldchenzhe@qq.com

周红燕(1997-),女,四川广安人,桂林理工大学硕士研究生,主要从事固体废物资源化研究.